OCS Study

MMS 2002-077

Coastal Marine Institute

Offshore Petroleum Platforms: Functional

Significance for Larval Fish Across Longitudinal

and Latitudinal Gradients

Authors

Richard F. Shaw

David C. Lindquist

Mark C. Benfield

Talat Farooqi

John T. Plunket

DECEMBER 2002

Prepared under MMS Contract

14-35-0001-30660-19961

by

Coastal Fisheries Institute

Louisiana State University

Baton Rouge, LA 70803

Published by

U.S. Department of the Interior Cooperative Agreement

Minerals Management Service Coastal Marine Institute

Gulf of Mexico OCS Region Louisiana State University
iii

DISCLAIMER

This report was prepared under contract between the Minerals Management Service (MMS) and

the Coastal Fisheries Institute (CFI), Louisiana State University (LSU). This report has been

technically reviewed by the MMS and it has been approved for publication. Approval does not

signify that the contents necessarily reflect the views and policies of LSU or the MMS, nor does

mention of trades names or commercial products constitute endorsement or recommendation for

use. It is, however, exempt from review and compliance with the MMS editorial standard.

REPORT AVAILABILITY

Extra copies of the report may be obtained from the Public Information Office (Mail Stop 5034)

at the following address:

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Boulevard

New Orleans, Louisiana 70123-2394

Telephone: (504) 736-2519 or

1-800-200-GULF

CITATION

Suggested Citation:

Shaw, R.F., D.C. Lindquist, M.C. Benfield, T. Farooqi and J.T. Plunket. 2001. Offshore

Petroleum Platforms: Functional Significance for Larval Fish Across Longitudinal and

Latitudinal Gradients. Prepared by the Coastal Fisheries Institute, Louisiana State

University. U.S. Department of the Interior, Minerals Management Service, Gulf of

Mexico OCS Region, New Orleans, LA. OCS Study MMS 2002-077. 107 pp.
v

ACKNOWLEDGMENTS

We gratefully acknowledge funding by the Minerals Management Service-Louisiana State

University-Coastal Marine Institute (Contract Number 30660-19961, OSR Number 10483). We

thank Jason Box, Danny Cline, Landon Franklin, Robin Hargroder, Sean Keenan, Brian Milan,

Rishi Ramtahal, Paul Rogers, and Christopher Whatley for their assistance in the field and

laboratory. We are indebted to James Ditty, Joseph Cope and Frank Hernandez, Jr. for their

efforts during our previous platform research, and during the initial stages of this study. We

gratefully acknowledge the assistance of Drs. Ann Scarborough-Bull, Charles Wilson and David

Stanley. We would also like to thank the Santa Fe-Snyder Oil Corporation and Murphy Oil for

access to their offshore oil and gas platforms and logistical support, and the crews of Santa FeSnyder’s MP 259A and Murphy Oil’s VK 203 for their assistance and hospitality.
vii

EXECUTIVE SUMMARY

The introduction and proliferation of offshore oil and gas platforms in the northern Gulf

of Mexico (Gulf) has undoubtedly affected the marine ecosystem. There are approximately

4,000 platforms in the federal waters of the Gulf. Because a mud/silt/sand bottom with little

relief or hard-substrate dominates the northern Gulf, especially west of the Mississippi River

Delta (Delta), any additional hard-substrate provided by platforms could prove significant.

Although platforms provide an estimated 11.7 km2

 (or 0.4%) of the total “reef” habitat in the

northern Gulf, platform significance may be enhanced by the vertical relief of their substrate,

which extends from the bottom through the photic zone to the surface. Because fish populations

are usually limited by available energy, recruitment, or habitat, it is important to determine if

platforms: 1) serve as new or additional spawning habitat; 2) provide critical habitat for early life

history stages; or 3) influence energy flow through the ecosystem by aggregating prey.

Some researchers believe that certain reef fish species may be experiencing nursery

habitat limitation, i.e., the availability of shelter at the time of settlement is the limiting factor for

reef fish populations. Therefore, an underlying rationale for artificial reef deployment is that

they provide critical habitat for settling reef fishes, and, subsequently, improve recruitment and

adult production. Despite research efforts, however, biologists still disagree over the paradigm

of whether artificial reefs contribute significantly to new fish production or simply attract and

concentrate individuals from surrounding habitats. Because the north-central Gulf has little reef

habitat, it is likely that the contribution of artificial reefs (e.g., platforms) has enhanced reef fish

populations, although the net impact of this augmentation is unknown.

Few baseline, ecological ichthyoplankton studies within the Gulf’s offshore oil and gas

fields have been published, and, therefore, the role that platforms may play as essential fish

habitat has not been adequately addressed. This study focused on three objectives designed to

evaluate the ecological significance of platforms to the early life history stages of fishes. The

first objective was to characterize the larval and juvenile fish assemblages at two platforms east

of the Delta. Data from these collections were analyzed with data collected previously from

three platforms west of the Delta to determine whether there were differences in the larval and

juvenile fish assemblages at platforms by depth (across-shelf) and by east/west of the Delta

(along-shelf), as seen for adult populations. The second objective was to assess how platforms

affect the local distribution and abundance of larval and juvenile fishes, and, therefore, whether

platforms have a nursery/refugia function for fishes. The third objective was to provide much

needed ecological information on the early life history stages of reef fishes, e.g., seasonality,

lunar periodicity, horizontal distribution, and relative abundance.

Larval and juvenile fishes were sampled at two platforms east of the Delta. Santa FeSnyder’s Main Pass (MP) 259, which stands in 120 m of water on the outer shelf (29°19’32” N,

88°01’12” W), was sampled over two-night periods, twice monthly (i.e., new and full moon

phases) during May-September 1999. Murphy Oil’s Viosca Knoll (VK) 203, which stands in 35

m of water at mid-shelf (29°46’53” N, 88°19’59” W), was also sampled over two-night periods,

twice monthly during May-October 2000. These platforms were selected to complement and

supplement previous sampling efforts at platforms west of the Delta [i.e., the inner shelf platform

South Timbalier (ST) 54, the mid-shelf platform Grand Isle (GI) 94, and the outer shelf platform
viii

Green Canyon (GC) 18]. Larval and juvenile fishes were collected within the platform structure

using passive plankton nets and light traps fished at the surface, and off-platform (about 20 m

down-current of the platform) using light traps. Data collected from MP 259 and VK 203 were

analyzed with data from the three platforms west of the Delta.

Patterns of larval and juvenile fish abundance and diversity were primarily influenced by

across-shelf gradients of increasing water depth. Larval fish total densities from plankton nets

were highest for the inner shelf platform ST 54, and decreased with increasing depth. Light trap

total CPUEs were highest for the mid-shelf platforms VK 203 and GI 94, and were generally low

at the outer shelf platforms. Diversity and taxonomic richness were both generally greatest at the

mid-shelf platforms. This higher diversity and abundance of postlarval and juvenile fishes at the

mid-shelf platforms may be attributed to an overlap in species distributions between coastal and

oceanic/tropical taxa. Furthermore, the platforms at intermediate depth may have higher

diversity because they reside in areas of highest platform concentration, i.e., more potential upcurrent sources of larvae.

The species composition of the non-clupeiform larval and juvenile fish assemblages at

platforms also seemed to be structured by depth. Cluster analysis found three general

assemblages: an inner shelf assemblage (ST 54), a mid-shelf assemblage (GI 94 and VK 203)

and an outer shelf assemblage (GC 18 and MP 259). High densities of sciaenids, synodontids

and other coastal taxa characterized the inner shelf assemblage. The mid-shelf assemblage was

characterized by blenniids, synodontids, and Bregmaceros cantori. The mid-shelf platforms also

had the highest abundance and diversity of reef fishes, particularly blenniids, lutjanids, and

pomacentrids. The outer shelf assemblage was primarily composed of oceanic pelagic taxa, such

as carangids and scombrids, and mesopelagic taxa. Because they generally agree with those

found for adult populations, the assemblages observed at platforms probably reflect adult

spawning behavior. The only differences observed in the larval and juvenile fish assemblages

across longitudinal gradients (i.e., east or west of the Delta) were differences in the abundance of

certain taxa. The higher abundances of these taxa collected at platforms east or west of the Delta

may be attributed to the dominant hydrographic conditions and the availability of habitat in the

northeastern and northwestern Gulf.

The waters within platforms had higher abundance and diversity of postlarval and

juvenile fishes than waters immediately down-current of the platform. Clupeiforms (clupeids

and engraulids) and synodontids were consistently collected in higher abundance within the

platform than off-platform. Taxa collected primarily in waters down-current of the platforms

included scombrids and possibly carangids. These taxa are highly predatory and, therefore, may

be utilizing the concentrations of zooplankton and ichthyoplankton that are attracted to the light

field of platforms. Reef fish were occasionally found in higher abundance within-platform, yet

pomacentrids and the blenniid Hypsoblennius invemar were often found in higher abundance offplatform. These specimens, though, were collected at settlement-size and may represent

individuals settling to the platform. The few differences observed between the “background”

continental shelf (SEAMAP) collections and platform abundances of reef taxa were confounded

by the large disparity in sampling effort between SEAMAP and platform samples. In general,

blenniids and pomacentrids were more abundant in platform plankton nets and light traps, while

serranids and lutjanids were more abundant in SEAMAP bongo nets and neuston nets.
ix

The larval and juvenile reef fishes collected at platforms exhibited distinct temporal

patterns of abundance. Most of the reef taxa were collected during months when surface water

temperatures were at their highest, i.e., June-August. The exceptions were the serranid

subfamilies Anthinae and Serraninae, which were predominantly collected during the late spring

and early fall, respectively. There were strong differences in the numbers and developmental

stages collected between new and full moons. Many preflexion reef larvae (larval stages before

the onset of caudal fin development) were collected in higher numbers on new moons, but

preflexion holocentrids, labrids and some blenniids were collected predominantly on full moons.

Postflexion reef larvae, however, were almost exclusively collected on new moons, which was

consistent with peak periods of settlement for many reef fishes. Coefficients of variation,

calculated from the mean plankton net and light trap catches per sampling night and sampling

trip, indicated pulses in the catches of several dominant taxa at platforms. Light trap-collected

blenniids and pomacentrids, in particular, were primarily found in very discrete pulses containing

the majority of settlement-size fishes collected at platforms. The occurrence of these pulses

during darker periods of the lunar month (i.e., first quarter and new moons) further suggests that

they were settlement-related. Settlement pulses, comprising the majority of a given year class,

have been observed for many species of reef fish.

Examination of the length frequency and developmental stages of reef taxa collected at

platforms provided indirect evidence of the potential spawning and nursery/recruitment habitat

provided by platforms. Recently-hatched and preflexion blenniid, holocentrids, labrid, lutjanid,

scarid, and serranid larvae were collected at every platform, indicating near by (recent)

spawning. Although it is possible that these larvae were spawned at natural reefs, the

preponderance of platforms within the transport envelope of these larvae, and the general

absence of natural hard bottoms off central Louisiana, make platforms the most probable source.

Blenniids and pomacentrids were the most abundant settlement-size reef taxa. Settlement-size

larvae of other reef taxa were relatively rare, as compared with blenniids and pomacentrids, and

were represented mostly by lutjanids, particularly at the mid-shelf platforms. The relative

abundance of settlement-size reef larvae at platforms may be influenced by two disparate

spawning strategies exhibited by reef fishes: broadcast spawning of pelagic eggs or benthic

spawning of demersal adhesive eggs. Pelagic spawners, e.g., serranids and labrids, generally

have larvae with longer larval durations that are more susceptible to predation and advective

loss. Meanwhile, benthic spawners, e.g., blenniids and pomacentrids, generally have larvae with

shorter larval durations and limited dispersal, and, thus, may be able to remain in areas where

suitable settlement habitat is more available.

Environmental conditions influenced the abundance of larval and juvenile fishes

collected at both MP 259 and VK 203. Seasonal variations in temperature influenced the

abundances of several taxa at the outer shelf platform MP 259. Several taxa were also associated

with different water masses that occurred at MP 259. There was less environmental variability at

the mid-shelf platform VK 203, and consequently microzooplankton biomass was the most

influential environmental variable for larval and juvenile fish abundances. Catch efficiency of

the gear was also affected by the prevailing environmental conditions. Light trap efficiency was

impaired by high water current conditions with CPUEs <10 fish/10 min at current speeds greater

than 30 cm s-1. At these current speeds, the catch was predominantly preflexion larvae (median

length <10 mm) that were passively entrained by the gear. Larger postlarvae and juveniles were
x

seemingly relatively unavailable to light traps at the highest current speeds, probably due to their

inability to maneuver to and enter the light trap at these current speeds.

This study represents an important step towards understanding the ecological significance

of oil and gas platforms to the early life history stages of fishes. Platforms represent hardsubstrate (i.e., “vertical benthos”) imposed on the pelagic environment. However, the most

common postlarval and juvenile fishes collected at platforms represented species that are pelagic

as adults, e.g., clupeiforms, carangids and scombrids, and the presettlement stages of soft-bottom

taxa, e.g., synodontids. Populations of these taxa may benefit from increased foraging

opportunities provided by concentrations of prey that may be found in the waters around

platforms. Platforms may also provide nursery/recruitment habitat for certain reef taxa, i.e.,

blenniids and pomacentrids. Because artificial reefs most likely benefit species that are habitatlimited, platforms should enhance the production of typical shallow water reef taxa (e.g.,

blenniids and pomacentrids) by providing additional spawning habitat and, to a much lesser

extent, nursery/recruitment habitat.
xi

TABLE OF CONTENTS

PAGE

Acknowledgments ............................................................................................................. v

Executive Summary ........................................................................................................... vii

List of Figures .................................................................................................................... xiii

List of Tables ..................................................................................................................... xvii

Introduction ....................................................................................................................... 1

Materials and Methods ...................................................................................................... 3

 Study Areas ................................................................................................................... 3

 Sampling Procedures .................................................................................................... 5

 Data Analyses .............................................................................................................. 7

Results ............................................................................................................................... 9

 Environmental Characterization of Sampling Sites ...................................................... 9

 Characterization of Fish Catches at MP 259 and VK 203 ............................................ 12

 Across- and Along-Shelf Characterization of Larval and Juvenile Fish Assemblages 12

 Within- vs. Off-Platform Patterns in Light Trap Catches ............................................ 33

 Comparisons of Within-, and 20 m and 50 m Off-Platform Light Trap Catches ......... 40

 Comparisons of SEAMAP vs. Platform Collections .................................................... 46

 Seasonal Patterns of Plankton Net and Light Trap Catches ......................................... 53

 Lunar Periodicity .......................................................................................................... 53

 Among Trip and Night Variability in Plankton Net and Light Trap Catches .............. 59

 Within-Night Variability in Plankton Net and Light Trap Catches .............................. 65

 Length-Frequency and Developmental Stages of Reef Taxa ....................................... 65

 Environmental Variables and Larval and Juvenile Fish Abundances .......................... 79

 Relationships between Water Current Speed and Light Trap CPUE ........................... 79

Discussion .......................................................................................................................... 87

 Across- and Along-Shelf Larval and Juvenile Fish Assemblages ................................ 87

 Within- and Off-Platform Distribution of Larval and Juvenile Fishes ......................... 89

 Seasonal Patterns of Larval and Juvenile Fish Abundance at Platforms ...................... 91

 Lunar Periodicity .......................................................................................................... 92

 Temporal Variability of Plankton Net and Light Trap Catches ................................... 92

 Length-Frequency and Developmental Stages of Reef Taxa ....................................... 94

 Environmental Variables and Larval and Juvenile Fish Abundances .......................... 95

 Effects of Water Current Speed on Light Trap Catches ............................................... 96

Conclusions ....................................................................................................................... 96

Literature Cited .................................................................................................................. 99
xiii

LIST OF FIGURES

Figure PAGE

1. Location of oil and gas platforms sampled in the northern Gulf of Mexico ........ 4

2. Mean surface temperatures, surface salinities, surface turbidities, and

microzooplankton biomass (with standard errors) for each sampling trip at the

outer shelf platform, MP 259 (depth = 120m) ...................................................... 10

3. Mean surface temperatures, surface salinities, suspended solids, and

microzooplankton biomass (with standard errors) for each sampling trip at the

mid-shelf platform, VK 203 (depth = 35m) .......................................................... 11

4. Mean total densities of fishes from plankton nets (with standard errors) at each

platform for data a) with clupeiforms and b) without clupeiforms ...................... 25

5. Mean total CPUEs of fishes from light traps (with standard errors) at each

platform for data a) with clupeiforms and b) without clupeiforms ....................... 26

6. Cluster diagram of the overall larval and juvenile fish assemblages, with

clupeiforms excluded, collected from all gears combined at each platform ........ 29

7. Mean CPUEs (with standard errors) from within- and off-platform light traps at

each platform for data a) with clupeiforms and b) without clupeiforms .............. 32

8. Significantly different size distributions of engraulids collected by off-platform

(OL) and within-platform (SL) light traps ............................................................ 36

9. Significantly different size distributions of synodontids collected by offplatform (OL) and within-platform (SL) light traps ............................................. 37

10. Significantly different size distributions of scombrids collected by off-platform

(OL) and within-platform (SL) light traps ............................................................ 38

11. Significantly different size distributions of Hypsoblennius invemar collected by

off-platform (OL) and within-platform (SL) light traps. Also presented are size

distributions of H. invemar from the inner shelf platform, ST 54 ........................ 39

12. Mean total CPUEs (with standard errors) by sampling date from light traps

deployed at three distances relative to the mid-shelf platform VK 203: within

the platform, 20 m down-current from the platform, and 50 m down-current

from the platform .................................................................................................. 41
xiv

Figure PAGE

13. Size distributions of Hypsoblennius invemar collected at VK 203 using light

traps deployed at three distances relative to the platform: within the platform,

20 m down-current from platform, and 50 m down-current from platform ......... 45

14. Significantly different size distributions of Serranidae collected at platforms

using passively-fished surface and subsurface plankton nets, and at selected

nearby SEAMAP sampling stations using oblique bongo net tows ..................... 49

15. Significantly different size distributions of Lutjanidae collected at platforms

using passively-fished surface and subsurface plankton nets, and at selected

nearby SEAMAP sampling stations using oblique bongo net tows ..................... 50

16. Mean plankton net densities (with standard errors) by sampling trip at the outer

shelf platform MP 259 (1999) for data a) with clupeiforms and b) without

clupeiforms included ............................................................................................. 54

17. Mean light trap CPUEs (with standard errors) by sampling trip at the outer shelf

platform MP 259 (1999) for data a) with clupeiforms and b) without

clupeiforms included ............................................................................................. 55

18. Mean plankton net densities (with standard errors) by sampling trip at the midshelf platform VK 203 (2000) for data a) with clupeiforms and b) without

clupeiforms included ............................................................................................. 56

19. Mean light trap CPUEs (with standard errors) by sampling trip at the mid-shelf

platform VK 203 (2000) for data a) with clupeiforms and b) without

clupeiforms included ............................................................................................. 57

20. Mean plankton net densities and light trap CPUEs (with standard errors) for

lunar phases sampled at MP 259 and VK 203 for data with clupeiforms (shaded

bars) and without clupeiforms included (open bars) ............................................ 58

21. Mean plankton net densities and light trap CPUEs (with standard errors) by

sampling night for fish taxa collected at GI 94 ..................................................... 62

22. Mean light trap CPUEs (with standard errors) by sampling night for fish taxa

collected at VK 203 .............................................................................................. 64

23. Mean plankton net densities and light trap CPUEs (with standard errors) by

number of hours after sunset that collections were taken in surface waters at

each platform, for data with and without clupeiforms included ........................... 66

24. Size distributions of blenniids collected at each platform .................................... 68
xv

Figure PAGE

25. Size distributions of lutjanids collected at each platform ..................................... 70

26. Size distributions of serranids collected at each platform .................................... 72

27. Size distributions of holocentrids collected at each platform ............................... 74

28. Size distributions of pomacentrids collected at each platform ............................. 75

29. Size distributions of labrids collected at MP 259 and scarids collected at GC 18. 77

30. Scatter plots of water current speed versus mean CPUE from light traps at

MP 259 and VK 203 combined, for data a) with clupeiforms and b) without

clupeiforms included ............................................................................................. 84

31. Size distributions of fishes, with clupeiforms included, collected in light traps

from MP 259 and VK 203 combined at different water current speeds ............... 85

32. Size distributions of fishes, without clupeiforms included, collected in light

traps from MP 259 and VK 203 combined at different water current speeds ...... 86
xvii

LIST OF TABLES

Table PAGE

1. Number of samples collected by date and gear for both platforms east of the

Mississippi River Delta ......................................................................................... 6

2. Mean light trap CPUE (fish/10 min) and plankton net density (fish/100 m3) for

fish collected at Main Pass 259, with standard error (SE), rank based on CPUEs

or densities, and percent of total catch (%N) ........................................................ 13

3. Mean light trap CPUE (fish/10 min) and plankton net density (fish/100 m3) for

fish collected at Viosca Knoll 203, with standard error (SE), rank based on

CPUE or densities, and percent of the total catch (%N) ....................................... 19

4. Taxonomic richness and mean Shannon-Weiner Diversity (with standard

errors) for the larval and juvenile fish assemblages collected at each platform ... 28

5. Relative abundance (% of total catch from plankton nets and light traps

combined) of the ten most abundant non-clupeiform taxa, and reef taxa that

comprised >0.5%, at each platform ...................................................................... 30

6. Mean densities and CPUEs (with standard errors) for taxa more abundant at

platforms either east or west of the Mississippi River Delta in both plankton

nets and light traps ................................................................................................ 31

7. Taxonomic richness and mean Shannon-Weiner Diversity (with standard

errors) for the larval and juvenile fish assemblages collected by within- (SL)

and off-platform (OL) light traps at each platform ............................................... 34

8. Mean CPUEs (with standard errors) of clupeiforms from within- and offplatform light traps at each platform ..................................................................... 34

9. Mean CPUEs of the dominant taxa, and reef fish families, from within- and offplatform light traps at each platform ..................................................................... 35

10. Mean CPUE (fish/10 min) for fish collected using light traps at three distances

relative to the mid-shelf platform VK 203, with standard error (SE), rank, and

percent of total catch (%N) ................................................................................... 42

11. Mean abundance (fish/m2

), with standard deviation (SD), for reef fish collected

at selected nearby SEAMAP ichthyoplankton sampling stations (oblique, 60-

cm bongo tows) and at three oil and gas platforms (passively-fished, 60-cm

plankton nets, surface and subsurface) across the continental shelf west of the

Delta .................................................................................................................... 47
xviii

Table PAGE

12. Mean abundance (fish/100 m3), with standard deviation (SD), for reef fish

collected at selected nearby SEAMAP ichthyoplankton sampling stations

(oblique, 60-cm bongo tows) and at two oil and gas platforms (passivelyfished, 60-cm surface plankton nets) across the continental shelf east of the

Delta ..................................................................................................................... 48

13. Mean number (fish/sample), with standard deviation (SD), for reef fish

collected at selected nearby SEAMAP ichthyoplankton sampling stations

(neuston tows) and at three oil and gas platforms (light traps, within- and offplatform) across the continental shelf west of the Delta ....................................... 51

14. Mean number (fish/sample), with standard deviation (SD), for reef fish

collected at selected nearby SEAMAP ichthyoplankton sampling stations

(neuston tows) and at two oil and gas platforms (light traps, within- and offplatform) across the continental shelf east of the Delta ........................................ 52

15. Total numbers of pre- and postflexion reef larvae collected on new and full

moons at all platforms combined (except GC 18 where sampling only occurred

on new moons ....................................................................................................... 60

16. Coefficients of variation of the mean densities and CPUEs by sampling trip and

sampling night for the ten most abundant taxa taken by plankton nets and light

traps, and reef families collected at the mid-shelf platform GI 94 ................. 61

17. Coefficients of variation of the mean densities and CPUEs by sampling trip and

sampling night for the ten most abundant taxa taken by plankton nets and light

traps, and reef families collected at the mid-shelf platform VK 203 .................... 63

18. Results of the canonical correlation analysis on log-transformed plankton net

densities (15 most abundant taxa, and reef fish families) and environmental

variables for MP 259 ............................................................................................. 80

19. Results of the canonical correlation analysis on log-transformed light trap

CPUEs (15 most abundant taxa) and environmental variables for MP 259 ......... 81

20. Results of the canonical correlation analysis on log-transformed plankton net

densities (15 most abundant taxa, and reef fish families) and environmental

variables for VK 203 ............................................................................................. 82

21. Results of the canonical correlation analysis on log-transformed light trap

CPUEs (15 most abundant taxa, and reef fish families) and environmental

variables for VK 203 ............................................................................................. 83
1

Introduction

The introduction and proliferation of offshore oil and gas platforms (hereafter referred to

as platforms) in the northern Gulf of Mexico (Gulf) has undoubtedly affected the marine

ecosystem. There are approximately 4,000 platforms in the federal waters of the Gulf (Stanley

and Wilson 2000). Because a mud/silt/sand bottom with little relief or hard-substrate dominates

the northern Gulf, especially west of the Mississippi River Delta (Delta), any additional hardsubstrate provided by platforms could prove significant. Although platforms provide an

estimated 11.7 km2

 (or 0.4%) of the total “reef” habitat in the northern Gulf (Gallaway 1999),

platform significance may be enhanced by the vertical relief of their substrate, which extends

from the bottom through the photic zone to the surface. Because fish populations are usually

limited by available energy, recruitment, or habitat, it is important to determine if platforms: 1)

serve as new or additional spawning habitat; 2) provide critical habitat for early life history

stages; or 3) influence energy flow through the ecosystem by aggregating prey.

Platforms can enhance fisheries by providing attachment substrate for habitat-limited

sessile invertebrates, thereby creating food and habitat for reef-dependent species that are

trophically dependent on these invertebrates (Gallaway 1981; Bohnsack and Sutherland 1985;

Bohnsack 1991). In addition, platforms may offer refugia for species that are trophically

independent of the biofouling community, but are ecologically important resident, seasonal, or

transient members of the fish community (Gallaway and Martin 1980). Less well known,

however, is whether platforms provide additional spawning habitat for fishes. Direct evidence of

spawning by resident fishes is difficult to obtain because of the patchiness of eggs and yolk-sac

larvae, the likelihood of significant and variable advection to and away from platforms by

passive pelagic forms, and the inaccessibility of demersal/adhesive eggs. However, at least one

species, the sergeant-major damselfish Abudefduf saxatilis, has been observed guarding nests on

platform structure (Scarborough-Bull and Kendall 1994). Spawning has also been observed on

other types of artificial reefs (Pickering and Whitmarsh 1997). Therefore, it is likely that

reproduction does occur at platforms, particularly for demersal spawners such as blenniids and

pomacentrids (Bohnsack 1989; Pickering and Whitmarsh 1997).

Because larval supply can often exceed the numbers able to settle on a reef, some

researchers believe that larval supply and mortality during the planktonic stage may limit adult

reef fish populations (Sale 1980; Victor 1983). However, others believe that some species of

fish may be experiencing nursery habitat limitation, i.e., the availability of shelter at the time of

settlement is the limiting factor (Ursin 1982; Shulman 1985; Hixon and Beets 1989). In the

absence of suitable shelter, the time of settlement for postlarval fishes is often characterized by

high predation rates by resident populations. An underlying rationale for artificial reef

deployment is that they provide critical habitat for early life history stages and, subsequently,

improve recruitment and adult production (Bohnsack 1989; Bohnsack et al. 1994).

Despite the potential for increased reef fish recruitment, biologists still disagree over the

paradigm of whether artificial reefs contribute significantly to new fish production or simply

attract and concentrate individuals from surrounding habitats, thereby making them potentially

more vulnerable to harvest (Pickering and Whitmarsh 1997; Bortone 1998). Bohnsack (1989)

theorized that reef effects fell along a continuum between attraction and production. Species

most likely to benefit from increased productivity are habitat-limited, territorial, obligatory reef
2

species. The attraction hypothesis is more likely in areas where natural reef habitat is abundant

and where species are recruitment-limited, highly mobile, and opportunistic reef species.

Because the north-central Gulf has little reef habitat, it is likely that the contribution of artificial

reefs (e.g., platforms) has enhanced reef fish populations, although the net impact of this

augmentation is unknown.

Few baseline ichthyoplankton studies within the oil field have been published (Finucane

et al. 1979a; Finucane et al. 1979b; Bedinger et al. 1980) and none have been published that

focus on platform infrastructure. The National Marine Fisheries Service (NMFS)/Southeastern

Area Monitoring and Assessment Program’s (SEAMAP) Gulf-wide fisheries surveys and the

Minerals Management Service Louisiana-Texas (MMS LATEX) Physical Oceanography

Program have historically not sampled in the immediate vicinity of platforms because of

conservative navigation/safety requirements. Therefore, fisheries-independent assessment of the

abundance of fish early life history stages within and immediately around these platforms and the

role they might play as essential fish habitat has not been adequately addressed.

The adult fish assemblages on natural and artificial reefs (including platforms), however,

are well known and often associated with water depth in the northern Gulf (Hastings 1976;

Sonnier et al. 1976; Stanley and Wilson 2000). Demersal fish assemblages have been divided

into depth zones corresponding to the distribution of major shrimp species (i.e., white shrimp

grounds: 3-22 m depth, brown shrimp grounds: 22-110 m depth; Chittenden and McEachern

1976). Using this information, Gallaway (1981) described distinct transitions between reef and

platform fish assemblages across these depths. Overall, the outer shelf (>60 m depth) reefs and

platforms seemed to be more speciose, followed by the mid-shelf (20-60 m) and the inner shelf

(3-20m). More tropical taxa were present at the outer shelf, such as haemulids, labrids and

scarids. There was some overlap between reef species at the outer and mid-shelf habitats, but in

general, tropical taxa were replaced by more temperate species, such as serranids and sparids, at

the shallower sites. This transition was presumably due to cooler winter temperatures limiting

the distribution of tropical species at inshore habitats.

 In addition to depth-related (across-shelf) transitions, differences in adult fish

assemblages east and west of the Delta (along-shelf) have been noted in the northern Gulf and

have been attributed to substrate characteristics and circulation patterns (Briggs 1958; Chittenden

and McEachern 1976; Hoese and Moore 1998). Terrigeneous muds from the Mississippi and

Atchafalaya Rivers are predominant in the northwestern Gulf, while sand, biogenic calcareous

sediments and limestone outcroppings are more common east of the Delta (Parker 1960;

Chittenden and McEachern 1976). The Mississippi River discharge, which predominantly is a

westerly flow from the Delta, and the Loop Current, which brings Caribbean waters into the

Gulf, dominate the circulation patterns in the Gulf. The large volume of freshwater leaving the

Mississippi River distributaries may present a barrier to the dispersal of certain coastal taxa

(Hildebrand 1954; McClure and McEachern 1992). Although the Loop Current, by way of

shedding warm-core rings, may seed offshore reefs in the northwestern Gulf with Caribbean

postlarval/juvenile fish, it generally favors eastward dispersal of these taxa (Shipp 1992). This

combination of circulation and more favorable reef-type habitats, along with warmer and more

saline hydrographic conditions, has resulted in tropical reef fish species being more common in

the northeastern Gulf (Briggs 1958; Smith 1976).
3

This study focused on three objectives designed to evaluate the ecological significance of

oil and gas platforms to the early life history stages of fishes. The first objective was to

characterize the larval and juvenile fish assemblages within and immediately around two

platforms east of the Delta. Data from these collections were analyzed with data collected

previously from three platforms west of the Delta (Hernandez 2001; Hernandez et al. 2001;

Hernandez et al. in press) to determine whether there were significant differences in the larval

and juvenile fish assemblages by depth (across-shelf) and by east/west of the Delta (along-shelf)

as found for adult populations in the northern Gulf. The second objective was to assess how

platforms affect the local distribution and abundance of larval and juvenile fishes. Near-field

collections within the platform infrastructure were compared with synoptic data taken from

waters 20 m down-current from the platform, and from background data taken further afield (i.e.,

SEAMAP data). The third objective was to provide much needed ecological information on the

early life history stages of reef fishes, e.g., seasonality, lunar periodicity and large-scale

distribution patterns, and to address whether platforms have a nursery/refugia function for these

fishes. Unlike the majority of species in ichthyoplankton collections (e.g., soft-bottom or pelagic

species), reef/hard-bottom species are often associated with platforms as adults, and, therefore,

populations of these fishes may benefit from the increased habitat provided by platforms. Reef

fishes were classified as either reef-dependent or reef-associated (Choat and Bellwood 1991).

Reef-dependent taxa are those that are associated with reef habitat for the duration of their adult

life and included individuals from the families: Acanthuridae (surgeonfishes), Chaetodontidae

(butterflyfishes), Labridae (wrasses), Pomacanthidae (angelfishes), Pomacentridae

(damselfishes), and Scaridae (parrotfishes). Reef-associated taxa are those that exploit the

resources of the reef but also occur in other habitats. Although this definition could encompass

certain pelagic (e.g., Carangidae, Scombridae and Sphyraenidae) and benthic taxa (Gobiidae,

Muraenidae and Synodontidae), we focused on the families: Blenniidae (blennies),

Holocentridae (squirrelfishes), Lutjanidae (snappers), and Serranidae (sea basses). 

Materials and Methods

Study Areas

Larval and juvenile fishes were sampled at two platforms east of the Delta: Santa FeSnyder’s Main Pass (MP) 259 and Murphy Oil’s Viosca Knoll (VK) 203 (Figure 1). These

platforms were selected based on characterizations of the adult fish assemblages by Gallaway

and Martin (1980), Gallaway (1981) and Continental Shelf Associates (1982) who reported that

nektonic communities around platforms could be categorized by water depth in the northern

Gulf. These platforms were also selected to complement and supplement previous sampling

efforts at platforms west of the Delta (Hernandez et al. 2001). Main Pass 259 (29°19’32” N,

88°01’12” W; installed in 1994), which stands in about 120 m of water on the outer shelf, was

sampled twice monthly from May to September 1999. Viosca Knoll 203 (29°46’53” N,

88°19’59” W; installed in 1993), which stands in about 35 m of water at mid-shelf, was sampled

twice monthly from May to October 2000. Inclement weather forced the cancellation of one trip

in early August and two trips during September at VK 203. Furthermore, the October trip took

place during the moon’s first quarter. Both platforms have similar structural complexity; MP

259 is a six-pile (or leg) platform, and while VK 203 has four piles it also has a well deck that

extends from the platform’s north side and down to about 10 m depth, creating de facto fifth and

six piles in the upper water column.
4

Longitude (°W)

Latitude (oN)

Figure 1. Location of oil and gas platforms sampled in the northern Gulf of Mexico. Circles represent platforms sampled during th Squares represent platforms sampled during previous studies, used for comparisons.
5

Sampling Procedures

Sampling protocols at MP 259 and VK 203 were similar to those conducted at platforms

west of the Delta (Hernandez et al. 2001). Samples were taken over two consecutive nights at

approximately 14-day intervals coincident with new and full moon phases (Table 1). These

moon phases were targeted because they have been associated with either peak spawning or

recruitment periods for many reef-dependent fishes (Johannes 1978; Robertson et al. 1988). The

main sampling station for each platform was located in the internal central region along a

stainless-steel guidewire tethered to the first set of the platform’s underwater cross-member

support structures. At this station, larval and juvenile fish collections were made using a nearsurface modified quatrefoil light trap with power supplied via an umbilical connected to a 12-

volt marine battery (Hernandez et al. 2001 for specifications), and a near-surface, passivelyfished, 60-cm diameter plankton net (333 μm mesh, dyed dark green) rigged with a flow meter

(General Oceanics flowmeter 2030 with slow velocity rotor). In addition, collections were made

using a light trap that was tethered and free-drifted away from the platform (off-platform) to a

distance of 20 m on the down-current side of the platform. All light traps were deployed with

their lights off, fished with their lights on and retrieved with their lights off. Each of these gears

was sampled during four sets each night, with two sets taken before midnight and two sets taken

after midnight. The order of light trap and plankton net collections was randomized within each

set. Also, on three occasions during July and August 2000 at VK 203, samples were collected

with light traps deployed at three distances relative to the platform: within the platform; 20 m

down-current from the platform; and 50 m down-current from the platform.

Temperature (°C), salinity (ppt) and turbidity (NTU) were measured during each set

using a Hydrolab DataSonde 3. Current speed and direction were recorded using an InterOcean

S4 Current Meter. During each set a plankton net (30-cm, 63 μm mesh), which was held rigidly

to the guidewire, was lowered cod-end first to the bottom of the guidewire, left at depth for 5

minutes for water column restabilization, then hauled vertically to the surface at about 1m/s to

ascertain microzooplankton biomass as a measure of prey availability. The resulting samples

were filtered through a pre-weighed, microfiber filter (1.2 μm), dried in an oven for 24 hr at

60°C, then weighed to determine the dry weight biomass (g m-3). Surface water samples were

also collected during each set to determine total suspended solids, an estimate of turbidity.

These samples were filtered, dried, and weighed to determine the suspended sediment load

(g/500 ml).

Samples of larval and juvenile fishes were preserved in 10% buffered formaldehyde and

changed over to ethanol within 8-12 hours. Fish were removed from all samples, enumerated

and identified to the lowest possible taxonomic level using the taxonomic classification of

Robins et al. (1991). Fish were measured under a dissecting microscope with the aid of an

ocular micrometer. Preflexion larvae were measured to the end of the notochord (notochord

length: NL) and postflexion larvae and juveniles were measured to the end of the vertebral

column (standard length: SL). In the event that the number of fish in a sample was greater than

50 for any single species, the largest, smallest and a random subsample of 50 individuals was

measured. Light trap samples were standardized to a catch-per-unit-effort (CPUE) of fish per 10

min. Plankton net samples were standardized to the number of fish per 100 m3

 (density).
6

 Table 1. Number of samples collected by date and gear for both 

 platforms east of the Mississippi River Delta. (Lunar phases: 

 N= new moon; F= full moon).

Off-platform Surface Surface

Light Trap Light Trap Plankton Net

Main Pass 259A

(1999)

 May 29-30 (F) 0 6 6

 Jun 12-14 (N) 8 8 8

 Jun 26-28 (F) 0 8 8

 Jul 10-12 (N) 8 8 8

 Jul 26-28 (F) 8 8 8

 Aug 10-12 (N) 8 8 8

 Aug 24-26 (F) 8 8 8

 Sep 7-9 (N) 8 8 8

 Sep 24-26 (F) 8 8 8

 Totals 56 70 70

Viosca Knoll 203

(2000)

 May 18-20 (F) 8 8 8

 May 31-Jun 2 (N) 888

 Jun 14-16 (F) 8 8 8

 Jun 29-Jul 1 (N) 8 8 8

 Jul 15-17 (F) 8 8 8

 Jul 30-Aug 1 (N) 8 8 8

 Aug 29-31 (N) 8 8 8

 Oct 6-7 ‡ 4 4 4

 Totals 60 60 60

 ‡ samples taken during first quarter moon.
7

Data Analyses

Data collected at MP 259 and VK 203 were compared with data previously collected at

platforms west of the Delta, i.e., the inner shelf platform South Timbalier (ST) 54, the mid-shelf

platform Grand Isle (GI) 94 and the outer shelf platform Green Canyon (GC) 18 (Hernandez et

al. 2001). To avoid bias, only data from common sampling periods and sampling gears were

included in the analyses, i.e., data from months other than May through August and from

subsurface light traps and plankton nets were dropped. Because most of the data sets analyzed

were non-normal in their distribution, nonparametric Kruskal-Wallis ANOVAs (α= 0.05) were

used to compare mean total plankton net densities, light trap CPUEs and Shannon-Wiener

diversity indices (Magurran 1988) from each platform. Nonparametric Tukey-type multiple

comparison tests were used to determine which means were significantly different (Zar 1984).

Because very large numbers of clupeiforms overwhelmed the trends of other taxa collected at the

platforms, these analyses were run with and without clupeiforms included. These and

subsequent analyses were performed using SAS version 6.12 (SAS Institute, Inc. 1989) unless

otherwise noted.

Similarity among the non-clupeiform larval and juvenile fish assemblages of each

platform was calculated using the Bray-Curtis index of similarity (= 1 – Bray-Curtis

dissimilarity; Krebs 1999). Fish collections from all gears, from May through August, were

combined for each platform and the relative contribution of each taxon to the total nonclupeiform catch was calculated. In general, only fishes identified to at least the genus level

were included in this analysis, however, unidentified Gobiidae and Myctophidae were also

included because they were numerically dominant at certain platforms. Each platform

assemblage was compared with every other platform assemblage and the resulting similarity

matrix was used to create a cluster diagram using the group-average sorting method in SYSTAT

version 9 (SPSS, Inc. 1999). The resulting dendrogram ordered platforms into groups of

similarly structured fish assemblages.

Mean plankton net densities and light trap CPUEs of the dominant fish taxa collected at

platforms were statistically compared relative to their collection east or west of the Delta.

Samples taken at MP 259 and VK 203 were categorized as east Delta, while samples taken at GC

18, GI 94 and ST54 were categorized as west Delta. Only samples taken from surface waters

during the months of May through August were included in this analysis. Means were compared

using the nonparametric Wilcoxon 2-sample test (Zar 1984).

The potential effect of platforms on the distribution and abundance of postlarval and

juvenile fishes was examined by comparing light trap catches from the surface waters withinplatform vs. off-platform at each platform (east and west of the Delta). Wilcoxon 2-sample tests

were used to compare mean total light trap CPUEs and Shannon-Wiener diversity indices

between locations. Similar analyses were run on the CPUEs of dominant taxa (including

clupeiform taxa) and reef fish families collected at each platform. Kruskal-Wallis ANOVAs

were used to compare the mean total CPUEs and diversity indices collected between the withinplatform, and the 20 m and 50 m off-platform light trap samples collected at VK 203.

Kolmogorov-Smirnov (K-S) length frequency analyses were performed for selected taxa (n ≥ 10

for each gear) at each platform to determine if there were differences in the size distributions of
8

these taxa collected within- vs. off-platform. All K-S tests were performed using SYSTAT

version 9 (SPSS, Inc. 1999).

Platform-collected reef fish larval abundances were compared to open-water or

“background” abundances from nearby SEAMAP surveys. Data were selected from SEAMAP

sampling stations, from data sets for the years 1995-1997 (the most recent available data sets),

that were relatively close in both proximity (location on shelf) and water depth to each platform,

and during the same months as platform collections (i.e., April-August). Oblique bongo net (60-

cm diameter; 333 μm mesh) and neuston net (1 x 2 m mouth; 948 μm mesh) samples were

collected at these stations using standard SEAMAP protocols (SEAMAP 2000). Because

abundances from bongo nets were expressed as fish per 10 m2

, abundances from paired passive

plankton nets (surface and subsurface) at platforms were standardized using the equation:

[(N1 + N2)/(V1 + V2)] x D x 10 where: N1 = number of fish in surface sample

 N2 = number of fish in subsurface sample

 V1 = volume of water filtered by surface net

 V2 = volume of water filtered by subsurface net

 D = total depth sampled.

Because subsurface plankton nets were not used at platforms east of the Delta, flowmeter

readings from the oblique bongo tows were used to calculate densities of fish per 100 m3

. Mean

abundances from bongo tows were then compared with mean abundances from plankton nets at

platforms for reef fish families. Similarly, the mean number of reef fish taxa collected in

SEAMAP neuston net samples was compared with the mean number collected in light trap

samples (within- and off-platform combined) for each platform. No statistical comparisons were

attempted due to differences in sampling effort and protocols between SEAMAP and platform

data. However, reef fish size distributions were compared between bongo nets and plankton

nets, and neuston nets and light traps, using K-S tests (<10 individuals for each gear).

The frequency of reef fishes collected during new vs. full moons was compared, from all

platforms combined (except GC 18 where sampling occurred only on new moons), using chisquare goodness of fit tests (α = 0.01; Zar 1984). To distinguish between possible spawning and

settlement events, separate analyses were performed for the total numbers of preflexion and

postflexion fish collected on new and full moons at platforms. Published data were used to make

the distinction between these developmental stages. Because the total number of samples

(plankton nets and light traps combined) taken on new moons was higher than on full moons (n=

714 and 527, respectively), the expected frequencies for each chi-square analysis were similarly

disproportional (58:42, new: full).

Several studies have noted that spawning and recruitment often occur in pulses of short

duration (Robertson et al. 1988; Doherty 1991). To identify taxa that exhibited pulses at

platforms, coefficients of variation (CVs; standardized to 100%) were calculated from the mean

densities and CPUEs among trips and among nights within a sampling season for the ten most

abundant non-clupeiforms in plankton nets and light traps, and for abundant families of reef fish.

These analyses were performed for data from GI 94 and VK 203, because these platforms had

the highest numbers of reef fish collected. Plots of the nightly mean density and CPUE for taxa
9

were used, in combination with the coefficients of variation, to further illustrate the variability in

catch, i.e., the taxa with the highest CVs should exhibit the most discrete pulse.

Temporal variability in larval and juvenile fish abundance was also analyzed within

nights. Mean total plankton net densities and light trap CPUEs were calculated for each hour

that sampling occurred after sunset at each platform. These means were compared using the

Kruskal-Wallis ANOVA and Tukey’s tests. Similar tests were performed on the hourly mean

densities and CPUEs of the most abundant non-clupeiform fish taxa and reef fish families.

Canonical correlations were used to determine relationships between larval and juvenile

fish abundance and environmental variables at MP 259 and VK 203. Log-transformed

(log10(x+1)) plankton net densities and light trap CPUEs of the top 15 non-clupeiform taxa, and

reef fish families, were analyzed along with temperature, salinity, microzooplankton biomass and

measurements of turbidity. The importance of an environmental variable was based on the

magnitude of its correlation with the environmental variate, with the sign of the correlation

indicating if the variable was directly or inversely related with the variate. A taxa was

considered to be related to the variate if the absolute value of the correlation was greater than

0.387 (i.e., the variate predicted ≥15% of the species’ variation within the model).

The effect of current speed on light trap catches was assessed by plotting the mean total

light trap CPUE against current speed (cm s-1), as measured by the S4 Current Meter. Because

there was no obvious difference in the relationship of light trap CPUE vs. current speed

depending on the location of the light trap (within- vs. off-platform), the mean light trap CPUE

per set (from both light traps combined) was plotted against the mean current speed during that

set. These means were calculated from MP 259 and VK 203 and the results from both platforms

were combined. The size distributions of fishes collected at different current levels, from both

MP 259 and VK 203 combined, were analyzed using K-S tests.

Results

Environmental Characterization of Sampling Sites

Environmental conditions were variable at the outer shelf platform MP 259 (Figure 2).

Mean surface temperatures increased over the season from 27.8 °C in May to a peak of 31.5 °C

during late August, and then decreased to a low of 27.7 °C during late September. Although

surface salinities ranged from 29-34 ppt during most of the sampling season, minimums were

seen in May and late August (26.7 and 23.5 ppt, respectively). Mean surface turbidities ranged

from 0.2 to 2.5 NTU, while microzooplankton biomass ranged from 0.2 to 1.4 g m-3. Peaks in

mean turbidity and microzooplankton biomass coincided with the lower salinities observed

during May and late August, which suggests the presence offshore of river plume water.

In contrast, most environmental variables were relatively stable at the mid-shelf platform

VK 203 (Figure 3). Mean surface temperatures showed a similar seasonal pattern as at MP 259,

with low temperatures during May (25.7 °C) and high temperatures during late August (30.2 °C).

However, mean surface salinities showed very little variation, ranging from 31.6 to 33.5 ppt.

Mean surface turbidity measurements from the Hydrolab were near zero at all times, except for
20

2224262830323436

May 29-30 Jun 12-14 Jun 26-28 Jul 10-12 Jul 26-28 Aug 10-12 Aug 24

Salinity (ppt)

2526272829303132

May 29-30 Jun 12-14 Jun 26-28 Jul 10-12 Jul 26-28 Aug 10-12 Aug 24-26 Sep 7-9 Sep 24-26

Temperature ( 

o

C)

0

0.5

1

1.5

2

2.5

3

May 29-30 Jun 12-14 Jun 26-28 Jul 10-12 Jul 26-28 Aug 10-12 Aug 24-26Sep 7-9 Sep 24-26

Turbidity (NTU) 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

1.80

May 29-30 Jun 12-14 Jun 26-28 Jul 10-12 Jul 26-28 Aug 10-12 Aug 

Microzooplankton 

Biomass (g/m

3

)

Figure 2. Mean surface temperatures, surface salinities, surface turbidities, and microzooplankton biomass (with standard erro trip at the outer shelf platform, MP 259 (depth = 120 m).

10
20

2224262830323436

May 18-20 May 31-Jun 2 Jun 14-16 Jun 29-Jul 1 Jul 15-17 Jul 30-Aug 

Salinity (ppt)

2526272829303132

May 18-20 May 31-Jun 2 Jun 14-16 Jun 29-Jul 1 Jul 15-17 Jul 30-Aug 1 Aug 29-31Oct 6-7

Temperature (

o

C)

0.04

0.045

0.05

0.055

0.06

0.065

0.07

May 18-20 May 31-Jun 2 Jun 14-16 Jun 29-Jul 1 Jul 15-17 Jul 30-Aug 1 Aug 29-31Oct 6-7

Suspended 

Solids (g/500ml) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

May 18-20 May 31-Jun 2 Jun 14-16 Jun 29-Jul 1 Jul 15-17 Jul 30-Aug 

Microzooplankton 

Biomass (g/m

3

)

Figure 3. Mean surface temperatures, surface salinities, suspended solids, and microzooplankton biomass (with standard error

 trip at the mid-shelf platform, VK 203 (depth = 35 m).

11
12

the nights of July 15-16 (mean turbidity = 1.8 NTU ± 0.05 SE) and October 6-7 (mean = 8.4 ±

0.13). Mean surface suspended solid concentrations were relatively stable, only ranging between

0.05-0.06 g/500 ml. Mean microzooplankton biomass was lower than at MP 259 and ranged

from 0.03 to 0.2 g m-3. Both the suspended solids and microzooplankton biomass curves showed

similar patterns with lower values between June and August interrupted by a peak on July 15-17.

Characterization of Fish Catches at MP 259 and VK 203

A total of 16,439 fish, from 43 families and 69 taxa (identified to at least the genus level),

was collected at the outer shelf platform MP 259 between May and September 1999 (Table 2).

Of this total, 14,856 fish were collected in plankton nets and 1,583 in light traps. Plankton net

collections represented 41 families and 62 taxa, with 23 families and 33 taxa being unique to this

gear type. Meanwhile, light traps collected 21 families and 36 taxa, of which 3 families and 7

taxa were unique to light traps. Mean Shannon-Wiener diversity indices were significantly

higher for plankton net collections (mean = 1.39 ± 0.07) than light trap collections (mean = 0.82

± 0.05; Wilcoxon Z = 5.77, p<0.0001). Clupeiform fishes, primarily Anchoa spp., Engraulis

eurystole and Opisthonema oglinum, dominated the catch for both plankton nets (85.6% of the

total catch) and light traps (80.3%). Caranx crysos, Auxis spp., and unidentified gobiids and

myctophids were the dominant non-clupeiforms in plankton nets, while Caranx crysos,

Hypsoblennius invemar, Synodus foetens and Euthynnus alletteratus were common in light traps.

At the mid-shelf platform VK 203, a total of 6,196 fish was collected between May and

October 2000, representing 38 families and 83 taxa (identified to at least the genus level; Table

3). Plankton nets collected 4,190 fish, while light traps collected 2,006 fish. Plankton nets

collected individuals from 33 families and 68 taxa, with 7 families and 15 taxa unique to nets.

Light traps collected 31 families and 68 taxa, with 5 families and 15 taxa collected only by light

traps. Mean Shannon-Wiener diversity indices were not significantly different (light trap mean =

1.26 ± 0.05 vs. plankton net mean = 1.13 ± 0.08). Clupeiforms were not as dominant at VK 203

as at MP 259, comprising only 60.0% of the total plankton net catch and 31.9% of the total light

trap catch. Bregmaceros cantori, Cynoscion arenarius, Micropogonias undulatus, and

unidentified gobiids were among the dominant non-clupeiform taxa collected in plankton nets,

while Saurida brasiliensis, Synodus foetens, Decapterus punctatus, Hypsoblennius invemar, and

Euthynnus alletteratus were common in light trap collections.

Across- and Along-Shelf Characterization of Larval and Juvenile Fish Assemblages

Mean densities of larval fishes from surface plankton nets were highest at the inner shelf

platform and decreased with increasing water depth (Figure 4). With clupeiforms included,

mean densities were significantly different between platforms (Kruskal-Wallis χ

2

 = 38.26,

p<0.0001), with highest densities at the inner shelf (ST 54 = 5001.1 fish 100m-3 ± 2467.9) and

lowest at the outer shelf (GC 18 = 180.1 ± 52.0; Figure 4a). When clupeiforms were excluded

from the analysis, mean densities were significantly different (χ

2

 = 33.66, p<0.0001) and the

same across-shelf pattern was observed, although no significant differences were found between

the inner shelf platform ST 54, and the two platforms east of the Delta, VK 203 and MP 259.
13

 Table 2. Mean light trap CPUE (fish/10 min) and plankton net density (fish/100 m3) for fish collected at

 Main Pass 259, with standard error (SE), rank based on CPUE or densities, and percent of total catch

 (%N). For ranks, tied values received the mean of the corresponding ranks.

 RA = reef associated taxa, RD = reef dependent taxa.

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

Osteichthyes

 Unidentified Jul, Aug 0 (0) 0.01 (0.01)

36.5 (<1.0)

0.06 (0.06)

67 (<1.0)

Anguilliformes

 Muraenidae

 Unidentified

 (moray eels) Aug 0 (0) 0 (0) 0.02 (0.02)80 (<1.0)

 Ophichthidae

 Ophichthus spp.

 (snake eels) May, Jun, Jul, Sep 0.02 (0.02)

38 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.30 (0.11)

43 (<1.0)

 Congridae

 Unidentified

 (conger eels) Jun 0 (0) 0 (0) 0.10 (0.06)

56 (<1.0)

Clupeiformes

 Clupeidae

 Harengula jaguana

 (scaled sardine) May, Jun, Jul, Aug 0.50 (0.23)3 (5.5)

0.53 (0.34)

4 (3.5)

33.06 (10.65)

3 (6.1)

 Opisthonema oglinum

 (Atlantic thread herring) May, Jun, Jul, Aug, Sep 2.39 (0.83)1 (27.0)

5.34 (1.74)

2 (34.7)

220.93 (82.81)

1 (64.52)

 Engraulidae

 Unidentified May, Jun, Jul, Aug, Sep 0.07 (0.03)

19.5 (<1.0)

0.14 (0.08)

10.5 (<1.0)

25.52 (12.34)

4 (5.1)

 Anchoa spp. May, Jun, Jul, Aug, Sep 0.21 (0.07)

7 (2.4)

0.11 (0.06)

14.5 (<1.0)

40.97 (13.84)

2 (8.4)

 Anchoa cubana

 (Cuban anchovy) Sep 0 (0) 0 (0) 0.11 (0.11)53 (<1.0)

 Anchoa hepsetus

 (striped anchovy) May, Jul, Aug, Sep 0.48 (0.19)4 (5.4)

0.14 (0.12)

10.5 (<1.0)

1.81 (0.92)

19 (<1.0)

 Anchoa mitchilli

 (bay anchovy) May, Jun, Jul, Aug, Sep 0.11 (0.07)14 (1.2)

0.16 (0.09)

8 (1.1)

2.22 (0.93)

16 (<1.0)

 Anchoa nasuta

 (longnose anchovy) May, Jun, Jul, Aug, Sep 0.18 (0.08)9 (2.0)

0.04 (0.02)

23.5 (<1.0)

3.40 (1.13)

12 (<1.0)

 Anchoa nasuta/hepsetus

 (longnose/striped anchovy) May, Jun, Jul, Sep 0.04 (0.03)29.5 (<1.0)

0.29 (0.27)

5 (1.9)

2.98 (1.30)

15 (<1.0)

 Anchoviella perfasciata

 (flat anchovy) Aug, Sep 0.18 (0.08) 9 (2.0)

0.07 (0.05)

19.5 (<1.0) 0 (0)

 Engraulis eurystole

 (silver anchovy) May, Jun, Jul, Aug, Sep 2.38 (0.65)2 (26.8)

5.98 (2.59)

1 (39.0)

5.08 (2.26)

9 (<1.0)

Stomiiformes

 Gonostomatidae

 Cyclothone spp.

 (bristlemouths) Aug, Sep 0 (0) 0 (0) 0.09 (0.07)59 (<1.0)
14

 Table 2. Main Pass 259 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

Aulopiformes

 Chlorophthalmidae

 Unidentified

 (greeneyes) Aug 0 (0) 0 (0) 0.03 (0.03)75 (<1.0)

 Synodontidae

 Saurida brasiliensis

 (largescale lizardfish) Jun, Jul, Aug, Sep 0 (0) 0.16 (0.09)8 (1.0)

1.18 (0.38)

23 (<1.0)

 Synodus foetens

 (inshore lizardfish) Jun, Jul, Sep 0.05 (0.04)24 (<1.0)

0.90 (0.63)

3 (5.8)

0.01 (0.01)

86 (<1.0)

 Trachinocephalus myops

 (snakefish) Jun, Jul, Sep 0.07 (0.06)19.5 (<1.0)

0.07 (0.04)

19.5 (<1.0)

0.04 (0.04)

71 (<1.0)

 Paralepidae

 Lestrolepis intermedia

 (barracudina) Jun 0 (0) 0 (0) 0.11 (0.09)54 (<1.0)

Myctophiformes

 Myctophidae

 Unidentified

 (lanternfishes) May, Jun, Aug, Sep 0.05 (0.05)

24 (<1.0)

0.06 (0.03)

21 (<1.0)

3.93 (1.45)

10 (<1.0)

Gadiformes

 Bregmacerotidae

 Bregmaceros cantori

 (codlet) May, Jun, Jul, Sep 0.11 (0.06)14 (1.2)

0.04 (0.02)

23.5 (<1.0)

3.58 (0.81)

11 (<1.0)

Ophidiiformes

 Ophidiidae

 Lepophidium staurophor

 (barred cusk-eel) Jun, Jul, Aug 0 (0) 0 (0) 0.19 (0.09)48 (<1.0)

Lophiiformes

 Caulophyrnidae

 Unidentified

 (anglerfishes) Jun 0 (0) 0 (0) 0.02 (0.02)84 (<1.0)

 Ceratiidae

 Unidentified

 (seadevils) Sep 0 (0) 0 (0) 0.06 (0.06)68 (<1.0)

Atheriniformes

 Exocoetidae

 Unidentified

 (flyingfishes) Jun, Jul, Sep 0.02 (0.02)38 (<1.0) 0 (0) 0.47 (0.24)33 (<1.0)

 Atherinidae

 Membras martinica

 (rough silverside) Jun 0 (0) 0.03 (0.03)28 (<1.0) 0 (0)

Beryciformes

 Holocentridae (RA)

 Holocentrus spp.

 (squirrelfishes) Jul, Sep 0.07 (0.03)19.5 (<1.0)

0.13 (0.06)

12 (<1.0) 0 (0)
15

 Table 2. Main Pass 259 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

Scorpaeniformes

 Scorpaenidae

 Scorpaena spp.

 (scorpionfishes) Jun, Jul, Aug, Sep 0 (0) 0 (0) 0.13 (0.07)50 (<1.0)

 Triglidae

 Prionotus spp.

 (searobins) May 0 (0) 0 (0) 0.09 (0.06)58 (<1.0)

Perciformes

 Unidentified Jun, Jul, Aug, Sep 0.09 (0.09)

16.50 (<1.0)

0.01 (0.01)

36.5 (<1.0)

3.01 (1.07)

14 (<1.0)

 Serranidae (RA)

 Anthinae

 (sea perches) Jun, Sep 0 (0) 0 (0) 0.35 (0.16)39 (<1.0)

 Epinephelinae

 (groupers) Jun, Jul, Aug 0.04 (0.04)29.5 (<1.0)

0.03 (0.02)

28 (<1.0)

0.09 (0.07)

57 (<1.0)

 Serraninae

 (sea basses) Jun 0 (0) 0 (0) 0.11 (0.07)52 (<1.0)

 Priacanthidae

 Priacanthus spp.

 (bigeyes) Jun 0 (0) 0 (0) 0.03 (0.03)73.5 (<1.0)

 Rachycentridae

 Rachycentron canadum

 (cobia) Jun 0 (0) 0 (0) 0.02 (0.02)82 (<1.0)

 Echeneidae

 Unidentified

 (remoras) Sep 0 (0) 0 (0) 0.07 (0.07)65 (<1.0)

 Carangidae

 Unidentified Jul, Aug 0.02 (0.02)

38 (<1.0)

0.09 (0.06)

17.5 (<1.0)

6.51 (3.72)

7 (2.1)

 Caranx spp. Jun, Jul, Sep 0.02 (0.02)

38 (<1.0) 0 (0) 0.55 (0.35)30 (<1.0)

 Caranx crysos

 (blue runner) May, Jun, Jul, Aug, Sep 0.18 (0.06)9 (2.0)

0.16 (0.05)

8 (1.0)

7.17 (1.40)

6 (1.53)

 Caranx hippos/latus

 (crevalle/horse-eye jack) Jun, Aug 0 (0) 0.03 (0.02)28 (<1.0)

0.02 (0.02)

82 (<1.0)

 Chloroscombrus chrysurus

 (Atlantic bumper) Jul, Aug, Sep 0.04 (0.03)29.5 (<1.0)

0.04 (0.02)

23.5 (<1.0)

0.68 (0.39)

27 (<1.0)

 Decapterus punctatus

 (round scad) Jun, Jul, Aug 0.11 (0.07)14 (1.2)

0.09 (0.04)

17.5 (<1.0)

0.42 (0.33)

36 (<1.0)

 Elagatis bipinnulata

 (rainbow runner) Jul 0 (0) 0 (0) 0.03 (0.03)

76 (<1.0)

 Selar crumenophthalmus

 (bigeye scad) Jul 0 (0) 0.01 (0.01)36.5 (<1.0)

0.05 (0.05)

69 (<1.0)

 Selene vomer

 (lookdown) May, Jun 0 (0) 0 (0) 0.07 (0.05)64 (<1.0)

 Seriola spp.

 (amberjacks) May, Jun, Jul 0 (0) 0 (0) 0.56 (0.31)29 (<1.0)
16

 Table 2. Main Pass 259 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Seriola zonata

 (banded rudderfish) Jun 0.02 (0.02)38 (<1.0) 0 (0) 0 (0)

 Trachurus lathami

 (rough scad) Aug 0 (0) 0 (0) 0.03 (0.03)78 (<1.0)

 Lutjanidae (RA)

 Unidentified Jun 0.02 (0.02)

38 (<1.0) 0 (0) 0 (0)

 Lutjanus campechanus

 (red snapper) May 0 (0) 0 (0) 0.05 (0.05)

70 (<1.0)

 Pristipomoides aquilonaris

 (wenchman) May, Jun, Jul, Sep 0 (0) 0 (0) 0.31 (0.16)

42 (<1.0)

 Rhomboplites aurorubens

 (vermilion snapper) Jun 0 (0) 0 (0) 0.03 (0.03)77 (<1.0)

 Sparidae

 Lagodon rhomboides

 (pinfish) Jun 0 (0) 0 (0) 0.02 (0.02)79 (<1.0)

 Sciaenidae

 Cynoscion arenarius

 (sand seatrout) Jul 0 (0) 0 (0) 0.46 (0.27)34 (<1.0)

 Menticirrhus spp.

 (kingfish) Aug 0 (0) 0 (0) 0.07 (0.05)62 (<1.0)

 Pomacentridae (RD)

 Pomacentrus spp.

 (damselfishes) Jul 0.07 (0.06)19.5 (<1.0)

0.04 (0.03)

23.5 (<1.0) 0 (0)

 Mugilidae

 Mugil curema

 (white mullet) May, Jun 0 (0) 0 (0) 0.15 (0.09)49 (<1.0)

 Sphyraenidae

 Sphyraena spp. May, Jun 0 (0) 0 (0) 0.28 (0.16)

44 (<1.0)

 Sphyraena barracuda

 (great barracuda) Sep 0 (0) 0 (0) 0.06 (0.06)66 (<1.0)

 Sphyraena guachancho

 (guaguanche) May, Jun, Jul 0 (0) 0 (0) 0.36 (0.18)38 (<1.0)

 Labridae (RD)

 Unidentified Jun, Aug, Sep 0 (0) 0 (0) 0.34 (0.14)

40 (<1.0)

 Thalassoma spp.

 (wrasses) Jun 0 (0) 0 (0) 0.07 (0.07)63 (<1.0)

 Scaridae (RD)

 Unidentified

 (parrotfishes) Aug 0 (0) 0 (0) 0.11 (0.08)55 (<1.0)
17

 Table 2. Main Pass 259 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Blenniidae (RA)

 Unidentified May, Jun, Jul, Aug, Sep 0 (0) 0.02 (0.02)

30.5 (<1.0)

2.06 (0.54)

18 (<1.0)

 Hypsoblennius invemar

 (tesselated blenny) Jul 0.38 (0.22)5 (4.2)

0.03 (0.02)

28 (<1.0) 0 (0)

 Scartella cristata

 (molly miller) Jul 0 (0) 0 (0) 0.02 (0.02)85 (<1.0)

 Callionymidae

 Callionymus pauciradiatus

 (spotted dragonet) Aug 0 (0) 0 (0) 0.08 (0.06)60 (<1.0)

 Gobiidae

 Unidentified May, Jun, Jul, Aug, Sep 0.09 (0.04)

16.5 (1.0)

0.03 (0.02)

28 (<1.0)

10.46 (1.51)

5 (1.63)

 Gobiosoma bosc

 (naked goby) May, Jun 0 (0) 0 (0) 0.95 (0.57)24 (<1.0)

 Microdesmidae

 Microdesmus spp. May, Jun, Jul, Aug 0.02 (0.02)

38 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.54 (0.26)

32 (<1.0)

 Microdesmus lanceolatus

 (lancetail wormfish) May, Jun, Jul, Aug, Sep 0.04 (0.03)29.5 (<1.0) 0 (0) 1.28 (0.32)22 (<1.0)

 Trichiuridae

 Trichiurus lepturus

 (Atlantic cutlassfish) May, Jun, Jul, Sep 0.02 (0.02)

38 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.40 (0.17)

37 (<1.0)

 Scombridae

 Unidentified May, Jun, Jul 0.02 (0.02)

38 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.26 (0.19)

47 (<1.0)

 Auxis spp.

 (frigate mackerels) May, Jun, Jul, Aug, Sep 0.13 (0.06)11.5 (1.4)

0.19 (0.07)

6 (1.5)

5.77 (2.14)

8 (2.2)

 Euthynnus alletteratus

 (little tunny) Jun, Jul, Aug, Sep 0.29 (0.11)6 (3.2)

0.10 (0.04)

14.5 (<1.0)

3.19 (1.12)

13 (<1.0)

 Katsuwonus pelamis

 (skipjack tuna) Jul 0.05 (0.05)

24 (<1.0) 0 (0) 0 (0)

 Scomberomorus cavalla

 (king mackerel) May, Jun, Jul, Aug 0.04 (0.03)

29.5 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.57 (0.17)

28 (<1.0)

 Scomberomorus maculatus

 (Spanish mackerel) May, Jun, Jul, Aug 0.05 (0.03)24 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.70 (0.27)

26 (<1.0)

 Thunnus spp.

 (tunas) Jun, Sep 0.13 (0.07)11.5 (1.4)

0.09 (0.05)

16 (<1.0)

0.33 (0.17)

41 (<1.0)

 Stromatoideae

 Unidentified Jun, Jul, Aug, Sep 0.05 (0.03)

24 (<1.0) 0 (0) 1.69 (0.66)20 (<1.0)

 Ariommidae

 Ariomma spp.

 (driftfish) May, Jul 0 (0) 0 (0) 0.08 (0.06)61 (<1.0)
18

 Table 2. Main Pass 259 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Nomeidae

 Unidentified Sep 0 (0) 0 (0) 0.12 (0.12)

51 (<1.0)

 Psenes spp.

 (driftfish) Jun 0 (0) 0 (0) 0.02 (0.02)

82 (<1.0)

 Cubiceps pauciradiatus

 (bigeye cigarfish) Jun, Jul, Sep 0 (0) 0 (0) 0.82 (0.39)25 (<1.0)

Pleuronectiformes

 Bothidae

 Bothus spp.

 (eyed/spottail flounder) Jun, Aug, Sep 0 (0) 0 (0) 1.47 (0.42)21 (<1.0)

 Citharichthys spp.

 (whiffs) Jun, Jul, Aug 0 (0) 0 (0) 0.28 (0.17)46 (<1.0)

 Etropus crossotus

 (fringed flounder) Jun, Jul, Aug 0 (0) 0 (0) 0.55 (0.25)31 (<1.0)

 Syacium spp.

 (dusky/shoal flounder) May, Jun, Jul, Aug 0.02 (0.02)38 (<1.0)

0.01 (0.01)

36.5 (<1.0)

0.43 (0.19)

35 (<1.0)

 Cynoglossidae

 Symphurus spp.

 (tonguefish) Jun, Jul, Aug, Sep 0.04 (0.04)29.5 (<1.0)

0.02 (0.02)

30.5 (<1.0)

2.20 (0.49)

17 (<1.0)

Tetraodontiformes

 Balistidae

 Unidentified Jun, Jul 0 (0) 0 (0) 0.28 (0.19)

45 (<1.0)

 Monacanthus spp.

 (filefish) Jun 0 (0) 0 (0) 0.03 (0.03)72 (<1.0)

 Monacanthus hispidus

 (planehead filefish) Jun 0 (0) 0 (0) 0.03 (0.03)

73.5 (<1.0)
19

 Table 3. Mean light trap CPUE (fish/10 min) and plankton net density (fish/100 m3) for fish collected at

 Viosca Knoll 203, with standard error (SE), rank based on CPUE or densities, and percent of total catch

 (%N). For ranks, tied values received the mean of the corresponding ranks.

 RA = reef associated, RD = reef dependent.

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

Anguilliformes

 Muraenidae

 Unidentified

 (moray eels) Jun, Oct 0 (0) 0.03 (0.03)40.5 (<1.0)

0.15 (0.11)

68 (<1.0)

 Ophichthidae

 Ophichthus spp.

 (snake eels) May, Jun, Jul, Aug 0.05 (0.03)27.5 (<1.0)

0.32 (0.09)

16 (1.5)

3.13 (0.93)

15 (1.5)

 Ophichthus gomesi

 (shrimp eel) May, Jun, Jul 0.02 (0.02)49.5 (<1.0)

0.12 (0.05)

27.5 (<1.0)

0.36 (0.28)

44 (<1.0)

Clupeiformes

 Clupeidae

 Harengula jaguana

 (scaled sardine) May, Jun, Jul, Aug 1.97 (0.89)2 (15.9)

2.05 (0.59)

3 (9.8)

23.69 (6.61)

4 (6.8)

 Opisthonema oglinum

 (Atlantic thread herring) May, Jun, Jul, Aug 0.08 (0.04)22 (<1.0)

1.27 (0.65)

6 (6.1)

12.17 (3.83)

10 (4.2)

 Sardinella aurita

 (Spanish sardine) May, Jun, Jul 0.02 (0.02)49.5 (<1.0)

0.07 (0.03)

35.5 (<1.0)

0.05 (0.05)

85 (<1.0)

 Engraulidae

 Unidentified May, Jun, Jul, Oct 0.04 (0.03)

31.5 (<1.0)

0.13 (0.06)

26 (<1.0)

16.89 (7.57)

6 (4.8)

 Anchoa spp. May, Jun, Jul, Aug, Oct 1.22 (0.32)

3 (9.9)

0.53 (0.21)

11 (2.5)

179.50 (75.10)

1 (26.1)

 Anchoa hepsetus

 (striped herring) May, Jun, Jul, Aug, Oct 0.05 (0.03)27.5 (<1.0)

0.83 (0.24)

8 (4.0)

15.85 (5.94)

7 (3.4)

 Anchoa mitchilli

 (bay anchovy) Jun, Jul, Aug 0.02 (0.02)49.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

0.09 (0.07)

79 (<1.0)

 Anchoa nasuta

 (longnose anchovy) May, Jun, Jul 0.02 (0.02)49.5 (<1.0)

0.10 (0.05)

29 (<1.0)

0.63 (0.50)

33 (<1.0)

 Anchoa nasuta/hepsetus

 (longnose/striped anchovy) Jun, Jul, Aug 0 (0) 0 (0) 36.28 (13.57)3 (12.9)

 Anchoviella perfasciata

 (flat anchovy) Jun, Jul 0 (0) 0.27 (0.19)17 (1.3) 0 (0)

 Engraulis eurystole

 (silver anchovy) May, Jun, Jul, Aug, Oct 0.17 (0.09)13.5 (1.3)

1.80 (0.38)

4 (8.6)

12.97 (6.45)

9 (1.5)

Stomiiformes

 Gonostomatidae

 Cyclothone spp.

 (bristlemouths) Jun 0 (0) 0 (0) 0.12 (0.12)74 (<1.0)

Aulopiformes

 Synodontidae

 Saurida brasiliensis

 (largescale lizardfish) May, Jun, Jul, Aug, Oct 0.36 (0.10)9 (2.9)

2.57 (1.66)

1.5 (12.3)

2.68 (0.84)

16 (1.0)

 Synodus foetens

 (inshore lizardfish) May, Jun, Jul, Aug, Oct 0.03 (0.03)35 (<1.0)

2.57 (0.91)

1.5 (12.3)

0.23 (0.17)

57 (<1.0)
20

 Table 3. Viosca Knoll 203 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Synodus poeyi

 (offshore lizardfish) Jun 0 (0) 0.02 (0.02)53.5 (<1.0) 0 (0)

 Trachinocephalus myops

 (snakefish) May, Jun, Aug 0.02 (0.02)49.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

0.06 (0.04)

84 (<1.0)

Myctophiformes

 Myctophidae

 Unidentified

 (lanternfishes) May, Jun, Jul 0.07 (0.03)23.5 (<1.0) 0 (0) 0.33 (0.15)47 (<1.0)

Gadiformes

 Bregmacerotidae

 Bregmaceros cantori

 (codlet) May, Jun, Jul, Aug, Oct 0.39 (0.16)8 (3.2)

0.20 (0.09)

24 (<1.0)

14.21 (4.68)

8 (7.9)

Ophidiiformes

 Ophidiidae

 Lepophidium staurophor

 (barred cusk-eel) May, Jun, Oct 0.02 (0.02)49.5 (<1.0) 0 (0) 0.27 (0.17)52 (<1.0)

 Ophidion spp.

 (cusk-eels) Jun, Jul, Oct 0.02 (0.02)49.5 (<1.0) 0 (0) 0.35 (0.29)46 (<1.0)

 Ophidion nocomis

 (cusk-eels) May, Jun, Jul, Oct 0.02 (0.02)49.5 (<1.0)

0.05 (0.03)

38.5 (<1.0)

1.65 (0.51)

22 (<1.0)

Atheriniformes

 Atherinidae

 Membras martinica

 (rough silverside) Jun 0 (0) 0.02 (0.02)53.5 (<1.0) 0 (0)

Scorpaeniformes

 Scorpaenidae

 Scorpaena spp.

 (scorpionfishes) May, Jun, Oct 0.02 (0.02)49.5 (<1.0)

0.08 (0.05)

31.5 (<1.0)

0.62 (0.24)

34 (<1.0)

 Triglidae

 Prionotus spp.

 (searobins) May 0 (0) 0 (0) 0.08 (0.08)81 (<1.0)

Perciformes

 Unidentified May, Jun, Jul, Aug, Oct 0.04 (0.03)

31.5 (<1.0) 0 (0) 3.48 (0.89)13 (1.1)

 Serranidae (RA)

 Anthinae

 (sea perches) Jun 0.02 (0.02)49.5 (<1.0) 0 (0) 0.10 (0.10)77 (<1.0)

 Epinephelinae

 (groupers) Jul 0 (0) 0 (0) 0.04 (0.04)87 (<1.0)

 Grammistinae

 (basslets) May, Jun 0 (0) 0.02 (0.02)53.5 (<1.0)

0.16 (0.13)

67 (<1.0)

 Serraninae

 (sea basses) May, Jun, Jul, Oct 0.02 (0.02)49.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

0.81 (0.30)

29 (<1.0)

 Apogonidae

 Unidentified

 (cardinalfishes) May, Jun 0 (0) 0.02 (0.02)53.5 (<1.0)

0.10 (0.10)

78 (<1.0)
21

 Table 3. Viosca Knoll 203 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Rachycentridae

 Rachycentron canadum

 (cobia) Jun 0 (0) 0 (0) 0.21 (0.16)60 (<1.0)

 Carangidae

 Unidentified Jul, Oct 0 (0) 0 (0) 0.18 (0.13)

62 (<1.0)

 Caranx spp Jun, Jul, Aug 0 (0) 0.02 (0.02)

53.5 (<1.0)

1.07 (0.48)

24 (<1.0)

 Caranx crysos

 (blue runner) May, Jun, Jul, Oct 0.17 (0.10)13.5 (1.3)

0.43 (0.21)

13 (2.1)

2.19 (0.75)

19 (<1.0)

 Caranx latus

 (horse-eye jack) Jun 0 (0) 0.02 (0.02)53.5 (<1.0) 0 (0)

 Chloroscombrus chrysurus

 (Atlantic bumper) May, Jun, Jul, Aug, Oct 0.03 (0.02)35 (<1.0)

0.07 (0.03)

35.5 (<1.0)

2.24 (0.77)

18 (<1.0)

 Decapterus punctatus

 (round scad) May, Jun, Jul, Aug, Oct 0.69 (0.22)5 (5.5)

1.28 (0.39)

5 (6.1)

0.48 (0.27)

38 (<1.0)

 Seriola spp.

 (amberjacks) May, Jun, Jul 0.02 (0.02)49.5 (<1.0) 0 (0) 0.91 (0.46)26 (<1.0)

 Seriola zonata

 (banded rudderfish) May 0 (0) 0.02 (0.02)53.5 (<1.0)

0.13 (0.13)

72.5 (<1.0)

 Trachurus lathami

 (rough scad) May 0 (0) 0.02 (0.02)53.5 (<1.0) 0 (0)

 Lutjanidae (RA)

 Lutjanus spp. May, Jun, Jul, Aug 0.05 (0.03)

27.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

0.99 (0.50)

25 (<1.0)

 Lutjanus campechanus

 (red snapper) May, Jun, Jul 0.01 (0.01)62.5 (<1.0)

0.23 (0.15)

20 (1.1)

0.28 (0.21)

50 (<1.0)

 Lutjanus griseus

 (gray snapper) Aug 0.33 (0.21)10 (2.7)

0.02 (0.02)

53.5 (<1.0)

0.26 (0.26)

53.5 (<1.0)

 Lutjanus synagris

 (lane snapper) Jul 0 (0) 0 (0) 0.11 (0.08)75 (<1.0)

 Ocyurus chrysurus

 (yellowtail snapper) Jun 0 (0) 0.03 (0.02)53.5 (<1.0) 0 (0)

 Rhomboplites aurorubens

 (vermilion snapper) Jul, Aug, Oct 0.16 (0.05)15 (1.2)

0.08 (0.04)

31.5 (<1.0)

0.18 (0.18)

63 (<1.0)

 Gerreidae

 Unidentified May, Jun 0.07 (0.05)

23.5 (<1.0)

0.04 (0.02)

53.5 (<1.0)

0.42 (0.23)

40 (<1.0)

 Eucinostomus spp.

 (mojarras) Jun, Jul, Aug, Oct 0.10 (0.05)18.5 (<1.0)

0.58 (0.26)

10 (2.8)

0.66 (0.40)

32 (<1.0)

 Sparidae

 Unidentified Jun, Jul, Aug 0 (0) 0 (0) 0.68 (0.38)

31 (<1.0)

 Lagodon rhomboides

 (pinfish) Jun 0 (0) 0 (0) 0.14 (0.14)70 (<1.0)
22

Table 3. Viosca Knoll 203 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Sciaenidae

 Cynoscion arenarius

 (sand seatrout) May, Jun, Jul, Aug, Oct 0.02 (0.02)49.5 (<1.0)

0.05 (0.02)

53.5 (<1.0)

7.92 (2.44)

11 (1.4)

 Cynoscion nothus

 (silver seatrout) May, Jun 0.02 (0.02)49.5 (<1.0)

0.06 (0.02)

53.5 (<1.0)

0.45 (0.22)

39 (<1.0)

 Menticirrhus spp.

 (kingfish) May, Jun 0.02 (0.02)49.5 (<1.0) 0 (0) 0.24 (0.15)56 (<1.0)

 Micropogonias undulatus

 (Atlantic croaker) Oct 0.10 (0.06)20 (<1.0)

0.12 (0.06)

27.5 (<1.0)

20.39 (9.94)

5 (4.7)

 Sciaenops ocellatus

 (red drum) Oct 0 (0) 0 (0) 0.09 (0.09)

80 (<1.0)

 Mullidae

 Unidentified

 (goatfishes) May 0 (0) 0.07 (0.07)35.5 (<1.0) 0 (0)

 Ephippidae

 Chaetodipterus faber

 (Atlantic spadefish) May 0.02 (0.02)49.5 (<1.0) 0 (0) 0 (0)

 Pomacentridae (RD)

 Pomacentrus spp.

 (damselfishes) Jun, Jul, Aug 0.51 (0.15)6 (4.1)

0.35 (0.10)

15 (1.7) 0 (0)

 Mugilidae

 Mugil cephalus

 (striped mullet) May 0.02 (0.02)49.5 (<1.0) 0 (0) 0 (0)

 Mugil curema

 (white mullet) May 0 (0) 0.02 (0.02)53.5 (<1.0) 0 (0)

 Sphyraenidae

 Sphyraena spp. May, Jun, Jul 0 (0) 0 (0) 0.59 (0.27)

36 (<1.0)

 Sphyraena borealis

 (northern sennet) Jun, Jul 0.05 (0.04)27.5 (<1.0) 0 (0) 2.27 (0.89)17 (<1.0)

 Sphyraena guachancho

 (guaguanche) Jun 0 (0) 0.08 (0.05)31.5 (<1.0) 0 (0)

 Opisthognathidae

 Opisthognathus spp.

 (jawfishes) May, Jun 0.02 (0.02)49.5 (<1.0) 0 (0) 0.21 (0.18)59 (<1.0)

 Blenniidae (RA)

 Unidentified May, Jun, Jul, Aug 0.01 (0.01)

62.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

0.61 (0.24)

35 (<1.0)

 Chasmodes spp.

 (Florida/striped blenny) Jun, Jul, Aug 0 (0) 0.03 (0.03)40.5 (<1.0)

0.50 (0.31)

37 (<1.0)

 Hypleurochilus multifilis

 (blenny) May, Jun 0 (0) 0.22 (0.11)23 (1.0)

0.14 (0.14)

69 (<1.0)

 Hypsoblennius hentzi

 (feather blenny) May, Jun 0 (0) 0.07 (0.04)35.5 (<1.0) 0 (0)

 Hypsoblennius invemar

 (tesselated blenny) May, Jun, Jul, Aug 2.46 (0.81)1 (19.7)

0.23 (0.07)

20 (1.1) 0 (0)
23

Table 3. Viosca Knoll 203 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Hypsoblennius ionthas

 (freckled blenny) Jun, Jul, Aug 0.12 (0.09)17 (<1.0)

0.23 (0.11)

20 (1.1)

0.05 (0.05)

86 (<1.0)

 Ophioblennius macclurei

 (redlip blenny) May, Jun 0 (0) 0 (0) 0.30 (0.22)49 (<1.0)

 Parablennius marmoreus

 (seaweed blenny) May, Jun, Jul, Aug 0.10 (0.07)18.5 (<1.0)

0.45 (0.11)

12 (2.1) 0 (0)

 Scartella cristata

 (molly miller) May, Jun, Jul, Aug 0.25 (0.09)12 (2.0)

0.17 (0.08)

25 (<1.0)

3.15 (1.01)

14 (<1.0)

 Callionymidae

 Unidentified

 (dragonets) Oct 0 (0) 0 (0) 0.37 (0.28)43 (<1.0)

 Gobiidae

 Unidentified May, Jun, Jul, Aug, Oct 0.29 (0.07)

11 (2.5)

0.23 (0.09)

20 (1.1)

44.72 (10.15)

2 (11.8)

 Gobionellus oceanicus

 (sharptail goby) Jun 0 (0) 0 (0) 0.10 (0.10)76 (<1.0)

 Gobiosoma bosc

 (naked goby) Jun 0.03 (0.03)35 (<1.0) 0 (0) 0.39 (0.39)42 (<1.0)

 Microgobius sp.

 (clown/green goby) May 0 (0) 0 (0) 0.26 (0.26)53.5 (<1.0)

 Microdesmidae

 Microdesmus spp. May, Jul 0 (0) 0 (0) 0.33 (0.17)

48 (<1.0)

 Microdesmus lanceolatus

 (lancetail wormfish) May, Jun, Jul 0.02 (0.02)49.5 (<1.0)

0.03 (0.02)

53.5 (<1.0)

3.64 (0.81)

12 (<1.0)

 Microdesmus longipinnis

 (pink wormfish) May, Jun 0.02 (0.02)49.5 (<1.0) 0 (0) 0.19 (0.10)61 (<1.0)

 Trichiuridae

 Trichiurus lepturus

 (Atlantic cutlassfish) Jul, Aug 0 (0) 0 (0) 0.17 (0.15)

65 (<1.0)

 Scombridae

 Auxis spp.

 (frigate mackerels) May, Jun, Jul, Oct 1.19 (0.43)4 (9.7)

0.67 (0.24)

9 (3.2)

0.36 (0.28)

45 (<1.0)

 Euthynnus alletteratus

 (little tunny) May, Jun, Jul, Aug 0.40 (0.15)7 (3.2)

1.17 (0.48)

7 (5.6)

0.85 (0.41)

27 (<1.0)

 Scomberomorus cavalla

 (king mackerel) May, Jun, Jul, Aug 0.05 (0.03)

27.5 (<1.0)

0.23 (0.09)

20 (1.1)

0.17 (0.13)

64 (<1.0)

 Scomberomorus maculatus

 (Spanish mackerel) May, Jun, Jul, Aug 0.14 (0.05)16 (1.2)

0.42 (0.18)

14 (2.0)

1.85 (1.01)

21 (<1.0)

 Thunnus spp.

 (tunas) Jun, Aug 0.02 (0.02)49.5 (<1.0)

0.05 (0.03)

38.5 (<1.0)

0.84 (0.61)

28 (<1.0)

 Thunnus thynnus

 (bluefin tuna) May, Jun 0.08 (0.04)21 (<1.0) 0 (0) 0 (0)

 Ariommidae

 Ariomma spp.

 (driftfish) Jun 0.02 (0.02)49.5 (<1.0)

0.04 (0.02)

53.5 (<1.0)

0.07 (0.05)

82.5 (<1.0)
24

 Table 3. Viosca Knoll 203 (continued)

Months

Off-Platform

Light Trap

Within-Platform

Light Trap

Surface

Plankton Net

TAXA

Collected CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Density (SE)

Rank (%N)

 Stromateidae

 Peprilus burti

 (gulf butterfish) Oct 0 (0) 0 (0) 0.13 (0.13)72.5 (<1.0)

 Peprilus paru

 (harvestfish) May, Aug 0.03 (0.02)35 (<1.0) 0 (0) 0.78 (0.78)30 (<1.0)

Pleuronectiformes

 Bothidae

 Bothus spp.

 (eyed/spottail flounder) Jun 0 (0) 0 (0) 0.07 (0.05)82.5 (<1.0)

 Etropus crossotus

 (fringed flounder) May, Jun, Jul, Aug 0.05 (0.03)27.5 (<1.0) 0 (0) 1.24 (0.56)23 (<1.0)

 Syacium spp.

 (dusky/shoal flounder) May, Jun, Jul, Oct 0.02 (0.02)49.5 (<1.0)

0.08 (0.04)

31.5 (<1.0)

0.41 (0.19)

41 (<1.0)

 Cynoglossidae

 Symphurus spp.

 (tonguefish) May, Jun, Jul, Aug 0.02 (0.02)49.5 (<1.0)

0.02 (0.02)

53.5 (<1.0)

1.87 (0.58)

20 (<1.0)

Tetraodontiformes

 Balistidae

 Monacanthus spp.

 (filefish) May, Jul 0.03 (0.02)35 (<1.0) 0 (0) 0.21 (0.21)58 (<1.0)

 Tetraodontidae

 Sphoeroides spp.

 (pufferfish) Aug 0 (0) 0 (0) 0.03 (0.03)89 (<1.0)

 Sphoeroides parvus

 (least puffer) Jul 0 (0) 0 (0) 0.17 (0.17)66 (<1.0)
25

Figure 4. Mean total densities of fishes from plankton nets (with standard errors) at

 each platform for data a) with clupeiforms and b) without clupeiforms.

 Different letters above bars indicate significant differences between

 platform densities using Tukey’s Studentized Range Test. Open bars

 represent platforms east of the Delta. Numbers in parentheses indicates

 the depth, in meters, of each platform.

0

100

200

300

400

500

600

ST54 VK203 GI94 MP259 GC18

Platform

Mean Total Density (#/100m3

)

5001.1 B

B

0

50

100

150

200

250

ST54 VK203 GI94 MP259 GC18

Platform

Mean Total Density (#/100m3

)

A

A

B

(20 m) (120 m) (35 m) (230 m) (60 m)

A) with clupeiforms 

B) without clupeiforms

A

B

C

A

B

(20 m) (230 m) (35 m) (60 m) (120 m)
26

Figure 5. Mean total CPUEs of fishes from light traps (with standard errors) at each

 platform for data a) with clupeiforms and b) without clupeiforms.

 Different letters above bars indicate significant differences between

 platform CPUEs using Tukey’s Studentized Range Test. Open bars

 represent platforms East of the Delta. Numbers in parentheses indicate the

 depth, in meters, of each platform.

0

20

40

60

80

100

120

ST54 VK203 GI94 MP259 GC18

Platform

Mean Total CPUE (#/10 min)

C

(20 m) (230 m) (35 m) (60 m) (120 m)

0

5

10

15

20

25

30

35

ST54 VK203 GI94 MP259 GC18

Platform

Mean Total CPUE (#/10 min)

A

A

B

C

D

(20 m) (230 m) (35 m) (60 m) (120 m)

A) with clupeiforms

B) without clupeiforms

A

B

C

D
27

Mean CPUEs from light traps were generally highest at platforms of intermediate depth (Figure

5). With clupeiforms included, mean CPUEs were significantly different between platforms (χ

2

= 158.85, p<0.0001), with highest catches at mid-shelf (GI 94 = 88.5 fish/10 min. ± 15.8) and

lowest at the outer shelf (GC 18 = 7.2 ± 1.2; Figure 5a). The inner shelf platform ST 54 had the

second highest mean CPUE (47.7 ± 8.7), due to high abundances of Opisthonema oglinum. With

clupeiforms excluded, mean CPUEs were significantly different (χ

2

 = 118.37, p<0.0001), with

highest catches at the mid-shelf platforms, GI 94 and VK 203 (Figure 5b).

Taxonomic richness and diversity were also highest, for both gear types, at platforms of

intermediate depth (35-120 m depth; Table 4). The number of taxa (identified to at least the

genus level) and families collected, for both plankton nets and light traps, peaked at the midshelf platform GI 94, and were generally lowest at the two depth extremes. Light trap collections

at the outer shelf platform MP 259, however, appeared to have exceptionally low taxonomic

richness, and this may be manifested in its low diversity value. The higher number of taxa and

families at GI 94 may be the result of the higher number of samples taken at this platform. Mean

Shannon-Wiener diversity indices, which accounted for unequal sample sizes, showed a similar

pattern of greatest diversity at intermediate depths. Plankton net diversity was significantly

higher for the outer shelf platform MP 259 (χ

2

 = 28.50, p<0.0001). Light trap diversity was

significantly higher for the mid-shelf platforms VK 203 and GI 94 (χ

2

 = 66.01, p<0.0001).

Numerical classification of the non-clupeiform larval and juvenile fish assemblages

collected at each platform indicated that highest similarity occurred between platforms of similar

depth (Figure 6). Although similarity was low for each platform pairing (mean similarity index

= 0.371 ± 0.03) three groups were described. The most similar pair was the two outer shelf

platforms, GC 18 and MP 259. Larval and juvenile fish assemblages at these platforms were

characterized by relatively high numbers of Auxis spp. (10.2% and 19.1% of the total nonclupeiform catch, respectively) and Caranx crysos (6.6% and 13.3%), and included taxa such as

Holocentrus spp. and unidentified myctophids and scarids (Table 5). The next most similar

group consisted of the mid-shelf platforms, GI 94 and VK 203. High numbers of Bregmaceros

cantori (6.5% and 13.5%, respectively), Hypsoblennius invemar (15.1% and 6.0%) and Saurida

brasiliensis (9.6% and 8.0%) occurred at these platforms. The mid-shelf platforms also had the

highest numbers of reef taxa, particularly blenniids, pomacentrids and lutjanids. The third group

consisted of the inner shelf platform ST 54. This platform was dominated by Cynoscion

arenarius (30.3%) and Synodus foetens (20.5%), and included other coastal taxa such as

Scomberomorus maculatus and the blenniid complex Scartella/Hypleurochilus.

Several taxa were more abundant at platforms either east or west of the Delta, regardless

of the sampling gear used (Table 6). Decapterus punctatus, Eucinostomus spp., Lutjanus spp.,

and Ophichthidae were significantly more abundant east of the Delta in both plankton nets and

light traps. Microdesmus lanceolatus was significantly more abundant east of the Delta only in

plankton nets, while Thunnus spp. was significantly more abundant east of the Delta only in light

traps. Caranx hippos/latus was the only taxa significantly more abundant west of the Delta in

both gear types. Cynoscion arenarius, Euthynnus alletteratus, Peprilus burti, Scomberomorus

cavalla, and Symphurus spp. were significantly more abundant west of the Delta in light traps.
28

 Table 4. Taxonomic richness and mean Shannon-Weiner Diversity

 (with standard errors) for the larval and juvenile fish assemblages 

 collected at each platform. Different letters beside diversity values

 indicate significant differences between indices using the Tukey's 

 Studentized Range Test. Numbers in parentheses indicate depth,

 in meters, of each platform. n = number of samples for a gear type. 

 ‡ denotes platforms east of the Delta.

Plankton Net n Families Taxa Diversity

GC18 (230 m) 47 38 54 0.726 ± 0.094 (C) 

MP259 (120 m) ‡ 54 38 60 1.347 ± 0.085 (A)

GI94 (60 m) 144 40 73 0.929 ± 0.054 (B/C)

VK203 (35 m) ‡ 56 32 65 1.181 ± 0.081 (A/B)

ST54 (20 m) 65 26 44 0.856 ± 0.081 (C) 

Light Trap n Families Taxa Diversity

GC18 (230 m) 96 31 48 0.761 ± 0.061 (B)

MP259 (120 m) ‡ 94 20 35 0.826 ± 0.064 (B)

GI94 (60 m) 267 37 80 1.105 ± 0.033 (A)

VK203 (35 m) ‡ 112 31 67 1.267 ± 0.053 (A)

ST54 (20 m) 122 31 55 0.887 ± 0.052 (B)
29

(20 m)

(230 m)

(120 m)

(35 m)

(60 m)

Figure 6. Cluster diagram of the overall larval and juvenile fish

 assemblages, with clupeiforms excluded, collected from all

 gears combined at each platform. Numbers in parentheses

 indicate the depth, in meters, of each platform. Samples were

 clustered using group-average sorting and the Bray-Curtis

 similarity index.

Bray-Curtis Similarity

MP259

ST54

GI94

GC18

VK203

1.0 0.8 0.9 0.3 0.7 0.6 0.5 0.4
30

Table 5. Relative abundance (% of total catch from plankton nets and light traps combined) of the ten most 

abundant non-clupeiform taxa, and reef taxa that comprised >0.5%, at each platform. Numbers in parentheses

 indicate the depth, in meters, of each platform. RA = reef associated, RD = reef dependent.

 ST 54 (20 m) - inner shelf %

Dominant Cynoscion arenarius 30.3

Taxa Synodus foetens 20.5

Gobiidae 5.4

Scomberomorus maculatus 4.9

Euthynnus alletteratus 4.1

Scartella/Hypleurochilus RA 3.0

Scomberomorus cavalla 2.6

Caranx hippos/latus 2.4

Hypsoblennius hentz/ionthas RA 2.4

Hypsoblennius invemar RA 2.0

 GI 94 (60 m) - mid-shelf % VK 203 (35 m) - mid-shelf %

Dominant Synodus poeyi 15.3 Dominant Gobiidae 19.4

Taxa Hypsoblennius invemar RA 15.1 Taxa Bregmaceros cantori 13.5

Parablennius marmoreus RA 11.2 Saurida brasiliensis 8.0

Saurida brasiliensis 9.6 Hypsoblennius invemar RA 6.0

Euthynnus alletteratus 8.0 Synodus foetens 5.7

Bregmaceros cantori 6.5 Auxis spp. 4.2

Symphurus spp. 4.9 Decapterus punctatus 4.0

Synodus foetens 4.2 Euthynnus alletteratus 3.7

Gobiidae 3.8 Ophichthus spp. 3.0

Auxis spp. 3.4 Scartella/Hypleurochilus RA 2.3

Reef Taxa Hypsoblennius hentz/ionthas RA 2.8 Reef Taxa Pomacentrus spp. RD 1.9

Scartella/Hypleurochilus RA 1.8 Lutjanus spp. RA 1.4

Pomacentrus spp. RD 0.8 Parablennius marmoreus RA 1.2

Chromis spp. RD 0.8 Hypsoblennius hentz/ionthas RA 1.0

Rhomboplites aurorubens RA 0.5 Lutjanus campechanus RA 0.8

 GC 18 (230 m) - outer shelf % MP 259 (120 m) - outer shelf %

Dominant Auxis spp. 10.2 Dominant Auxis spp. 19.1

Taxa Gobiidae 10.1 Taxa Gobiidae 13.7

Caranx hippos/latus 9.7 Caranx crysos 13.3

Caranx crysos 6.6 Euthynnus alletteratus 8.6

Symphurus spp. 6.4 Myctophidae 6.6

Ariomma spp. 6.0 Bregmaceros cantori 3.8

Cynoscion arenarius 5.6 Synodus foetens 3.8

Euthynnus alletteratus 3.6 Symphurus spp. 3.4

Pristipomoides aquilonaris RA 2.9 Decapterus punctatus 2.5

Epinephelinae RA 2.6 Microdesmus lanceolatus 2.1

Reef Taxa Anthinae RA 2.3 Reef Taxa Hypsoblennius invemar RA 1.3

Holocentrus spp. RA 1.8 Holocentrus spp. RA 0.7

Pomacentrus spp. RD 1.8 Pomacentrus spp. RD 0.5

Scaridae RD 1.0
31

Table 6. Mean densities and CPUEs (with standard errors) for taxa more abundant at platforms

 either east or west of the Mississippi River Delta in both plankton nets and light traps. 

 Underlined means are significantly greater. 

 Mean plankton net densities (#/100 m3

)

Taxa East of Delta West of Delta Wilcoxon 2-sample test

Decapterus punctatus 0.42 ± 0.24 0.02 ± 0.01 Z = 2.81, p=0.005

Eucinostomus spp. 1.15 ± 0.48 0 Z = 6.11, p=0.0001

Lutjanus spp. 1.45 ± 0.59 0.61 ± 0.20 Z = 2.83, p=0.005

Microdesmus lanceolatus 2.74 ± 0.49 1.19 ± 0.33 Z = 4.74, p=0.0001

Ophichthidae 2.06 ± 0.54 0.14 ± 0.07 Z = 5.67, p=0.0001

Thunnus spp. 0.50 ± 0.33 0.04 ± 0.03 Z = 1.16, p=0.25

Caranx hippos/latus 0.01 ± 0.01 1.77 ± 0.83 Z = -2.71, p=0.007

Cynoscion arenarius 4.54 ± 1.38 11.03 ± 2.37 Z = -0.65, p=0.51

Euthynnus alletteratus 2.27 ± 0.74 3.28 ± 0.70 Z = -0.33, p=0.74

Peprilus burti 0 0.56 ± 0.40 Z = -1.33, p=0.18

Scomberomorus cavalla 0.46 ± 0.13 1.84 ± 1.02 Z = -0.04, p=0.95

Symphurus spp. 2.37 ± 0.44 7.51 ± 1.47 Z = -0.77, p=0.44

 Mean light trap CPUE (#/10 min.)

Taxa East of Delta West of Delta Wilcoxon 2-sample test

Decapterus punctatus 0.58 ± 0.13 0.03 ± 0.01 Z = 8.26, p=0.0001

Eucinostomus spp. 0.38 ± 0.14 0.04 ± 0.01 Z = 6.09, p=0.0001

Lutjanus spp. 0.22 ± 0.11 0.01 ± 0.005 Z = 4.30, p=0.0001

Microdesmus lanceolatus 0.02 ± 0.01 0.006 ± 0.004 Z = 1.59, p=0.11

Ophichthidae 0.16 ± 0.03 0.03 ± 0.01 Z = 5.70, p=0.0001

Thunnus spp. 0.06 ± 0.02 0.02 ± 0.01 Z = 2.48, p=0.01

Caranx hippos/latus 0.01 ± 0.008 0.19 ± 0.04 Z = -4.02, p=0.0001

Cynoscion arenarius 0.01 ± 0.01 0.12 ± 0.03 Z = -3.14, p=0.002

Euthynnus alletteratus 0.55 ± 0.15 1.44 ± 0.23 Z = -2.71, p=0.007

Peprilus burti 0 0.05 ± 0.01 Z = -2.18, p=0.03

Scomberomorus cavalla 0.09 ± 0.03 0.15 ± 0.04 Z = -2.42, p=0.02

Symphurus spp. 0.02 ± 0.01 0.09 ± 0.02 Z = -2.67, p=0.008
32

Figure 7. Mean CPUEs (with standard errors) from within- and off-platform light

 traps at each platform for data a) with clupeiforms and b) without

 clupeiforms. Arrows above the bars point to the mean for that gear.

 Asterisk denotes significant differences between light traps at a platform

 using the Wilcoxon 2-sample test. Numbers in parentheses indicate the

 depth, in meters, of each platform. ‡ denotes platforms East of the Delta.

0

10

20

30

40

50

60

ST54 VK203 ‡ GI94 MP259 ‡ GC18

Platform

Mean CPUE (#/10 min)

Off-platform Light Trap

Within-platform Light Trap

148.2

*

*

A) with clupeiforms

(20 m) (230 m) (35 m) (60 m) (120 m)

0

2

4

6

8

10

12

14

16

18

20

ST54 VK203 ‡ GI94 MP259 ‡ GC18

Platform

Mean CPUE (#/10 min)

Off-platform Light Trap

Within-platform Light Trap

*

58.2 

B) without clupeiforms

*

(20 m) (230 m) (35 m) (60 m) (120 m)
33

Within- vs. Off-Platform Patterns in Light Trap Catches

Light traps fished within the platform structure generally had higher mean CPUEs than

light traps fished down-current from the platform (Figure 7). With clupeiforms included, VK

203 (Z = -1.97, p<0.05) and GI 94 (Z = 8.58, p<0.0001) had significantly higher total catch rates

within-platform. When clupeiforms were removed from these analyses, mean total CPUEs

remained higher for within-platform light traps, with GI 94’s significantly higher (Z = 5.74,

p<0.0001). The exception was ST 54, where off-platform catch rates were significantly higher

(Z = 2.31, p<0.02).

Comparisons of taxonomic richness and diversity between within- and off-platform light

trap collections showed no clear patterns among platforms (Table 7). Within-platform light traps

collected more families and taxa (identified to at least the genus level) than off-platform light

traps at GC 18 and GI 94. However, slightly higher numbers of families and taxa were identified

from off-platform collections at ST 54, VK 203 and MP 259 (number of taxa at MP 259 were

equal). Mean Shannon-Wiener diversity indices were greater for within-platform light trap

collections at GC 18 and GI 94, and significantly greater at VK 203 (Z = 4.05, p<0.0001). Offplatform light trap collections had greater diversity indices at ST 54 and MP 259.

Higher measures of similarity between the within- and off-platform fish assemblages

coincided with relatively greater abundances of clupeiforms off-platform at ST 54 and MP 259.

The fish assemblages collected by within- and off-platform light traps were most similar at ST

54 and MP 259 (Bray-Curtis similarity = 0.76 and 0.72, respectively) than at the other platforms

(GC 18 = 0.49, GI 94 = 0.47, VK 203 = 0.47). Furthermore, only at ST 54 and MP 259 were

mean off-platform CPUEs of clupeiforms not significantly lower than within-platform CPUEs

(Table 8).

Fishes from the families Clupeidae (Harengula jaguana and Opisthonema oglinum),

Engraulidae (Anchoa mitchilli, A. nasuta/hepsetus and Engraulis eurystole), Synodontidae

(Saurida brasiliensis and Synodus foetens), and Blenniidae (Hypsoblennius hentz/ionthas and

Scartella/Hypleurochilus) were found, with a few exceptions, to have higher mean CPUEs in

within-platform light trap samples at each platform (Table 9). Conversely, Scombridae

(Euthynnus alletteratus, Scomberomorus cavalla and S. maculatus) were more abundant offplatform at each platform, except at VK 203 where CPUEs for these species were higher withinplatform. Pomacentrus spp. and Hypsoblennius invemar also seemed to have higher mean

CPUEs off-platform, with the exception of GC 18 and GI 94, respectively. Mean CPUEs for

Caranx hippos/latus were significantly higher in off-platform light traps at ST 54 and GC 18. Of

the most abundant reef fish families, only Lutjanidae and Serranidae at GI 94 showed significant

differences between mean light trap CPUEs, with both taxa being more abundant withinplatform. Otherwise, reef fish CPUEs were low (<0.13 fish/10 min.) at each platform, except

VK 203 where lutjanid CPUEs for within- and off-platform light traps were 0.35 and 0.55

fish/10 min., respectively.

Several species of engraulids showed significantly different size distributions between

within- and off-platform light trap collections at platforms (Figure 8). Most Anchoa mitchilli

collected within-platform at GI 94 were between 13 and 16 mm, while those collected offplatform were mainly 16 to 19 mm in length. Much larger Engraulis eurystole were collected in
34

Table 8. Mean CPUEs (with standard errors) of clupeiforms from within- and offplatform light traps at each platform. Underlined means are significantly greater.

 Numbers in parentheses indicate the depth, in meter, of each platform. 

 ‡ denotes platforms east of the Delta.

Within-platform Off-platform

Light Trap Light Trap Wilcoxon 2-sample test

GC18 (230 m) 1.73 ± 0.39 0.77 ± 0.22 Z = -2.33, p<0.02

MP259 (120 m) ‡ 12.80 ± 4.17 6.55 ± 1.52 Z = -0.71, p<0.48

GI94 (60 m) 90.06 ± 25.83 10.85 ± 2.14 Z = 3.57, p<0.0004

VK203 (35 m) ‡ 7.07 ± 1.22 3.57 ± 0.91 Z = -2.77, p<0.006

ST54 (20 m) 35.55 ± 10.56 31.04 ± 9.35 Z = 0.58, p<0.56

Table 7. Taxonomic richness and mean Shannon-Weiner Diversity (with

standard errors) for the larval and juvenile fish assemblages collected by 

within- (SL) and off-platform (OL) light traps at each platform. Numbers

 in parentheses indicate the depth, in meters, of each platform.

 ‡ denotes platforms east of the Delta.

Light Trap Families Taxa Diversity

GC18 (230 m) SL 32 47 0.803 ± 0.090

OL 28 37 0.699 ± 0.071

MP259 (120 m) ‡ SL 18 31 0.748 ± 0.067

OL 20 31 0.923 ± 0.085

GI94 (60 m) SL 34 71 1.162 ± 0.041

OL 26 54 1.044 ± 0.051

VK203 (35 m) ‡ SL 26 53 1.454 ± 0.073

OL 27 56 1.069 ± 0.058

ST54 (20 m) SL 26 40 0.801 ± 0.064

OL 27 51 0.875 ± 0.075
35

Table 9. Mean CPUEs of the dominant taxa, and reef fish families, from within- and off-platform light traps at each platform. Unde significantly greater for taxa at a platform (Wilcoxon 2-sample test, p<0.05). Numbers in parentheses indicate the depth, in mete ‡ denotes platforms east of the Delta. RA = reef associated taxa; RD = reef dependent taxa. 

 ST54 (20 m) VK203 (35 m) ‡ GI94 (60 m) MP259 (120 m) ‡ 

Within Off Within Off Within Off Within Off 

Dominant Taxa Platform Platform Platform Platform Platform Platform Platform Platform Harengula jaguana 0.56 0.55 2.05 1.97 0.69 0.68 0.53 0.50 Opisthonema oglinum 23.26 25.53 1.27 0.08 6.04 4.11 5.34 2.39 

Anchoa mitchilli 0.38 0.31 0.02 0.02 1.89 0.47 0.16 0.11 Anchoa nasuta/hepsetus 9.96 3.69 0.93 0.07 41.91 3.26 0.47 0.70 

Engraulis eurystole 0.26 0.03 1.80 0.17 38.79 1.25 5.98 2.38 

Saurida brasiliensis 0.06 0.08 2.57 0.36 3.35 0.50 0.16 0 

Synodus foetens 3.16 0.27 2.57 0.03 22.11 0.20 0.90 0.05 Caranx crysos 0.07 0.24 0.43 0.17 0.08 0.08 0.16 0.18 Caranx hippos/latus 0.04 0.61 0.02 0 0.11 0.09 0.03 0 

Decapterus punctatus 0.02 0.02 1.28 0.69 0.06 0.01 0.09 0.11 

Pomacentrus spp. RD 0 0.02 0.35 0.51 0.12 0.30 0.04 0.07 Hypsoblennius hentz/ionthas RA 0.21 0.48 0.30 0.12 1.76 0.04 0 0 Hypsoblennius invemar RA 0.11 0.48 0.23 2.46 6.33 3.58 0.03 0.38 

Scartella/Hypleurochilus RA 0.49 0.16 0.38 0.25 1.14 0.11 0 0 

Auxis spp. 0.02 0.26 0.67 1.19 0.76 0.24 0.19 0.13 Euthynnus alletteratus 0.37 1.08 1.17 0.40 0.92 2.83 0.10 0.29 Scomberomorus cavalla 0.05 0.10 0.23 0.05 0.09 0.27 0.01 0.04 Scomberomorus maculatus 0.18 0.81 0.42 0.14 0.04 0.17 0.01 0.05 

Reef familiesHolocentridae RA 0 0 0 0 0 0.01 0.13 0.07 

Lutjanidae RA 0 0.02 0.35 0.55 0.11 0.02 0 0.02 

Serranidae RA 0.01 0 0.03 0.03 0.12 0 0.03 0.04 
36

Figure 8. Significantly different size distributions of engraulids collected by off-platform (OL) and within-

 platform (SL) light traps. Number of samples (n) and the p-values from K-S Tests are included.

Engraulis eurystole @ VK203

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 4 7 10 13 16 19 22 25 28 31 34

Length (mm)

Frequency

SL n=108 OL n=10

p<0.001 Anchoa mitchilli @ GI94

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 3 5 7 9 11 13 15 17 19 21

Length (mm)

Frequency

SL n=182 OL n=63

p<0.001

Anchoa nasuta/hepsetus @ GI94

0

0.05

0.1

0.15

0.2

1 4 7 10 13 16 19 22 25 28 31 34 37 40+

Length (mm)

Frequency

SL n=883 OL n=395

p<0.001

Engraulis eurystole @ GI94

0

0.05

0.1

0.15

0.2

1 4 7 10 13 16 19 22 25 28 31 34 37 40+

Length (mm)

Frequency

SL n=900 OL n=158

p<0.001

Anchoa nasuta/hepsetus @ MP259

0

0.1

0.2

0.3

0.4

0.5

1 3 5 7 9 11 13 15 17 19 21 23 25 27

Length (mm)

Frequency

SL n=33 OL n=38

p=0.009

Engraulis eurystole @ MP259

0

0.05

0.1

0.15

1 4 7 10 13 16 19 22 25 28 31 34 37 40+

Length (mm)

Frequency

SL n=297 OL n=112

p=0.005
37

Figure 9. Significantly different size distributions of synodontids collected by off-platform (OL) and within-

 platform (SL) light traps. Number of samples (n) and the p-values from K-S Tests are included.

Synodus foetens @ ST54

0

0.05

0.1

0.15

1 4 7 10 13 16 19 22 25 28 31 34 37 40+

Length (mm)

Frequency

SL n=228 OL n=17

p=0.05

Saurida brasiliensis @ VK203

0

0.05

0.1

0.15

0.2

1 4 7 10 13 16 19 22 25 28 31 34 37 40+

Length (mm)

Frequency

SL n=92 OL n=22

p<0.001 Saurida brasiliensis @ GI94

0

0.1

0.2

0.3

0.4

0.5

5 10 15 20 25 30 35 40 45 50 55 60 65

Length (mm)

Frequency

SL n=463 OL n=77

p<0.001

Synodus foetens @ GI94

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 4 7 10 13 16 19 22 25 28 31 34 3740+

Length (mm)

Frequency

SL n=759 OL n=32

p=0.003
38

Figure 10. Significantly different size distributions of scombrids collected by off-platform (OL) and within-

 platform (SL) light traps. Number of samples (n) and the p-values from K-S Tests are included.

Euthynnus alletteratus @ ST54

0

0.05

0.1

0.15

0.2

0.25

0.3

1 3 5 7 9 11 13 15 17 19 21 23 25

Length (mm)

Frequency

SL n=31 OL n=67

p<0.001

Auxis spp. @ GI94

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37

Length (mm)

Frequency

SL n=101 OL n=35

p<0.001

Euthynnus alletteratus @ GI94

0

0.02

0.04

0.06

0.08

0.1

0.12

1 4 7 10 13 16 19 22 25 28 31 34 3740+

Length (mm)

Frequency

SL n=143 OL n=388

p<0.001

Scomberomorus cavalla @ GI94

0

0.05

0.1

0.15

0.2

0.25

0.3

1 4 7 10 13 16 19 22 25 28 31 34 3740+

Length (mm)

Frequency

SL n=15 OL n=44

p<0.001
39

p=0.13 Hypsoblennius invemar @ ST 54

0

0.1

0.2

0.3

0.4

1 3 5 7 9 11 13 15 17

Length (mm)

Frequency

SL n=10 OL n=30

p=0.13

Hypsoblennius invemar @ VK203

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 3 5 7 9 11 13 15

Length (mm)

Frequency

SL n=14 OL n=148

p<0.001

Hypsoblennius invemar @ GI94

0

0.1

0.2

0.3

0.4

1 3 5 7 9 11 13 15

Length (mm)

Frequency

SL n=532 OL n=514

p<0.001

Figure 11. Significantly different size distributions of Hypsoblennius invemar collected by off-platform (OL)

 and within-platform (SL) light traps. Also presented are size distributions of H. invemar from the

 inner shelf platform, ST 54. Number of samples (n) and the p-values from K-S Tests are included.
40

within-platform light traps at VK 203, while off-platform light traps only collected fish ≤ 8 mm.

Although the size distributions of E. eurystole at GI 94 and MP 259 overlapped considerably, the

median length was slightly larger off-platform than within-platform (27 vs. 24 mm at GI 94, 23

vs. 22 at MP 259). Median lengths of Anchoa nasuta/hepsetus were larger off-platform at GI 94

(22 vs. 21 mm), while at MP 259 the median length was larger within-platform (20 vs. 18 mm).

Significantly different size distributions were also found for synodontids in comparisons

of within- and off-platform light trap collections (Figure 9). Synodus foetens collected offplatform at ST 54 and GI 94 had a greater frequency of larger individuals (>28 mm) than withinplatform collections; however, caution should be exercised considering the disproportionately

larger numbers taken within-platform at both locations. Larger individuals of Saurida

brasiliensis were also more frequent in off-platform collections at VK 203 and GI 94.

Size distributions of scombrids in off-platform light trap collections generally

encompassed larger size classes than within-platform light traps (Figure 10). Euthynnus

alletteratus collected in within-platform light traps at ST 54 and GI 94 were generally taken in

two modes, including a mode around 7 to 13 mm that was absent in off-platform light traps. Size

distributions of Auxis spp. and Scomberomorus cavalla collected off-platform at GI 94 had

greater frequencies of larger individuals (>15 mm) than within-platform collections.

Larger Hypsoblennius invemar were found in off-platform light traps, as compared with

within-platform light traps (Figure 11). The size distributions of H. invemar in off-platform

collections had a median length of 12 mm at each platform, while in within-platform collections

the median was 10 mm, except at GI 94 where the median length was 7 mm for within-platform

collections.

Few reef taxa were collected in sufficient numbers to compare size distributions between

within- and off-platform light traps. No statistically significant size differences were found for

the blenniids Hypsoblennius hentz/ionthas and Scartella/Hypleurochilus at ST 54, VK 203 or GI

94, as was also the situation for Pomacentrus spp. at VK 203 and GI 94. Although they were not

collected in sufficient numbers to analyze statistically, the median lengths of Holocentrus spp.

were larger within-platform at MP 259 (13 vs. 10 mm) and GC 18 (31 vs. 22.5 mm). All other

reef taxa were collected at similar sizes between within- and off-platform light traps.

Comparisons of Within-, and 20 m and 50 m Off-Platform Light Trap Catches

On three occasions at VK 203, samples were collected using light traps deployed at three

distances relative to the platform: within the platform; 20 m down-current from the platform; and

50 m down-current from the platform. Total mean CPUEs were highest for the 50 m offplatform (19.00 ± 6.66) and 20 m off-platform light traps (17.10 ± 5.36), as compared with the

within-platform light trap (13.20 ± 2.25). The higher catch rates off-platform were mostly the

result of high CPUEs during the July 29 sampling effort, when large numbers of the blenniid

Hypsoblennius invemar were collected (Figure 12). When this species was removed from the

data, total mean CPUEs were significantly higher for the within-platform light trap (12.40 ±

2.23), as compared with the 20 m off-platform (6.40 ± 0.85) and 50 m off-platform light traps

(6.20 ± 1.36; Kruskal-Wallis χ

2

 = 7.29, p<0.03).
41

Figure 12. Mean total CPUEs (with standard errors) by sampling date from light

 traps deployed at three distances relative to the mid-shelf platform VK

 203: within the platform, 20 m down-current from the platform, and 50

 m down-current from the platform.

0

5

10

15

20

25

30

35

40

45

50

July 17 July 29 August 31

Date

Mean CPUE (#/10 min)

Within-platform

Off-platform 20 m

Off-platform 50 m
42

 Table 10. Mean CPUE (fish/10 min) for fish collected using light traps at three

 distances relative to the mid-shelf platform VK 203, with standard error (SE), rank,

 and percent of total catch (%N). RA= reef associated taxa, RD= reef dependent taxa.

Within-Platform

Light Trap

Off-Platform

Light Trap

(20 m)

Off-Platform

Light Trap

(50 m)

TAXA CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Anguilliformes

 Ophichthidae

 Ophichthus gomesi

 (shrimp eel)

0.20 (0.13)

14 (1.53) 0 (0) 0 (0)

Clupeiformes

 Clupeidae

 Harengula jaguana

 (scaled sardine)

1.10 (0.41)

5 (8.40)

0.10 (0.10)

14(0.57)

0.30 (0.15)

4.5 (1.57)

 Opisthonema oglinum

 (Atlantic thread herring)

2.50 (0.97)

1(19.08)

1.00 (0.54)

3 (5.68) 0 (0)

 Sardinella aurita

 (Spanish sardine)

0.10 (0.10)

19 (0.76) 0 (0) 0 (0)

 Engraulidae

 Unidentified 0.40 (0.22)

10 (3.05)

2.67 (0.51)

2 (16.48)

3.77 (1.03)

2 (20.42)

 Anchoa nasuta/hepsetus

 (longnose/striped anchovy)

1.70 (0.63)

2.5 (12.98)

0.30 (0.16)

5 (2.27) 0 (0)

 Anchoa mitchilli

 (bay anchovy)

0.40 (0.31)

10 (3.05)

0.10 (0.10)

14 (0.57) 0 (0)

 Anchoviella perfasciata

 (flat anchovy)

1.40 (0.62)

4 (10.69) 0 (0) 0 (0)

 Engraulis eurystole

 (silver anchovy)

0.70 (0.33)

6.5 (5.34) 0 (0) 0 (0)

Aulopiformes

 Synodontidae

 Saurida brasiliensis

 (largescale lizardfish)

1.70 (0.88)

2.5 (12.98)

0.07 (0.07)

19.5 (0.57)

0.07 (0.07)

13.5 (0.52)

 Synodus foetens

 (inshore lizardfish)

0.50 (0.17)

8 (3.82) 0 (0) 0 (0)

Gadiformes

 Bregmacerotidae

 Bregmaceros cantori

 (codlet) 0 (0) 0 (0) 0.07 (0.07)13.5 (0.52)

Scorpaeniformes

 Triglidae

 Unidentified

 (searobins)

0.10 (0.10)

19 (0.76) 0 (0) 0 (0)
43

 Table 10. (continued)

Within-Platform

Light Trap

Off-Platform

Light Trap

(20 m)

Off-Platform

Light Trap

(50 m)

TAXA CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Perciformes

 Carangidae

 Caranx crysos

 (blue runner) 0 (0) 0 (0) 0.10 (0.10)9.5 (0.52)

 Caranx latus

 (horse-eye jack) 0 (0) 0 (0) 0.10 (0.10)9.5 (0.52)

 Chloroscombrus chrysurus

 (Atlantic bumper)

0.10 (0.10)

19 (0.76)

0.20 (0.20)

6.5 (1.14)

0.90 (0.35)

3 (4.71)

 Decapterus punctatus

 (round scad)

0.40 (0.22)

10 (3.05)

0.10 (0.10)

14 (0.57) 0 (0)

 Lutjanidae (RA)

 Lutjanus spp. 0 (0) 0.70 (0.33)

4 (3.98) 0 (0)

 Lutjanus synagris

 (lane snapper) 0 (0) 0.10 (0.10)14 (0.57) 0 (0)

 Rhomboplites aurorubens

 (vermilion snapper)

0.10 (0.10)

19 (0.76) 0 (0) 0.10 (0.10)9.5 (0.52)

 Gerreidae

 Unidentified

 (mojarras) 0 (0) 0.10 (0.10)14 (0.57) 0 (0)

 Sciaenidae

 Cynoscion arenarius

 (sand seatrout)

0.10 (0.10)

19 (0.76) 0 (0) 0.10 (0.10)9.5 (0.52)

 Pomacentridae (RD)

 Pomacentrus spp.

 (damselfishes)

0.10 (0.10)

19 (0.76) 0 (0) 0.20 (0.13)6 (1.05)

 Sphyraenidae

 Sphyraena guachancho

 (guaguanche) 0 (0) 0.10 (0.10)14 (0.57) 0 (0)

 Blenniidae (RA)

 Unidentified 0 (0) 0.10 (0.10)

14 (0.57) 0 (0)

 Hypsoblennius hentz/ionthas

 (feather/freckled blenny)

0.10 (0.10)

19 (0.76) 0 (0) 0 (0)

 Hypsoblennius invemar

 (tesselated blenny)

0.70 (0.30)

6.5 (5.34)

10.70 (4.72)

1 (60.80)

12.70 (6.28)

1 (66.49)

 Gobiidae

 Unidentified

 (gobies)

0.20 (0.13)

14 (1.53)

0.13 (0.09)

9 (1.14)

0.10 (0.10)

9.5 (0.52)

 Scombridae

 Euthynnus alletteratus

 (little tunny)

0.20 (0.20)

14 (1.53)

0.20 (0.13)

6.5 (1.14) 0 (0)

 Scomberomorus maculatus

 (Spanish mackerel)

0.30 (0.21)

12 (2.29)

0.17 (0.11)

8 (1.14)

0.10 (0.10)

9.5 (0.52)
44

 Table 10. (continued)

Within-Platform

Light Trap

Off-Platform

Light Trap

(20 m)

Off-Platform

Light Trap

(50 m)

TAXA CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

CPUE (SE)

Rank (%N)

Pleuronectiformes

 Cynoglossidae

 Symphurus civitatus

 (offshore tonguefish) 0 (0) 0.10 (0.10)14 (0.57) 0 (0)

 Bothidae

 Syacium spp.

 (dusky/shoal flounder) 0 (0) 0.10 (0.10)14 (0.57)

0.30 (0.21)

4.5 (1.57)

Tetraodontiformes

 Tetraodontidae

 Unidentified

 (puffers) 0 (0) 0.07 (0.07)

19.5 (0.57) 0 (0)
45

Figure 13. Size distributions of Hypsoblennius invemar collected at VK 203 using light traps deployed

 at three distances relative to the platform: within the platform, 20 m down-current from the

 platform, and 50 m down-current from the platform.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Length (mm)

Frequency

Within-platform n=7

Off-platform 20 m n=107

Off-platform 50 m n=127
46

A total of 498 fishes, from 18 families, was collected from 30 light trap samples (10 for each

light trap distance; Table 10). Within-platform light traps collected a total of 19 taxa (identified

to at least the genus level), while the 20 m off-platform collected 15 taxa and the 50 m offplatform collected 12 taxa. Mean Shannon-Wiener diversity indices were significantly higher

for the within-platform light trap (1.49 ± 0.14) than the 20 m off-platform (0.67 ± 0.08) or the 50

m off-platform (0.56 ± 0.20; χ

2

 = 12.75, p<0.002). The within-platform catch was

predominantly composed of clupeiforms (63.4% of the total catch) and synodontids (16.8%),

while Hypsoblennius invemar, Decapterus punctatus and Scomberomorus cavalla were minor

components of the catch. The 20 m and 50 m off-platform catches were dominated by H.

invemar (60.8% and 66.5%, respectively), although preflexion engraulids, Lutjanus spp. and

Chloroscombrus chrysurus were also abundant.

Because of the low number of samples, few taxa were found in sufficient numbers to

compare size distributions across the three light trap distances. Kolmogorov-Smirnov tests of the

sizes of Hypsoblennius invemar found each size distribution significantly different from each

other (P<0.0005; Figure 13). Sizes of H. invemar within-platform were between 6 and 10 mm,

while in both 20 m and 50 m off-platform light traps the sizes were predominantly 11 to 12 mm.

Anchoa nasuta/hepsetus were larger in within-platform (median = 16 mm) than the 20 m offplatform light trap collections (6 mm). Although sample size was low, the sizes of Euthynnus

alletteratus collected in the 20 m off-platform light trap were >40 mm in length, while withinplatform individuals collected were 9 and 11 mm. The sizes of other taxa were generally similar

across light trap distances.

Comparisons of SEAMAP vs. Platform Collections

Reef-dependent taxa were relatively rare, as compared with reef-associated taxa, in both

platform plankton nets and SEAMAP bongo nets west of the Delta (Table 11). On the outer

shelf, reef-dependent families Pomacentridae, Labridae, Acanthuridae, and Scaridae were

present only in SEAMAP collections, while chaetodontids were only found in platform

collections. In contrast, reef-dependent taxa were only found in platform collections at mid-shelf

(i.e., Chaetodontidae, Pomacentridae and Labridae) and inner shelf (i.e., Scaridae), albeit in low

densities. Reef-associated taxa were taken in larger densities in platform collections at all shelf

locations with the exception of serranids, which had higher densities in SEAMAP collections.

East of the Delta, labrids and scarids were the predominant reef-dependent taxa collected, and

were more common in SEAMAP bongo net collections (Table 12). Of the reef-associated taxa,

lutjanids and serranids were found in higher densities in SEAMAP collections, while blenniids

had higher densities in platform collections.

Significantly different size distributions were found for serranids between SEAMAP

bongo net and platform plankton net collections, with slightly smaller median lengths in

SEAMAP collections (2 mm vs. 3; Figure 14). Lutjanids also displayed significantly different

size distributions, having once again slightly smaller median lengths in SEAMAP collections (2

mm vs. 4; Figure 15). Statistical comparisons between ST 54, GI 94 and their respective

SEAMAP samples were not possible for serranids and lutjanids due to limited numbers

collected. No significant differences were found between the size distributions of labrids

collected at MP 259 and nearby SEAMAP stations, as both distributions ranged from 1 to 6 mm.
Table 11. Mean abundance (fish/m2), with standard deviation (SD), for reef fish collected at selected nearby SEAMAP ichthyop

stations (oblique, 60-cm bongo tows) and at three oil and gas platforms (passively-fished, 60-cm plankton nets, surface and 

 across the continental shelf west of the Delta.

Inner Shelf Mid-shelf Outer 

SEAMAP† ST 54‡ SEAMAP† GI 94‡ SEAMAP† 

Mean (SD) Mean (SD) Mean (SD) Mean (SD) Mean (SD) 

Reef-dependent Chaetodontidae 0 (0) 0 (0) 0 (0) <0.1 (0.5) 0 (0)  Pomacanthidae 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)  Pomacentridae 0 (0) 0 (0) 0 (0) 1.1 (6.7) 1.3 (4.0)  Labridae 0 (0) 0 (0) 0 (0) <0.1 (0.8) 4.4 (7.2)  Acanthuridae 0 (0) 0 (0) 0 (0) 0 (0) 0.3 (1.5)  Scaridae 0 (0) 0.5 (1.3) 0 (0) 0 (0) 1.6 (3.7) Reef-associated Holocentridae 0 (0) 0 (0) 0 (0) 0.3 (1.7) 0.3 (1.2)  Serranidae 0.2 (0.7) 0 (0) 5.6 (5.0) 1.2 (3.4) 72.3 (83.2)  Lutjanidae 0 (0) 0 (0) 1.9 (5.4) 2.1 (6.7) 3.2 (5.1)  Blenniidae 0.7 (2.0) 1.2 (2.4) 3.2 (5.4) 17.6 (19.3) 0 (0) 

 †Means calculated for five outer shelf (n=21), two mid-shelf (n=8) and three inner shelf (n=12) SEAMAP sampling stations (1995-1 ‡Means calculated for paired (subsurface and surface), passive plankton net samples collected at GC 18 (n=14), GI 94 (n=161) and 

 1995-1997.

47
 Table 12. Mean abundance (fish/100 m3), with standard deviation (SD), for reef fish collected at selected nearby

SEAMAP ichthyoplankton sampling stations (oblique, 60-cm bongo tows) and at two oil and gas platforms

 (passively-fished, 60-cm surface plankton nets) across the continental shelf east of the Delta.

Mid-shelf Outer Shelf

SEAMAP† VK 203‡ SEAMAP† MP 259‡

Mean (SD) Mean (SD) Mean (SD) Mean (SD)

Reef-dependent Chaetodontidae 0 (0) 0 (0) 0 (0) 0 (0) Pomacanthidae 0 (0) 0 (0) 0 (0) 0 (0) Pomacentridae 0 (0) 0 (0) <0.1 (0.1) 0 (0) Labridae 3.9 (8.8) 0 (0) 2.2 (2.9) 0.4 (1.3) Acanthuridae 0 (0) 0 (0) 0 (0) 0 (0) Scaridae 0 (0) 0 (0) 0.5 (0.7) 0.1 (0.6)Reef-associated Holocentridae 0 (0) 0 (0) 0 (0) 0 (0) Serranidae 18.3 (28.3) 1.1 (2.5) 1.0 (0.9) 0.6 (1.6) Lutjanidae 3.5 (4.8) 1.8 (4.7) 2.6 (3.5) 0.4 (1.4) Blenniidae <0.1 (0.2) 4.8 (7.9) 0.1 (0.3) 2.1 (4.5)

 †Means calculated for three outer shelf (n=5) and three mid-shelf (n=9) SEAMAP sampling stations (1995-1997 ‡Means calculated for passive surface plankton net samples collected at MP 259 (n=70) and VK 203 (n=60),

 during 1999-2000.

48
49

Figure 14. Significantly different size distributions of Serranidae

 collected at platforms using passively-fished surface and

 subsurface plankton nets, and at selected nearby SEAMAP

 sampling stations using oblique bongo net tows. ‡ denotes

 platforms east of the Delta where only surface collections

 were taken. Number of samples (n) and the p-values from

 K-S tests are included.

0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6 7 8 9 10

Length (mm)

Frequency

VK203 n=17

SEAMAP n=64

VK203 p=0.05

0

0.2

0.4

0.6

0.8

1 2 3 4 5 6 7 8 9 10

Length (mm)

Frequency

MP259 n=13

SEAMAP n=11

MP259 p<0.001

0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5 6 7 8 9 10

Length (mm)

Frequency

GC18 n=67

SEAMAP n=171

GC18 p<0.001
50

Figure 15. Significantly different size distributions of Lutjanidae

 collected at platforms using passively-fished surface and

 subsurface plankton nets, and at selected nearby SEAMAP

 sampling stations using oblique bongo net tows. ‡ denotes

 platforms east of the Delta where only surface collections

 were taken. Number of samples (n) and the p-values from

 K-S tests are included.

0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Length (mm)

Frequency

VK203 n=21

SEAMAP n=26

VK203 p<0.001

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3 4 5 6 7 8 9 10

Length (mm)

Frequency

MP259 n=10

SEAMAP n=21

MP259 p=0.004

0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5 6 7 8 9 10

Length (mm)

Frequency

GC18 n=41

SEAMAP n=12

GC18 p=0.001
Table 13. Mean number (fish/sample), with standard deviation (SD), for reef fish collected at selected nearby SEAMAP ichthyopl

 stations (neuston tows) and at three platforms (light-traps, within- and off-platform) across the continental shelf west of th

Inner Shelf Mid-shelf Outer 

SEAMAP† ST 54‡ SEAMAP† GI 94‡ SEAMAP† 

Mean (SD) Mean (SD) Mean (SD) Mean (SD) Mean (SD) 

Reef-dependent Chaetodontidae 0 (0) 0 (0) 0.1 (0.4) 0 (0) 0 (0)  Pomacanthidae 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)  Pomacentridae 0 (0) <0.1 (0.4) 0 (0) 0.5 (1.9) 0.2 (1.0)  Labridae 0 (0) 0 (0) 0 (0) <0.1 (0.1) <0.1 (0.2)  Acanthuridae 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)  Scaridae 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) Reef-associated Holocentridae 0 (0) 0 (0) 0 (0) <0.1 (0.1) 0 (0)  Serranidae 0 (0) <0.1 (0.1) 0 (0) 0.1 (0.4) 1.7 (5.6)  Lutjanidae 0.3 (0.6) <0.1 (0.1) 0.9 (2.1) 0.1 (0.3) 0.2 (0.7)  Blenniidae 0.6 (1.0) 1.0 (3.5) 1.0 (1.4) 11.2 (31.1) 0.4 (1.6) 

 †Means calculated for five outer shelf (n=25), two mid-shelf (n=8) and three inner shelf (n=12) SEAMAP sampling stations (1995-1 ‡Means calculated for light-trap (surface and off-platform) samples collected at GC 18 (n=154), GI 94 (n=319) and ST 54 (n=146), 

51
 Table 14. Mean number (fish/sample), with standard deviation (SD), for reef fish collected at selected nearby

 SEAMAP ichthyoplankton sampling stations (neuston tows) and at two platforms (light traps, within- and

 off-platform) across the continental shelf east of the Delta.

Mid-shelf Outer Shelf

SEAMAP† VK 203‡ SEAMAP† MP 259‡

Mean (SD) Mean (SD) Mean (SD) Mean (SD)

Reef-dependent Chaetodontidae 0 (0) 0 (0) 0 (0) 0 (0) Pomacanthidae 0 (0) 0 (0) 0 (0) 0 (0) Pomacentridae 0.2 (0.4) 0.4 (1.0) 1.4 (3.2) <0.1 (0.3) Labridae 0.1 (0.3) 0 (0) 1.0 (2.2) 0 (0) Acanthuridae 0 (0) 0 (0) 0 (0) 0 (0) Scaridae 0 (0) 0 (0) 0 (0) 0 (0)Reef-associated Holocentridae 0 (0) 0 (0) 0.2 (0.5) 0.1 (0.4) Serranidae 0 (0) <0.1 (0.2) 2.0 (3.9) <0.1 (0.2) Lutjanidae 0 (0) 0.5 (1.5) 1.0 (1.4) <0.1 (0.1) Blenniidae 11.2 (33.7) 2.2 (4.9) 0 (0) 0.2 (1.1)

 †Means calculated for three outer shelf (n=5) and three mid-shelf (n=9) SEAMAP sampling stations (1995-1997 ‡Means calculated for light-trap (within- and off-platform) samples collected at MP 259 (n=126) and MP 259 (n

 during 1999-2000.

52
53

 Other reef taxa such as scarids, holocentrids and blenniids were collected at similar sizes

between platform and SEAMAP samples.

Pomacentrids and labrids were the most common reef-dependent taxa collected by either

SEAMAP neuston nets or platform light traps west of the Delta (Table 13). These taxa were

more abundant in SEAMAP collections on the outer shelf and in platform collections on the mid

and inner shelf, except for labrids which were absent on the inner shelf altogether. Of the reefassociated taxa only lutjanids were more abundant in SEAMAP collections than in platform

collections. Blenniids and serranids were more abundant at platforms at the mid and inner shelf

locations, but were more abundant at SEAMAP stations on the outer shelf. East of the Delta,

pomacentrids and labrids were the only reef-dependent taxa collected (Table 14). Pomacentrids

were more abundant in SEAMAP collections on the outer shelf, and in platform collections at

mid-shelf. Labrids were only found in SEAMAP collections at either location. Reef-associated

taxa were relatively rare on the outer shelf, and all but blenniids were more abundant in

SEAMAP collections. In contrast, at mid-shelf, blenniids were the only taxa more abundant in

SEAMAP neuston nets than in platform light traps.

No taxa were collected in sufficient numbers to statistically compare size distributions

between neuston nets and light traps. In general, neuston nets and light traps collected similar

size ranges, although light traps did collect more, larger individuals. For example, lengths of

serranids collected in light traps at MP 259 ranged from 14 to 20 mm, while in corresponding

SEAMAP neuston nets lengths ranged from 3 to 9 mm.

Seasonal Patterns of Plankton Net and Light Trap Catches

Mean total plankton net densities by trip at the outer shelf platform MP 259 ranged from

69-1706 fish/100 m3

 and peaked during late July (Figure 16). After clupeiforms were removed

the July peak remained, while total densities were less than 80 fish/100 m3 for all other sampling

dates. Mean total light trap CPUEs by trip ranged from 1-68 fish/10 min. for both within- and

off-platform collections and peaked during early July and late August (Figure 17). As with the

plankton nets, when clupeiforms were removed light trap CPUEs remained highest during July,

while CPUEs were low (<3 fish/10 min) on all other dates.

Mean total plankton net densities by trip at the mid-shelf platform VK 203 ranged from

67-1207 fish/100 m3

 and peaked at the end of August (Figure 18). When clupeiforms were

removed, the highest mean densities occurred during early October. Mean total light trap CPUEs

ranged from 3-62 fish/10 min. for both within- and off-platform collections and peaked during

June (Figure 19). Removing clupeiforms from the catches did little to change these patterns.

Lunar Periodicity

Mean total plankton net densities were higher, though not significantly, during full moons

than new moons at the outer shelf platform MP 259 (Figure 20). When clupeiforms were

removed, the mean density was only slightly higher during full moons. Mean total light trap

CPUEs were also higher during full moons, but when clupeiforms were removed CPUEs were

higher during new moons. At the mid-shelf platform VK 203, mean total plankton net densities
54

Figure 16. Mean plankton net densities (with standard errors) by sampling trip at the

 outer shelf platform MP 259 (1999) for data a) with clupeiforms and b)

 without clupeiforms included. Arrow above the bar points to the mean for

 that trip.

A) with clupeiforms 

0

100

200

300

400

500

600

700

800

900

1000

May 29-30

Jun 12-14Jun 26-28

Jul 10-12Jul 26-28

Aug 10-12Aug 24-26

Sep 7-9

Sep 24-26

Sampling Dates

Density (#/100 m3

) B) without clupeiforms

0

20

40

60

80

100

120

140

160

180

200

May 29-30

Jun 12-14Jun 26-28

Jul 10-12Jul 26-28

Aug 10-12Aug 24-26

Sep 7-9

Sep 24-26

Sampling Dates

Density (#/100 m3

)

1706.14
55

Figure 17. Mean light trap CPUEs (with standard errors) by sampling trip at the outer

 shelf platform MP 259 (1999) for data a) with clupeiforms and b) without

 clupeiforms included. NA = data not collected due to deployment

 problems. Arrow above the bar points to the mean for that gear.

A) with clupeiforms

0

5

10

15

20

25

30

35

40

45

50

May 29-30

Jun 12-14Jun 26-28

Jul 10-12Jul 26-28

Aug 10-12Aug 24-26

Sep 7-9

Sep 24-26

Sampling Dates

CPUE (#/10 min)

Off-platform

Light Trap

Within-platform

Light Trap

68.25

B) without clupeiforms

0

2

4

6

8

10

12

14

16

18

May 29-30

Jun 12-14Jun 26-28

Jul 10-12

Jul 26-28

Aug 10-12Aug 24-26

Sep 7-9

Sep 24-26

Sampling Dates

CPUE (#/10 min)

Off-platform Light Trap

Within-platform Light Trap

NA NA

N

NA NA
56

A) with clupeiforms

0

100

200

300

400

500

600

May 18-20

May 31-Jun 2

Jun 14-16

Jun 29-Jul 1

Jul 15-17

Jul 30-Aug 1

Aug 29-31

Oct 6-7

Sampling Dates

Density (#/100 m3

)

1206.98

B) without clupeiforms

0

100

200

300

400

500

600

May 18-20

May 31-Jun 2

Jun 14-16

Jun 29-Jul 1

Jul 15-17

Jul 30-Aug 1

Aug 29-31

Oct 6-7

Sampling Dates

Density (#/100 m3

)

Figure 18. Mean plankton net densities (with standard errors) by sampling trip at the

 mid-shelf platform VK 203 (2000) for data a) with clupeiforms and b)

 without clupeiforms included. Arrow above the bar points to the mean for

 that trip.
57

Figure 19. Mean light trap CPUEs (with standard errors) by sampling trip at the

 mid-shelf platform VK 203 (2000) for data a) with clupeiforms and b)

 without clupeiforms included

A) with clupeiforms

0

10

20

30

40

50

60

70

80

90

100

May 18-20

May 31-Jun 2

Jun 14-16

Jun 29-Jul 1

Jul 15-17

Jul 30-Aug 1

Aug 29-31

Oct 6-7

Sampling Dates

CPUE (#/10 min)

Off-platform LT

Within-platform LT

B) without clupeiforms

0

10

20

30

40

50

60

70

80

90

100

May 18-20

May 31-Jun 2

Jun 14-16

Jun 29-Jul 1

Jul 15-17

Jul 30-Aug 1

Aug 29-31

Oct 6-7

Sampling Dates

CPUE (#/10 min)

Off-platform LT

Within-platform LT
58

Figure 20. Mean plankton net densities and light trap CPUEs (with standard errors) for lunar phases sampled

 at MP 259 and VK 203 for data with clupeiforms (shaded bars) and without clupeiforms included

 (open bars). Asterisk denotes significant differences between lunar phases using the Wilcoxon 2-

 sample test.

MP 259

VK 203

Plankton Net

0

100

200

300

400

500

600

700

Full Moon New Moon

Moon Phase

Density (#/100 m3) 

Light Trap 

0

5

10

15

20

Full Moon New Moon

Moon Phase

CPUE (#/10 min)

Plankton Net

0

100

200

300

400

500

600

700

800

Full Moon New Moon

Moon Phase

Density (#/100 m3

)

Light Trap

0

5

10

15

20

25

Full Moon New Moon

Moon Phase

CPUE (#/10 min)

*
59

were significantly higher during new moons than full moons (Wilcoxon Z = -2.46, p<0.02;

Figure 20). When clupeiforms were removed, mean densities were still higher during new

moons, though not significantly. Mean total light trap CPUEs were relatively equal between full

and new moons, however, catch rates were slightly higher during full moons. When clupeiforms

were excluded, this difference was somewhat larger. 

Preflexion reef fish larvae were found in higher frequencies during either new or full

moons, depending on the taxa, while postflexion larvae were collected almost exclusively during

new moons (Table 15). Preflexion unidentified blenniids and Hypsoblennius hentz/ionthas had

significantly higher frequencies collected during full moons. Preflexion holocentrids and labrids

were also collected more frequently during full moons. In contrast, preflexion lutjanids,

serranids, and other blenniids such as Hypsoblennius invemar and Scartella/Hypleurochilus were

taken more frequently during new moons, while the pomacentrid Chromis spp. was only

collected on new moons. Postflexion blenniids (H. hentz/ionthas, H. invemar, Parablennius

marmoreus, and Scartella/Hypleurochilus), lutjanids (Lutjanus spp. and Rhomboplites

aurorubens), pomacentrids (Chromis spp. and Pomacentrus spp.), and holocentrids were

collected in significantly higher frequencies during new moons. Other postflexion taxa, such as

labrids, scarids, epinephelines and serranines, were generally low in number, but were still

collected in higher frequencies during new moons. Lutjanus campechanus was the only

postflexion taxon that was collected in significantly higher frequencies during full moons.

Among Trip and Night Variability in Plankton Net and Light Trap Catches

Variability among the mean plankton net and light trap catches per night was consistently

greater than among the mean plankton net and light trap catches per trip for each taxon at the

mid-shelf platform GI 94 (Table 16). The taxa with the highest coefficients of variation (CVs)

exhibited discrete pulses, spanning one to two nights, in their nightly mean densities or CPUEs.

These pulses accounted for a high percentage of the total number of a taxon collected by a gear.

For example, Auxis spp. and Cynoscion arenarius had among the highest CVs (both trip and

night) and exhibited large pulses that accounted for 68% and 52%, respectively, of their total

numbers collected by plankton nets (Figure 21). In contrast, plankton net-collected Serranidae

and Bregmaceros cantori did not exhibit as discrete a pulse, were generally more common

throughout the sampling season, and consequently had lower CVs. Light trap-collected

Hypsoblennius invemar and Pomacentridae had high CVs and exhibited noticeable pulses over

one to two nights that accounted for 43% and 65%, respectively, of their total catches. Saurida

brasiliensis and Lutjanidae had relatively low CVs and though peak catches were observed they

were not as distinct.

Coefficients of variation (CVs) for taxa collected at the mid-shelf platform VK 203 were

generally lower than at GI 94, and displayed smaller differences between CVs among nights and

among trips (Table 17). However, variability among nights remained slightly higher than the

variability among trips in nearly all situations, and in the few exceptions the CVs were nearly

equal. Despite this, distinct pulses were still evident. Large single pulses were observed for both

Saurida brasiliensis (73% of the total catch) and Synodus foetens (65%) collected in light traps.

As at GI 94, light trap-collected Hypsoblennius invemar at VK 203 exhibited a discrete pulse in

nightly mean CPUEs spanning one night, however, a second smaller pulse occurred two trips
60

 Table 15. Total numbers of pre- and postflexion reef larvae collected on new and full moons at all 

 platforms combined (except GC18 where sampling only occurred on new moons). Asterisk denotes 

 significant differences between moon phases using the chi-square goodness of fit test (p<0.01). 

Preflexion Postflexion

Reef Taxa Full Moon New Moon Full Moon New Moon

Blenniidae

 Unidentified 205 * 119 0 1

 Hypsoblennius hentz/ionthas 10 * 1 16 * 63

 Hypsoblennius invemar 1 12 32 * 734

 Parablennius marmroeus 3 0 21 * 622

 Scartella/Hypleurochilus 12 21 28 * 147

Holocentridae 8 1 1 * 15

Labridae 6 0 0 2

Lutjanidae

 Lutjanus campechanus 4 6 16 * 2

 Lutjanus spp. 15 26 1 * 22

 Pristipomoides aquilonaris 3 11 0

 Rhomboplites aurorubens 1 4 7 * 48

Pomacentridae

 Abudefduf saxatilis 0 00 9

 Chromis spp. 0 * 16 0 * 12

 Pomacentrus spp. 1 2 11 * 86

Scaridae 0 2 0 3

Serranidae

 Anthinae 7 12 0 1

 Epinephelinae 2 3 1 6

 Grammistinae 3 1 1 1

 Serraninae 11 16 1 7
61

Table 16. Coefficients of variation of the mean densities and CPUEs by sampling 

 trip and sampling night for the ten most abundant taxa taken by plankton nets 

 and light traps, and reef families collected at the mid-shelf platform GI 94. 

 Plankton Net Light Trap

Trip Night Trip Night

Dominant Taxa

Auxis spp. 237.7 347.5 206.7 245.4

Bregmaceros cantori 89.0 134.1 . .

Cynoscion arenarius 292.7 420.7 . .

Etropus crossotus 88.9 146.8 . .

Euthynnus alletteratus 171.4 207.2 151.3 191.8

Gobiidae 46.8 78.2 . .

Hypsoblennius invemar . . 202.3 245.1

Hypsoblennius hentz/ionthas . . 268.2 325.4

Parablennius marmoreus . . 135.8 164.0

Peprilus paru 322.2 457.0 . .

Saurida brasiliensis 144.6 150.6 91.2 115.4

Scomberomorus cavalla . . 207.1 258.7

Scomberomorus maculatus . . 162.2 214.8

Symphurus spp. 132.2 138.9 . .

Synodus foetens 228.2 243.7 205.7 224.1

Synodus poeyi . . 124.0 155.9

Reef Families

Lutjanidae 131.1 179.5 173.5 221.9

Pomacentridae 292.7 432.9 221.4 231.8

Serranidae 99.8 159.3 196.3 224.2
62

Figure 21. Mean plankton net densities and light trap CPUEs (with standard errors) by sampling night for

 fish taxa collected at GI 94. Longer tick marks on x-axis delimit sampling trips. Open circles

 represent full moons, and dark circles represent new moons. The second and fifth sampling trips

 occurred during first quarter moons, while the third sampling trip occurred during a last quarter

 moon.

B) Light Trap

Se rranidae

0

2

4

6

8

10

M e an De ns ity (#/100 m3)

Auxis s pp. 

0

20

40

60

80

100

120

M e an De ns ity (#/100 m3)

Br egm a ceros ca ntori

0

20

40

60

80

100

M e an De ns ity (#/100 m3)

Cynosci on a rena ri us

0

10

20

30

40

50

60

70

M e an De ns ity (#/100 m3)

Sampling Night Sampling Night

A) Plankton Net

Hypsoblennius invem a r

0

10

20

30

40

50

60

M e an CPUE (#/10 m in.)

Pom acentridae

0

1

2

3

4

5

M e an CPUE (#/10 m in.)

Lutjanidae

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

M e an CPUE (#/10 m in.)

Sa urida bra siliensis

0

2

4

6

8

10

12

14

16

M e an CPUE (#/10 m in.)

Sampling Night Sampling Night

B) Light Trap

Se rranidae

0

2

4

6

8

10

M e an De ns ity (#/100 m3)

Auxis s pp. 

0

20

40

60

80

100

120

M e an De ns ity (#/100 m3)

Br egm a ceros ca ntori

0

20

40

60

80

100

M e an De ns ity (#/100 m3)

Cynosci on a rena ri us

0

10

20

30

40

50

60

70

M e an De ns ity (#/100 m3)

Sampling Night Sampling Night

A) Plankton Net

Se rranidae

0

2

4

6

8

10

M e an De ns ity (#/100 m3)

Auxis s pp. 

0

20

40

60

80

100

120

M e an De ns ity (#/100 m3)

Br egm a ceros ca ntori

0

20

40

60

80

100

M e an De ns ity (#/100 m3)

Cynosci on a rena ri us

0

10

20

30

40

50

60

70

M e an De ns ity (#/100 m3)

Sampling Night Sampling Night

A) Plankton Net

Hypsoblennius invem a r

0

10

20

30

40

50

60

M e an CPUE (#/10 m in.)

Pom acentridae

0

1

2

3

4

5

M e an CPUE (#/10 m in.)

Lutjanidae

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

M e an CPUE (#/10 m in.)

Sa urida bra siliensis

0

2

4

6

8

10

12

14

16

M e an CPUE (#/10 m in.)

Sampling Night Sampling Night

Hypsoblennius invem a r

0

10

20

30

40

50

60

M e an CPUE (#/10 m in.)

Pom acentridae

0

1

2

3

4

5

M e an CPUE (#/10 m in.)

Lutjanidae

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

M e an CPUE (#/10 m in.)

Sa urida bra siliensis

0

2

4

6

8

10

12

14

16

M e an CPUE (#/10 m in.)

Sampling Night Sampling Night
63

Table 17. Coefficients of variation of the mean densities and CPUEs by sampling

 trip and sampling night for the ten most abundant taxa taken by plankton nets 

 and light traps, and reef families collected at the mid-shelf platform VK 203. 

 Plankton Net Light Trap

Trip Night Trip Night

Dominant Taxa

Auxis spp. . . 244.8 262.0

Bregmaceros cantori 202.1 228.2 . .

Chloroscombrus chrysurus 194.8 192.9 . .

Cynoscion arenarius 141.2 204.6 . .

Decapterus punctatus . . 127.4 145.2

Eucinostomus spp. . . 133.5 178.4

Euthynnus alletteratus . . 225.6 264.8

Gobiidae 134.7 168.0 114.4 127.4

Hypsoblennius invemar . . 165.3 207.7

Microdesmus lanceolatus 121.5 118.9 . .

Ophichthus spp. 178.9 176.1 . .

Parablennius marmoreus . . 125.9 133.3

Saurida brasiliensis 196.4 186.3 217.1 279.0

Scartella/Hypleurochilus 116.0 154.7 . .

Scomberomorus maculatus 179.0 247.4 129.1 183.3

Symphurus spp. 148.6 163.4 . .

Synodus foetens . . 169.1 243.1

Reef Families

Lutjanidae 123.2 160.4 103.4 152.8

Pomacentridae . . 179.3 166.9

Serranidae 92.2 115.0 151.2 222.6
64

Hypsoblennius invemar

0

2

4

6

8

10

12

14

16

18

Mean CPUE (#/10 m in.)

Pom acentridae

0

0.5

1

1.5

2

2.5

3

Mean CPUE (#/10 m in.)

Saurida brasiliensis

0

5

10

15

20

25

30

Mean CPUE (#/10 m in.)

Synodus foetens

0

5

10

15

20

Mean CPUE (#/10 m in.)

Sampling Night Sampling Night

Hypsoblennius invemar

0

2

4

6

8

10

12

14

16

18

Mean CPUE (#/10 m in.)

Pom acentridae

0

0.5

1

1.5

2

2.5

3

Mean CPUE (#/10 m in.)

Saurida brasiliensis

0

5

10

15

20

25

30

Mean CPUE (#/10 m in.)

Synodus foetens

0

5

10

15

20

Mean CPUE (#/10 m in.)

Sampling Night Sampling Night

Figure 22. Mean light trap CPUEs (with standard errors) by sampling night for fish taxa collected at

 VK 203. Longer tick marks on x-axis delimit sampling trips. Open circles represent full

 moons, and dark circles represent new moons. The last sampling trip occurred during a

 first quarter moon.
65

later (two pulses combined = 88% of the total catch; Figure 22). Although it was the only taxa

collected by light traps to have a CV higher among trips than among nights, Pomacentridae at

VK 203 still exhibited two discrete pulses (combined = 90% of the total catch).

Pulses were also observed to coincide among taxa, particularly in light trap collections.

At GI 94, the pulses of Hypsoblennius invemar and Pomacentridae (which itself consisted of

both Pomacentrus spp. and Chromis spp.) both occurred over the same one to two night span

during the fifth sampling trip (Figure 21). Similarly, at VK 203 H. invemar and Pomacentridae

had pulses that coincided during both the fourth and sixth sampling trips, which both occurred

during new moon phases (Figure 22). Furthermore, pulses of the synodontids Saurida

brasiliensis and Synodus foetens coincided during the third sampling trip. A similar multi-taxa

pulse occurred during the fourth sampling trip at MP 259, when 100% of the H. invemar and

Pomacentridae catch, and greater than 95% of the Holocentrus spp. and Synodus foetens total

catch was taken.

Within-Night Variability in Plankton Net and Light Trap Catches

Mean total plankton net and light trap catches by within-night time of capture (averaged

across all sampling trips at each platform) were considerably variable, and no consistent patterns

were observed among platforms (Figure 23). Peaks in mean plankton net densities ranged from

four hours after sunset at ST 54 to ten hours at GI 94. The within-night variability in plankton

net densities mostly reflected episodic catches of clupeiform larvae. When clupeiforms were

removed, variability was reduced and the mean densities were surprisingly uniform throughout

the night at each platform, with the exception of GC 18 where non-clupeiform densities peaked

during the middle of the night. There were no significant differences in mean densities by hours

sampled after sunset at any of the platforms. Light trap catches at ST 54 and MP 259 peaked

about 3-4 hours after sunset and gradually declined thereafter, while at other platforms CPUEs

peaked later in the night (VK 203: 5-6 hours; GI 94 and GC 18: 8 hours). As in plankton nets,

the within-night variability in light trap CPUEs was largely attributable to clupeiforms. There

were no significant differences in mean CPUEs by hour after sunset, except at GC 18 (χ

2

 =

45.68, p<0.0001, with clupeiforms; χ

2

 = 41.50, p<0.0001, without clupeiforms). Attempts were

made to estimate the within-night variability of the dominant taxa and reef families, however,

mean densities and CPUEs were extremely variable and no notable patterns were observed.

Length-Frequency and Developmental Stages of Reef Taxa

Blenniids were the most abundant reef-associated taxa collected at platforms, and were

represented by a wide range of sizes and developmental stages. At the inner shelf platform

ST 54, most of blenniids were postflexion and settlement size Hypsoblennius invemar, H.

hentz/ionthas and Scartella/Hypleurochilus between 9 and 13 mm (Figure 24). A large number

of recently-hatched and preflexion unidentified blenniids were also collected. The most common

blenniids collected at the mid-shelf platform VK 203 were postflexion/settlement size H.

invemar 10 to 12 mm in length. A wide range of Scartella/Hypleurochilus, from recentlyhatched to settlement size, were also collected at VK 203. Blenniids were most numerous at GI

94 and were dominated by H. invemar and Parablennius marmoreus, representing preflexion

larvae of 4 to 5 mm up to postflexion/settlement larvae of 23 mm. Flexion and postflexion H.
66

 Plankton Nets Light Traps

VK 203

0

100200300400500600

34567

Hours after sunset

Mean Density

(#/100 m

3

)

(18) (10) (17) (10) (3)

ST 54

0

500

100015002000

23456789

Hours after sunset

Mean Density

(#/100 m

3

)

with clupeiformswithout clupeiforms

(11) (18) (11) (7) (17) (11) (5) (2)

GI 94

0

100200300400

1 2 3 4 5 6 7 8 9 10

Hours after sunset

Mean Density

(#/100 m

3

)

(4) (44) (52) (43) (48) (39) (48) (36) (11) (4)

ST 54

0

20406080

234567

Hours after sunset

Mean CPUE 

(#/10 min)

with witho

(21) (28) (13) (21) (28) (22) 

19155.9

1613.1 VK 203

05

1015202530

23456

Hours after sunset

Mean CPUE

(#/10 min)

(8) (27) (23) (28) (30

GI 94

0

50

100150200

234567

Hours after sunset

Mean CPUE

(#/10 min)

(60) (72) (66) 

(71) (78) (57)

5285.6

521.6 478.7

Figure 23. Mean plankton net densities and light trap CPUEs (with standard errors) by number of hours after sunset tha

 were taken in surface waters at each platform, for data with and without clupeiforms included. Arrows abov point to the mean for that hour. Numbers in parentheses indicate the number of samples for each hour.
67

Figure 23. (continued)

 Plankton Nets Light Traps

MP 259

0

100200300400500

34567

Hours after sunset

Mean Density

(#/100m

3

)

with clupeiformswithout clupeiforms

(17) (14) (16) (19) (4)

GC 18

0

50

100150200250300

123456789

Hours after sunset

Mean Density

(#/100 m

3

)

(3) (18) (23) (22) (23) (13) (12) (7) (4)

MP 259

05

101520

3456

Hours after sunset

Mean CPUE 

(#/10 min)

with witho

(23) (31) (26) (35) 

GC 18

05

10152025

2 3 4 5 6 7 8 

Hours after sunset

Mean CPUE

(#/10 min)

(46) (51) (62) (54) (45) (27) (16) 
Figure 24. Size distributions of blenniids collected at each platform. Lines above the bars denote the size ranges of different ear

 based on published literature. n = total number of fishes measured.

68

ST 54

05

1015202530354045

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion Postflexion

Settlement 58 mm 58 mm

n =187

0

102030405060708090

100

1 2 3 4 5 6 7 8 9 10 11 12 13 1

Length (mm)

Number of individuals 

Preflexion

Hatch

Flexion

Postflexion

Settlement

GI 94

0

50

100150200250300350

1 3 5 7 9 11 13 15 17 19 21 23

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

58 mm58 mm

n =2737

05

10152025

1 2 3 4 5 6 7 8 9 10 11 12 

Length (mm)

Number of individuals 

Hatch

Preflexion

Flexion

Postflexion

Settlement

Hypsoblennius hentz/ionthas Hypsoblennius invemar Parablennius marmoreus Scartella/Hypleurochilus 
Figure 24. (continued)

69

GC 18

05

101520253035

1 2 3 4 5 6 7 8 9 10

 Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

58 mm58 mm

31 32

n =41

Ophioblennius atlanticus Hypsoblennius invemar Unidentified
Figure 25. Size distributions of lutjanids collected at each platform. Lines above the bars denote the size ranges of different earl

 based on published literature. n = total number of fishes measured.

70

ST 54

0123

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

40 mm

n =12

GI 94

02468

101214161820

1 3 5 7 9 11 13 15 17 19 21 23 25

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement 40 mm

n =90

0123456

1 2 3 4 5 6 7 8 9 10 11 12 

Length (mm)

Number of individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

02468

101214

1 3 5 7 9 11 13 15 17 19 21 

Length (mm)

Number of individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

Lutjanus campechanus Lutjanus spp. Pristipomoides aquilonaris Rhomboplites aurorubens 
Figure 25. (continued)

71

GC 18

02468

1012141618

1 3 5 7 9 11 13 15 17 19

Length (mm)

Number of Individuals

40

Hatch

Preflexion

Flexion

Postflexion

Settlement

n =46

40 mm

Lutjanus spp. Pristipomoides aquilonaris
Figure 26. Size distributions of serranids collected at each platform. Lines above the bars denote the size ranges of different earl

 based on published literature. n = total number of fishes measured.

72

ST 54

01234

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

31 mm

17 mm

n =8

0123456789

1 3 5 7 9 11 13 15 

Length (mm)

Number of individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

GI 94

02468

10121416

1 3 5 7 9 11 13 15 17 19 21

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion Postflexion

Settlement 31 mm

n =63

01234567

1 3 5 7 9 11 13 15 

Length (mm)

Number of individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

Anthinae Epinephelinae Grammistinae Serraninae 
Figure 26. (continued)

73

GC 18

05

10152025303540

1 2 3 4 5 6 7 8 9 10 11 12

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

17 mm31 mm

n =73

Anthinae Epinephelinae Grammistinae Unidentified
74

Figure 27. Size distributions of holocentrids collected at each platform. Lines above the bars denote the size ranges of different 

 stages based on published literature. n = total number of fishes measured.

GI 94

01234

1 2 3 4 5 6 7 8 9 10

Length (mm)

Number of Individuals

Preflexion

Flexion

Postflexion

Rhynchichthys

Settlement

15.9 mm66 mm26- 66 mm

36

n =12

0123456

1 3 5 7 9 11 13 15 

Length (mm)

Number of individuals

Preflexion

Flexion

Postflexion

Rhynchichthys 

GC 18

0123

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37

Length (mm)

Number of Individuals

Preflexion

Flexion

Postflexion

Rhynchichthys 66 mm

Settlement 66 mm

n =25
75

Figure 28. Size distributions of pomacentrids collected at each platform. Lines above the bars denote the size ranges of different  stages based on published literature. n = total number of fishes measured.

05

10152025

1 3 5 7 9 11 13 15 

Length (mm)

Number of individuals

HatchPreflexion

FlexionPostflexion

Settlement

GI 94

05

1015202530354045

1 3 5 7 9 11 13 15 17 19 21 23 25

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

n =181

012345

1 2 3 4 5 6 7 8 9 10 11 12 

Length (mm)

Number of individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

ST 54

01234

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

n =7

Pomacentrus spp. Chromis spp. Abudefduf spp. 
Figure 28. (continued)

76

GC 18

02468

101214

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Length (mm)

Number of Individuals

Hatch

Preflexion

Flexion

Postflexion

Settlement

n =21

Pomacentrus spp.
Figure 29. Size distributions of labrids collected at MP 259 and scarids collected at GC 18. Lines above the bars denote the size 

 early life history stages based on published literature. n = total number of fishes measured.

77

05

10152025

1 2 3 4 5 6 7 

Length (mm)

Number of Indivivduals

Hatch Flexion

S

Labridae at MP 259

0123

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Length (mm)

Number of individuals

Hatch

Flexion

Postflexion

Settlement

n =8

21 mm

25 mm
78

hentz/ionthas, and recently-hatched unidentified blenniids were also common at GI 94. Fewer

numbers of blenniids were taken at the outer shelf platforms, MP 259 and GC 18, and these were

predominantly recently-hatched unidentified blenniids. However, a small number of postflexion

and settlement size H. invemar were taken at MP 259, and two postflexion/settlement

Ophioblennius atlanticus were taken at GC 18.

Lutjanids were also collected in a wide range of sizes and developmental stages;

however, most were preflexion and flexion larvae <7 mm in length (Figure 25). Very few

lutjanids (n = 12) were collected at the inner shelf platform ST 54, and most were settlement size

Rhomboplites aurorubens between 19 to 29 mm in length. Settlement size Lutjanus spp. (12-14

mm in length) and R. aurorubens (20-26 mm) were common at the mid-shelf platform VK 203.

A relatively large number of pre- to postflexion Lutjanus spp. and L. campechanus were also

collected at VK 203. Lutjanids at GI 94 were represented by pre- to post flexion Lutjanus spp.

and L. campechanus, and two size groups of R. aurorubens (3-12 mm pre- to postflexion larvae

and 20-25 mm settlement size juveniles). Most of lutjanids collected at the outer shelf platforms,

MP 259 and GC 18, were pre- to postflexion Pristipomoides aquilonaris; however, a small

number of settlement size Lutjanus spp. and a 40-mm P. aquilonaris were collected at GC 18.

Serranids collected at platforms were almost exclusively pre- to postflexion larvae

between 2 to 6 mm in length (Figure 26). Larger settlement size serranids were rare and

included: single specimens of Serraninae at ST 54 and VK 203 (15 and 12 mm in length,

respectively), a number of serranines and epinephelines at GI 94 and MP 259 (9-21 mm), and a

single anthiine collected at GC 18 (12 mm).

While relatively few in number, holocentrids collected at platforms were represented by

several developmental stages (Figure 27). At the mid-shelf platform GI 94, most holocentrids

were preflexion to postflexion larvae between 3 and 5 mm in length, however three

postflexion/rhynchichthys larvae were also collected. The holocentrids collected at the outer

shelf platform MP 259 were generally postflexion/rhynchichthys larvae. At GC 18, two groups

of holocentrids were collected: preflexion to postflexion larvae 2 to 9 mm in length, and

rhynchichthys/settlement size larvae 20 to 37 mm in length.

Pomacentrids were the most abundant reef-dependent taxa collected at platforms, and

were predominantly collected as settlement size juveniles 7 to 15 mm in length (Figure 28).

Most of these juveniles were Pomacentrus spp., however, a small number of Abudefduf spp.

were collected at ST 54 and GI 94, and Chromis spp. were common at GI 94. Preflexion to

postflexion Chromis spp. and Pomacentrus spp. larvae were also abundant at GI 94.

Larvae of other reef-dependent taxa were relatively rare at platforms, and of those

collected most were recently-hatched to postflexion larvae. A small number of labrids were

collected at MP 259, ranging from 1 to 6 mm in length (Figure 29). Scarids were most numerous

at the GC 18, and were represented by preflexion to settlement size larvae (Figure 29). Four

scarids 5 to 6 mm in length were also taken at MP 259. The only other reef-dependent taxa

collected at platforms were single 4-mm chaetodontids collected at GI 94 and GC 18.
79

Environmental Variables and Larval and Juvenile Fish Abundances

Salinity, temperature, turbidity, and microzooplankton biomass all were influential in

describing trends in the abundances of the top 15 dominant taxa and reef fish families at the

outer shelf platform MP 259. For plankton net collections, densities of Auxis spp., Euthynnus

alletteratus and Symphurus spp. were positively associated with the first environmental

canonical variate, which was primarily influenced by temperature (Table 18). Bregmaceros

cantori and Myctophidae were negatively associated with the first environmental variate.

Densities of Scomberomorus maculatus were positively associated with the second

environmental variate, which was positively influenced by turbidity and microzooplankton

biomass. Myctophidae and Serranidae densities were positively associated with the third

environmental variate, which was positively correlated with salinity and negatively correlated

with temperature and turbidity. Within light trap collections, Decapterus punctatus, Holocentrus

spp., Hypsoblennius invemar, Pomacentrus spp., and Synodus foetens were positively associated

with the first environmental variate, which was positively correlated with salinity and negatively

correlated with turbidity and microzooplankton biomass (Table 19). Chloroscombrus chrysurus

was negatively associated with the second environmental variate, which was positively

correlated with salinity and negatively correlated with temperature.

Salinity and microzooplankton biomass were the most influential environmental variables

in describing trends in larval and juvenile fish abundance at the mid-shelf platform VK 203. In

plankton net collections, densities of Microdesmus lanceolatus and Sphyraena borealis were

negatively correlated with the first environmental canonical variate, which was positively

influenced by salinity and microzooplankton biomass (Table 20). Etropus crossotus and

Serranidae were positively associated and Scartella/Hypleurochilus was negatively associated

with the second environmental, which was positively correlated with microzooplankton biomass

and negatively correlated with temperature. Bregmaceros cantori, E. crossotus, Saurida

brasiliensis, Scomberomorus maculatus, and Symphurus spp. were positively associated with the

third environmental variate, which was positively correlated with suspended solids and

microzooplankton biomass, and negatively correlated with salinity. For light trap collections,

Auxis spp., Caranx crysos and Euthynnus alletteratus were positively associated, and

Hypsoblennius invemar and Pomacentrus spp. were negatively associated with the first

environmental variate, which was positively correlated with salinity (Table 21). Scomberomorus

maculatus was positively associated, and H. invemar, Pomacentrus spp. and Lutjanidae were

negatively associated with the second environmental variate, which was positively correlated

with microzooplankton biomass.

Relationship Between Water Current Speed and Light Trap CPUE

Mean light trap CPUEs, from MP 259 and VK 203 combined, decreased with increasing

water current speed (Figure 30). Highest mean light trap catches (>20 fish/10 min.) were at

current speeds less than 30 cm s-1, with or without clupeiforms included. CPUEs at current

speeds above 30 cm s-1 were consistently less than 10 fish/10 min., although this observation is

somewhat limited by the low number of samples at these current speeds.
80

Table 18. Results of a canonical correlation analysis on log-transformed plankton net densities

(15 most abundant taxa, and reef fish families) and environmental variables for MP 259. Loadings 

in bold under statistically significant canonical variables V1, V2 and V3 explain at least 15% of the

variation for that taxon. Loadings in bold under the environmental canonical variates W1, W2 and 

 W3 indicate the most influential environmental variables. RA = reef associated taxa, RD = reef 

 dependent taxa.

Canonical Correlation Likelihood Ratio Approximate F Pr > F

1) 0.774921 0.08197861 1.8958 0.0002

2) 0.680626 0.20520408 1.6117 0.0117

3) 0.671726 0.38230996 1.5270 0.0514

Taxa Correlations between plankton net densities and their

 canonical variates

V1 V2 V3

Auxis spp. 0.5656 0.0427 0.2794

Bothus spp. -0.2224 0.0555 0.2860

Bregmaceros cantori -0.5841 0.0437 0.3801

Caranx crysos 0.2056 -0.2525 -0.0471

Chloroscombrus chrysurus 0.0132 -0.2931 -0.1435

Cubiceps pauciradiatus 0.1655 0.0914 0.2145

Euthynnus alletteratus 0.5891 0.1888 0.2196

Gobiidae -0.0724 -0.0855 0.0680

Labridae (RD) -0.2339 -0.1597 0.0736

Lutjanidae (RA) -0.2405 -0.1188 0.0540

Microdesmus lanceolatus 0.0701 0.3701 0.2202

Myctophidae -0.5082 -0.0136 0.4812

Saurida brasiliensis 0.0517 -0.3211 0.2782

Scaridae (RD) 0.1807 0.2114 0.0040

Scomberomorus cavalla 0.0777 0.0855 0.2661

Scomberomorus maculatus -0.1688 0.5995 -0.0356

Seriola spp. -0.1831 0.1808 0.1269

Serranidae (RA) -0.2921 -0.2370 0.4581

Symphurus spp. 0.4941 -0.0633 0.0672

Environmental Variables Correlations between environmental variables and their

 canonical variates

W1 W2 W3

Surface Salinity 0.0801 -0.3369 0.9343

Surface Temperature 0.5709 -0.1679 -0.7915

Surface Turbidity -0.0621 0.8492 -0.4348

Microzooplankton Biomass 0.3781 0.6226 -0.1321
81

Table 19. Results of a canonical correlation analysis on log-transformed light trap CPUEs (15 most

 abundant taxa) and environmental variables for MP 259. Loadings in bold under statistically 

 significant canonical variables V1 and V2 explain at least 15% of the variation for that taxon. 

 Loadings in bold under the environmental canonical variates W1 and W2 indicate the most

 influential environmental variables. RA= reef associated taxa, RD = reef dependent taxa.

Canonical Correlation Likelihood Ratio Approximate F Pr > F

1) 0.575643 0.34934089 1.6434 0.0013

2) 0.507755 0.52246836 1.4006 0.0422

Taxa Correlations between light trap CPUEs 

 and their canonical variates

V1 V2

Auxis spp. -0.0518 0.3702

Bregmaceros cantori 0.0312 0.2488

Caranx crysos 0.2316 -0.0651

Chloroscombrus chrysurus -0.0856 -0.6214

Decapterus punctatus 0.4450 0.0627

Epinephelinae (RA) 0.1394 -0.1870

Euthynnus alletteratus 0.3537 -0.2673

Gobiidae -0.0518 -0.2595

Holocentrus spp. (RA) 0.6577 0.0678

Hypsoblennius invemar (RA) 0.4158 0.0321

Myctophidae -0.1813 0.2465

Pomacentrusspp. (RD) 0.3872 0.0554

Saurida brasiliensis 0.0906 -0.2457

Scomberomorus maculatus -0.0967 0.1340

Synodus foetens 0.4239 0.0134

Environmental Variables Correlations between environmental 

 variables and their canonical variates

W1 W2

Surface Salinity 0.6789 0.6435

Surface Temperature -0.0958 -0.9602

Surface Turbidity -0.6706 -0.0270

Microzooplankton Biomass -0.4754 -0.2961
82

Table 20. Results of the canonical correlation analysis on log-transformed plankton net densities

(15 most abundant taxa, and reef fish families) and environmental variables for VK 203. Loadings 

in bold under statistically significant canonical variables V1, V2 and V3 explain at least 15% of the

 variation for that taxon. Loadings in bold under the environmental canonical variates W1, W2 

 and W3 indicate the most influential environmental variables. RA = reef associated taxa.

Canonical Correlation Likelihood Ratio Approximate F Pr > F

1) 0.819470 0.03894454 3.1261 0.0001

2) 0.777732 0.11856371 2.7905 0.0001

3) 0.753257 0.30005993 2.4177 0.0011

Taxa Correlations between plankton net densities and their

 canonical variates

V1 V2 V3

Bregmaceros cantori -0.0151 0.3816 0.4228

Caranx crysos 0.2905 -0.0590 -0.2477

Chloroscombrus chrysurus -0.2691 0.3119 0.2830

Cynoscion arenarius 0.0670 0.0950 0.0020

Etropus crossotus 0.0170 0.3880 0.4254

Gobiidae -0.2605 0.0293 0.2109

Lutjanidae (RA) -0.3487 -0.1906 0.0367

Microdesmus lanceolatus -0.4152 0.1520 0.1602

Micropogonias undulatus 0.3102 0.2026 -0.2157

Ophichthus spp. -0.3290 0.3397 0.1944

Saurida brasiliensis -0.1018 0.3291 0.4753

Scartella/Hypleurochilus (RA) -0.0819 -0.3553 0.1492

Scomberomorus maculatus 0.0790 -0.2379 0.3922

Serranidae (RA) 0.1269 0.4456 0.2980

Sphyraena borealis -0.6769 0.0186 0.1416

Symphurus spp. 0.0996 0.1954 0.7694

Environmental Variables Correlations between environmental variables and their

 canonical variates

W1 W2 W3

Surface Salinity 0.8923 -0.0707 -0.4225

Surface Temperature -0.1522 -0.7550 0.2641

Suspended Solids 0.2617 -0.1356 0.6124

Microzooplankton Biomass 0.4352 0.5929 0.6644
83

Table 21. Results of the canonical correlation analysis on log-transformed light trap CPUEs (15 most 

 abundant taxa, and reef fish families) and environmental variables for VK 203. Loadings in bold 

 under statistically significant canonical variables V1 and V2 explain at least 15% of the variation 

 for that taxon. Loadings in bold under the environmental canonical variates W1 and W2 indicate

 the most influential environmental variables. RA = reef associated taxa, RD = reef dependent taxa.

Canonical Correlation Likelihood Ratio Approximate F Pr > F

1) 0.773259 0.19717552 2.6970 0.0001

2) 0.518727 0.49039947 1.5354 0.0161

Taxa Correlations between light trap CPUEs 

 and their canonical variates

V1 V2

Auxis spp. 0.6754 -0.1477

Bregmaceros cantori 0.0468 0.3345

Caranx crysos 0.5244 -0.2060

Decapterus punctatus -0.2085 0.0656

Eucinostomus spp. -0.1354 0.0685

Euthynnus alletteratus 0.5372 -0.1012

Gobiidae -0.0533 -0.0639

Hypsoblennius hentz/ionthas (RA) 0.3618 -0.1968

Hypsoblennius invemar (RA) -0.4034 -0.5151

Lutjanidae (RA) 0.2125 -0.4366

Parablennius marmoreus (RA) 0.3097 0.1019

Pomacentrusspp. (RD) -0.6707 -0.4938

Saurida brasiliensis 0.3361 -0.1655

Scartella/Hypleurochilus (RA) 0.0976 0.1729

Scomberomorus maculatus 0.0114 0.5530

Serranidae (RA) -0.0705 0.1852

Synodus foetens 0.2703 0.0248

Environmental Variables Correlations between environmental 

 variables and their canonical variates

W1 W2

Surface Salinity 0.9937 -0.0675

Surface Temperature -0.3390 -0.3715

Suspended Solids 0.1844 0.2321

Microzooplankton Biomass 0.1147 0.9700
84

Figure 30. Scatter plots of water current speed versus mean CPUE from light traps at MP

 259 and VK 203 combined, for data a) with clupeiforms and b) without

 clupeiforms included.

0

10

20

30

40

50

60

70

0 20 40 60 80 100

Mean water current speed (cm/s)

Mean CPUE (#/10 min.)

0

10

20

30

40

50

60

70

0 20 40 60 80 100

Mean water current speed (cm/s)

Mean CPUE (#/10 min.)

B) without clupeiforms

A) with clupeiforms
Figure 31. Size distributions of fishes, with clupeiforms included, collected in light traps from MP 259 and VK 2

 at different water current speeds. Number of samples (n) and median lengths are included. Arrow abo

 points to the frequency for that length.

85

Current = 0-9 cm/s

n= 1277 median= 12 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 10-19 cm/s

n= 417 median= 18 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 20-29 cm/s

n= 1134 median= 17 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 30-39 cm/s

n= 138 median= 8 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

Current = 40-49 cm/s

n= 65 median= 4 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

Current >49 cm/s

n= 26 median= 4 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

0.35
Figure 32. Size distributions of fishes, without clupeiforms included, collected in light traps, from MP 259 and V

 combined, at different water current speeds. Number of samples (n) and median lengths are included. 

 the bar points to the frequency for that length.

86

Current = 0-9 cm/s

n= 788 median= 10 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 10-19 cm/s

n= 146 median= 10 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 20-29 cm/s

n= 461 median= 12 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 34 37 40

Length

Frequency

Current = 30-39 cm/s

n= 62 median= 7 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

Current = 40-49 cm/s

n= 50 median= 5 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

Current >49 cm/s

n= 21 median= 3 mm

0

0.05

0.1

0.15

0.2

0.25

1 4 7 10 13 16 19 22 25 28 31 

Length

Frequency

0.43
87

Size distributions of fishes, with and without clupeiforms included, collected by light traps at MP

259 and VK 203 combined were significantly different (p<0.05) between all current speed

intervals, except the two highest (i.e., 40-49 vs. >49 cm s-1; Figures 31 and 32). At the highest

current speeds the median lengths of fishes collected were below 5 mm, while at current speeds

<30 cm s-1 the median lengths were 12-17 mm, with clupeiforms included, and 10-12 mm,

without clupeiforms, and a wide range of sizes were collected.

Discussion

Across- and Along-Shelf Larval and Juvenile Fish Assemblages

Patterns of larval and juvenile fish abundance and diversity were primarily influenced by

across-shelf gradients of increasing water depth. Densities of larval fishes from plankton nets

were highest at the inner shelf platform ST 54 due mostly to high numbers of clupeiform larvae,

which comprised 97% of the fish collected at this site (Figure 4a). However, when clupeiforms

were removed densities remained highest at the shallowest platforms (Figure 4b). High numbers

of ichthyoplankton are typical of coastal waters, as compared with offshore waters (Govoni et al.

1989; Grimes and Finucane 1991; Sabatés and Olivar 1996). Catch rates of larger postlarval and

juvenile fishes showed a different pattern with highest catches at mid-shelf (Figure 5). Diversity

and taxonomic richness also exhibited a peak at intermediate depths (Table 4). High diversity at

intermediate depths may be the result of an overlap in the distributions of coastal and

oceanic/tropical taxa, as observed for both adult demersal and platform fish assemblages

(Sonnier et al. 1976; Gallaway 1981; Stanley and Wilson 2000). The inshore distribution of

oceanic and tropical taxa may be limited by seasonal fluctuations in the nearshore environment,

while the more oligotrophic conditions of the outer shelf may selectively remove coastal taxa

that require higher prey concentrations.

The species composition of the non-clupeiform larval and juvenile fish assemblages

collected at platforms also seemed to be structured by depth. Cluster analysis found three

general assemblages: an inner shelf assemblage (ST 54), a mid-shelf assemblage (GI 94 and VK

203) and an outer shelf assemblage (GC 18 and MP 259; Figure 6). The inner shelf assemblage

was dominated by Cynoscion arenarius, Synodus foetens and Scomberomorus maculatus (Table

5), and included high numbers of other typical coastal taxa, such as Etropus crossotus,

Menticirrhus spp., Peprilus burti and P. paru. The mid-shelf assemblages were characterized by

blenniids (particularly Hypsoblennius invemar), synodontids (particularly Saurida brasiliensis)

and the bregmacerotid Bregmaceros cantori (Table 5). Unlike the other assemblages, which

were dominated by demersal taxa, the outer shelf assemblages were primarily composed of

pelagic (Ariomma spp., Auxis spp., Caranx crysos, and Euthynnus alletteratus) and mesopelagic

taxa (Cyclothone braueri and Myctophidae; Tables 2 and 5).

These assemblages were similar to those found for across-shelf larval fish assemblages

collected elsewhere (Houde et al. 1979; Sabatés and Olivar 1996; Thorrold and Williams 1996).

In each of these studies there were strong differences between the larval fish assemblages inshore

and those offshore. Several factors have been suggested to determine these large-scale

distributions of larval fishes including: spawning behavior of adults, hydrographic conditions and

differential mortality (Leis 1991). Because they generally agree with those found for adult
88

populations (Houde 1981; Robins et al. 1986; Hoese and Moore 1998), the larval and juvenile

fish distributions observed probably reflected the spawning behavior of adult fishes from nearby

benthic, pelagic, and platform habitats. However, hydrographic conditions, such as the

occasional offshore intrusion of low-salinity water, can alter larval fish assemblages by

transporting coastal larvae to outer shelf regions (e.g., Sciaenops ocellatus and C. arenarius at

GC 18; Hernandez et al. 2001).

Across-shelf changes in abundance were also observed for the reef fish larvae collected at

platforms. On the inner shelf, blenniids (particularly Scartella/Hypleurochilus and

Hypsoblennius hentz/ionthas) were the dominant reef taxa collected (Table 5). Other reef taxa

were relatively rare (<10 individuals collected) and consisted of Lutjanus campechanus,

Rhomboplites aurorubens and Serraninae. Reef taxa were most abundant at mid-shelf, and

assemblages at these platforms included a high diversity of blenniids (Hypsoblennius invemar,

H. hentz/ionthas, Scartella/Hypleurochilus and Parablennius marmoreus), lutjanids (Lutjanus

spp., L. campechanus, and R. aurorubens), serranids (primarily Diplectrum spp., Serranus spp.,

and unidentified serranines) and pomacentrids (Abudefduf spp., Chromis spp. and Pomacentrus

spp.). On the outer shelf, blenniids were not as abundant and reef taxa common at the shallower

platforms were replaced by different taxa (i.e., Pristipomoides aquilonaris was the most

abundant lutjanid, while anthiines and epinephelines were the most abundant serranids). Scarid,

labrid and holocentrids larvae were also most common on the outer shelf. Higher abundance and

diversity of larval and juvenile reef fishes at mid-shelf may be attributed to the high platform

concentration in these areas (Tolan 2001). Because recruitment may be dependent on supply

from nearby upstream reefs (Sale 1980; Doherty 1991), the proximity of the mid-shelf platforms

to a greater number of potential sources (e.g., platforms) may favor a wider variety of taxa.

Similar transitions were observed in the adult reef fish assemblages at platforms in the

northern Gulf. In the faunal assemblages described by Gallaway (1981), relatively low numbers

of lutjanids and serranids characterized shallow platforms (<20 m), while a diverse assemblage

of blenniids, lutjanids, pomacentrids and serranids were found at mid-depth (20-60 m) and

chaetodontids, labrids, scarids, pomacentrids, and anthiine and epinepheline serranids were

common at the outer shelf platforms (>60 m). From visual surveys at the platforms west of the

Delta (GC 18, GI 94 and ST 54), lutjanids and serranids were most abundant and diverse at the

mid-shelf platform, while reef taxa at the outer and inner shelf were dominated by Paranthias

furcifer, and Lutjanus campechanus and L. griseus, respectively (Stanley and Wilson 2000).

The higher abundances of specific larvae in collections east or west of the Delta may be

attributed to the proximity of the platforms to different habitats in the northeastern and

northwestern Gulf. Larvae more abundant east of the Delta included: Decapterus punctatus,

Eucinostomus spp., Lutjanus spp., Microdesmus lanceolatus, Ophichthidae, and Thunnus spp.

Adults of several of these taxa are associated with habitats that are more common east of the

Delta. For example, Eucinostomus spp. and certain species of Ophichthidae are among the

dominant fishes in the pink shrimp-ground fish assemblage, which is associated with calcareous

sediments of the northeastern Gulf (Chittenden and McEachern 1976, Robins et al. 1986).

Furthermore, juvenile lutjanids (particularly Lutjanus griseus and L. synagris) are common in

seagrass beds and mangroves, habitats that are also more common in the eastern Gulf (Patillo et

al. 1997). Larvae more abundant west of the Delta included: Caranx hippos/latus, Cynoscion
89

arenarius, Euthynnus alletteratus, Peprilus burti, Scomberomorus cavalla, and Symphurus spp.

(Table 6). Populations of many of these species, particularly highly predaceous and fast growing

taxa such as C. hippos/latus, E. alletteratus and S. cavalla, may be enhanced by the greater

biological productivity associated with the Mississippi River discharge, which predominantly is a

westerly flow from the Delta (Grimes and Kingsford 1996; Grimes 2001). Furthermore, the

more estuarine conditions and softer sediments west of the Delta may provide more suitable

habitat for adult C. arenarius and Symphurus spp. (Parker 1960; Hoese and Moore 1998).

Despite the more favorable hydrographic conditions and the availability of more suitable

habitat in the northeastern Gulf (Briggs 1958; Smith 1976), differences in the abundance of reefdependent larvae (e.g., pomacentrids, labrids and scarids) were not observed in collections east

or west of the Delta. Juvenile and adult reef fish assemblages observed at platforms west of the

Delta have been found to be more similar to shallow-water natural reef communities, which are

more predominant in the northeastern Gulf, than they were to geographically-closer deep-water

natural reef communities (Tolan 2001). Platforms, therefore, may facilitate the range-extension

of shallow water reef taxa by providing shallow-water, hard substrate regardless of the water

depth or location on the continental shelf (Dennis and Bright 1988; Tolan 2001).

Within- and Off-Platform Distribution of Larval and Juvenile Fishes

Greater numbers and higher diversity of postlarval and juvenile fishes were collected

within the platform structure than in open waters up to 20 m away from the platform (Figure 7;

Table 7). Similar results were found when within-platform light trap catches were compared to

20-m and 50-m off-platform light trap catches at VK 203, after the dominant Hypsoblennius

invemar was removed from the data set. Studies on adult fish distributions have shown that

platforms attracted greater densities and diversities than that typically found in open waters

(Rooker et al. 1997; Stanley and Wilson 2000). Furthermore, this “platform-effect” decreased

rapidly with distance from the platform (within 18-50 m; Stanley and Wilson 2000). Therefore,

considering the relatively low CPUEs found off-platform, it is possible that the “platform-effect”

for larval and juvenile fishes was within 20 m of the structure.

Clupeiforms (clupeids and engraulids) and synodontids were consistently collected in

higher abundance within the platform than off-platform (Table 9). Though usually more

abundant in pelagic habitats, clupeids and engraulids have been shown to be particularly

photopositive (Choat et al. 1993; Brogan 1994) and would be strongly attracted to the platform’s

nighttime light field. Clupeiforms were collected in significantly greater abundance withinplatform than off-platform, except at ST 54 and MP 259 (Table 8). The higher relative

abundance of clupeiforms off-platform at ST 54 and MP 259 was probably responsible for the

greater similarity between within- and off-platform light trap collections, and the higher

diversities seen for off-platform light traps at these platforms (Table 7). Despite being

significantly different in several instances, the size distributions of engraulids between withinand off-platform were generally very similar (Figure 8). The size distributions of synodontids

also overlapped considerably, however, greater frequencies of larger individuals (>30 mm) were

collected off-platform (Figure 9). Synodontids are pelagic until they metamorphose and settle to

sand and mud bottoms at 30 to 45 mm in length (Jones et al. 1978; Leis and Rennis 2000a).
90

Therefore, it is probable that the individuals >30 mm collected off-platform represent individuals

that are settling to benthic habitats adjacent to platforms.

Taxa collected primarily in waters down-current from the platform included scombrids

and possibly carangids. Scombrids, particularly Euthynnus alletteratus, Scomberomorus cavalla

and S. maculatus, were generally more abundant off-platform at each platform sampled (Table

9). The scombrids collected off-platform also occurred at larger sizes than within-platform

(Figure 10). While the size distributions of E. alletteratus broadly overlapped, more individuals

between 8 and 13 mm were found within-platform. Carangids were more evenly distributed

between within- and off-platform, although Caranx hippos/latus was significantly more

abundant off-platform at ST 54 and GC 18 (Table 9). Adult scombrids and carangids are large,

predatory fishes that are commonly associated with the open waters around reefs, platforms or

other structure, primarily because of the greater feeding opportunities at these locations

(Gallaway 1981; Grimes et al. 1990; Choat and Bellwood 1991). The results from this study

suggested that the open waters adjacent to platforms were the main habitat for postlarvae and

juveniles. As larvae and juveniles, scombrids and carangids are highly predaceous and fast

growing, and prey availability may be a critical factor for their early growth and survival

(DeVries et al. 1990; Finucane et al. 1990; Lang et al. 1994). Therefore, scombrids and

carangids may be utilizing concentrations of zooplankton and ichthyoplankton that may be

attracted to the light field of platforms (Keenan et al. in press).

Reef fish, in general, did not show a consistent pattern of higher abundance within- or

off-platform. Several blenniids (i.e., Hypsoblennius hentz/ionthas and Scartella/Hypleurochilus)

were collected in higher numbers within-platform (Table 9). Hypsoblennius invemar, however,

was more abundant off-platform at each platform except GI 94. Furthermore, this species was

the dominant species collected in light traps deployed 50 m off-platform at VK 203 (Table 10).

The size distributions of H. invemar were also significantly different between within- and offplatform (Figure 11). In each instance, fish collected off-platform were between 11 and 13 mm

in length, while the lengths within-platform were generally smaller. Pomacentrus spp. were also

found in higher abundances off-platform, but there was no difference in their size distributions

(Table 9). The H. invemar and Pomacentrus spp. collected off-platform may represent settling

fish as their sizes correspond to the sizes at settlement found for congeners (Robertson et al.

1988; Watson 1996a, 1996b, 2000). Once settled at platforms, these taxa would become more

less available to light traps (Doherty 1991). Other reef taxa did not show consistent differences

in their abundances between within- and off-platform. Lutjanids and serranids were significantly

more abundant within-platform at GI 94, however, they were often more abundant off-platform

(e.g., lutjanids at VK 203; Tables 9). The lack of consistent differences among platforms could

be attributed to the grouping of individuals to the family level, however, separate analyses at

lower taxonomic levels failed to show any clear patterns.

The few differences observed between the open water “background” (SEAMAP) and

platform abundances of reef larvae were confounded by the large disparity in sampling effort

between SEAMAP and platform surveys. In general, greater abundances of blenniids,

holocentrids and pomacentrids were collected in platform plankton nets and light traps than in

SEAMAP bongo tows and neuston nets (Tables 11-14). In contrast, serranids and labrids, and to

some extent lutjanids, were generally more abundant in SEAMAP samples. The latter
91

differences may be attributed to the larger volumes of water filtered per SEAMAP sample, which

would increase the probability of encountering patches of reef fish larvae. For example, the

mean volume of water filtered by plankton nets at GC 18 was 39.2 m3 (± 30.1, SD), while bongo

nets at nearby SEAMAP stations filtered 259.2 m3. Although estimates were unavailable,

actively-towed neuston nets also probably sampled a greater volume of water than light traps.

However, the larger number of samples taken at a platform may have counteracted the greater

mean volume of water sampled at SEAMAP stations (e.g., 161 plankton net samples at GI 94 vs.

8 bongo net tows at SEAMAP stations; Table 11).

SEAMAP and platform samples were similar in that reef larvae, and especially reefdependent larvae, were relatively rare. This rarity has been attributed to high mortality during

the pelagic larval phase (near 100%; Leis 1991). Some researchers, however, have found that

certain reef taxa, such as scarids, labrids, and acanthurids, occurred in greatest abundance in

offshore waters up to 100 km from their natal reefs (Leis and Miller 1976; Victor 1987; Boehlert

and Mundy 1993). While this may help explain the rarity of reef taxa at platform locations, it

doesn’t explain their low relative abundance in ichthyoplankton surveys in the open waters of the

eastern Gulf and Caribbean (Richards 1984; Richards et al. 1993; Limouzy-Paris et al. 1994).

Lengths of larval reef fish collected by SEAMAP bongo nets were slightly smaller than

those collected by platform plankton nets. Size distributions of serranids collected by bongo nets

had a median length of 2 mm, while serranids collected by plankton nets had a median length of

3 mm (Figure 14). Similarly, the median length of lutjanids collected by bongo nets was 2 mm,

compared to 4 mm for plankton net-collected lutjanids (Figure 15). These observed differences

may be attributed to differences in the gears utilized to collect these larvae. Larvae were

collected during SEAMAP surveys in a 333 μm mesh bongo net towed obliquely through the

water column from near bottom (or to 200 m at the deeper stations) to the surface. Because the

bongo nets were actively towed, larger larvae may have been able to avoid the net because of the

pressure wave created (Choat et al. 1993). In contrast, the passively-fished, 333 μm mesh

plankton net used at platforms may have encountered fewer avoidance problems.

Seasonal Patterns of Larval and Juvenile Fish Abundance at Platforms

The seasonality observed in larval and juvenile fish catches at the mid-shelf (VK 203)

and outer-shelf (MP 259) platforms was similar to that observed from other studies in the

northern Gulf. Peak plankton net densities and light trap CPUEs occurred during June-August at

both platforms (Figures 16-19). Much of these higher catches were due to large abundances of

clupeiforms, which have peak spawning during the spring and summer months (Finucane et al.

1979b; Houde et al. 1979; Ditty et al. 1988). High non-clupeiform catches during these months,

at both VK 203 and MP 259, were attributed to other late spring and summer spawners such as

Auxis spp., Caranx crysos and Euthynnus alletteratus (Ditty et al. 1988). A similar trend was

observed during year-round sampling conducted at the outer shelf platform GC 18, although a

secondary peak occurred during November due to high numbers of the fall-winter spawning

Mugil cephalus (Hernandez 2001). High ichthyoplankton densities were also observed during

the fall at VK 203 due to high numbers of the fall-winter spawning Micropogonias undulatus

(Figure 18). Sciaenops ocellatus and Peprilus burti were also collected only during October at

VK 203 (Table 3).
92

Most of the reef larvae collected at MP 259 and VK 203 were also generally found during

the months of June-August (Tables 2 and 3). This was also the case during year-round sampling

at GC 18 (Hernandez 2001), quarterly sampling at platforms in the northern Gulf (Tolan 2001),

and other studies from the eastern and western Gulf (Finucane et al. 1979b; Houde et al. 1979;

Ditty et al. 1988). However, anthiines were most abundant during September at MP 259, while

serranines were most abundant during May at VK 203. These results were consistent with the

seasonality reported for Anthinae and Serraninae larvae from other surveys of the Gulf (i.e.,

Anthinae in fall, Serraninae in spring; Houde 1982; McGowan 1985).

 

Lunar Periodicity

There was little difference in the total plankton net or light trap catches between full and

new moons at MP 259 and VK 203, except for the total plankton net densities at VK 203, which

were significantly higher on full moons (Figure 20). Without clupeiforms, new and full moon

plankton net densities and light trap CPUEs were relatively similar to each other at both

platforms. Other studies involving light-aggregation devices have found lower catches on full

moons than on new moons, presumably due to the competitive interaction of lunar vs. lighted

gear illumination (Gregory and Powles 1985; Rooker et al. 1996). It is possible that the

platform’s nighttime light field may have overridden any lunar effect that may have otherwise

been present. Furthermore, differences in the total catch by lunar phase do not take into account

the lunar-related behavior of individual taxa.

In a closer examination of reef taxa collected at platforms, there were strong differences

in the numbers and developmental stages collected between new and full moons. Preflexion

holocentrids and labrids were almost exclusively collected on full moons (Table 15). Preflexion

Hypsoblennius hentz/ionthas and unidentified blenniids were found in significantly higher

numbers on new moons, although these taxonomic groupings probably include many, if not all,

of the species of blenniids collected at platforms. In contrast, preflexion H. invemar,

Scartella/Hypleurochilus and many lutjanids, pomacentrids and serranids were collected in

higher frequencies on new moons. Of the postflexion larvae, most blenniids, holocentrids,

lutjanids, and pomacentrids were found in significantly higher numbers on new moons. Lutjanus

campechanus were the only postflexion larvae found in higher numbers during full moons. The

predominance of postflexion larvae collected on new moons was not surprising, because most

were collected at, or near, settlement size; and numerous studies have documented higher rates of

settlement during new moons (Victor 1986; Sponaugle and Cowen 1994; Rooker et al. 1996).

Blenniids and pomacentrids, in particular, have settlement cycles that are associated with the

darker periods of the lunar month, presumably to avoid visual predation (Robertson et al. 1988).

Many reef taxa also exhibit lunar spawning cycles, which are often synchronized to maximize

the amount of potential settlers available during the most favorable lunar phases (Robertson

1991). The relative lack of significant differences between lunar phases for preflexion larvae, as

compared with postflexion larvae, was probably related to the effects of dispersal obscuring

evidence of lunar spawning cycles (Doherty and Williams 1988; Meekan et al. 1993).

Temporal Variability of Plankton Net and Light Trap Catches

Coefficients of variation (CVs), calculated from the mean plankton net and light trap

catches per sampling night and sampling trip, described pulses (or discrete peaks) in the catches
93

of several dominant taxa and reef families at GI 94 and VK 203. Coefficients of variations

among sampling nights were generally higher than, or approximately equal to, the CVs among

sampling trips for nearly every taxa, indicating that mean catches on a nightly time-scale were

more variable than mean catches by trip (Tables 16 and 17). However, the relative magnitude of

both among night and among trip CVs was more informative in describing taxa with pulses. For

example, plankton net-collected Bregmaceros cantori had CVs among the lowest at GI 94 and

did not exhibit a discrete pulse (Figure 21). In contrast, light trap-collected Hypsoblennius

invemar had CVs among the highest at both GI 94 and VK 203 and exhibited a large pulse, or

two pulses at VK 203, in their nightly catch (Figures 21 and 22). This was also the case for light

trap-collected Pomacentridae at both GI 94 and VK 203, even though among trip CVs were

higher than among night CVs for this taxa at VK 203.

The pulses observed for Hypsoblennius invemar and Pomacentridae at both GI 94 and

VK 203 probably represented settlement pulses. Most H. invemar and Pomacentridae collected

during these pulses, at both platforms, were between 10-13 mm and 8-11 mm respectively,

corresponding to the sizes at settlement reported for congeners (Robertson et al. 1988; Watson

1996a, 1996b, 2000). The occurrence of these pulses during darker periods of the lunar month

(i.e., first quarter moon at GI 94 and new moons at VK 203) further suggests that they were

settlement-related. Several studies have shown that reef fish year-classes were formed from

settlement pulses lasting 1-3 nights (Robertson et al. 1988; Doherty 1991). One settlement pulse

was found to contribute >40% of the year-class of a pomacentrid on the Great Barrier Reef

(Pitcher 1988). While it is highly probable that the pulses observed at platforms represent

settling fish, it is unknown how much, if at all, these pulses contribute to resident populations.

Another feature of these pulses was that they often coincided for different species and

even different families. This was particularly noticeable for Hypsoblennius invemar and

Pomacentridae, which simultaneously exhibited pulses in light trap samples at both GI 94 and

VK 203 (Figures 21 and 22). A similar pulse at MP 259 comprised most of the holocentrids,

pomacentrids, synodontids, and H. invemar collected at this platform. Multitaxa pulses have

also been documented during other studies. For example, a recruitment pulse observed on the

Great Barrier Reef was composed of three species of pomacentrids (Williams 1983). Also near

the Great Barrier Reef, most pomacentrids, lethrinids and mullids collected by light traps during

a survey were taken at the same station during the same 3-night period (Thorrold and Williams

1996). Such pulses have been attributed to the lunar entrainment of settlement cycles (Doherty

1991). However, the synodontids, holocentrids and H. invemar collected during the multitaxa

pulse at MP 259 were not at settlement size. It is possible that these taxa were entrained within a

common water mass that led to a presettlement aggregation of larvae composed of several

species (Sweatman 1988; Kingsford and Choat 1989).

Much of the within-night variability in total plankton net and light trap catches could be

attributed to variability in catches of clupeiforms. Mean densities and CPUEs by hour after

sunset were extremely variable with peaks occurring throughout the night at any specific

platform (Figure 23). Without clupeiforms, densities and CPUEs were surprisingly equal

throughout the night, except at VK 203 and GC 18 where clupeiforms were not as abundant

(Figure 24; Table 8). There was no obvious evidence of nocturnal vertical migration by larval

fish. Many taxa of larval fish undertake vertical migrations up into the neuston at dusk, followed
94

by a return to depth at dawn (Kendall and Naplin 1981; Leis 1986, 1991). Such movements may

have been missed during this study due to the absence of samples at either dusk or dawn.

Furthermore, analyses of the total ichthyoplankton catches (with or without clupeiforms) may

have masked the behavior of individual taxa. Separate analyses of the within-night catches of

dominant taxa and reef taxa, however, were also inconclusive, with high degrees of variability

throughout the night across the different platforms.

Length-Frequency and Developmental Stages of Reef Taxa

One of the proposed fisheries benefits of platforms is that they may provide new

spawning habitat. Although not directly observed, recent and nearby spawning by reef fish was

inferred by the collection of recently-hatched or very small larvae at platforms. Many recentlyhatched and preflexion blenniid, lutjanid and serranid larvae were collected at every platform,

indicating local spawning events (Figures 24-26). In addition, relatively small preflexion

holocentrids and pomacentrids were collected at the mid-shelf platform GI 94, and preflexion

holocentrids, labrids and scarids were collected at the outer shelf platforms, MP 259 and GC 18

(Figures 27-29). Although the presence of these young larvae is indicative of local spawning, it

is not known whether this spawning occurred at platforms or nearby natural reefs. Seasonal

transport envelopes, created from satellite altimetry data and otolith-derived pelagic transport

durations, have been created for serranid larvae collected at three mid- and outer shelf platforms

(Tolan 2001). Within these envelopes, other oil and gas platforms were numerically dominant

over natural reefs, statistically making platforms the most probable source of the larvae.

Another proposed fishery benefit of platforms is that they may provide new

nursery/settlement habitat or refugia. Again using the length frequency of reef fish collected at

platforms, the presence of settlement-size larvae could provide indirect evidence for a potential

nursery habitat function. Of the potential settlers, blenniids were the most abundant settlementsize reef larvae, and pomacentrids were the most common settlement-size reef-dependent larvae

collected at platforms (Figures 24 and 28). Considering that blenniids and pomacentrids were

often collected in discrete pulses during the darker lunar phases (new and first quarter; Figures

21 and 22), as reported for other settling fish from these families, it is highly likely that these

taxa were settling on platforms. Furthermore, efforts to collect new settlers in settlement traps

were attempted at MP 259 and VK 203 (unpublished data). Although catches were low,

blenniids and pomacentrids were the most common taxa and were collected at size ranges

beginning where sizes from light traps ended (i.e., ≥14 mm).

Settlement-size larvae of other reef taxa were relatively rare, as compared with blenniids

and pomacentrids, and were represented mostly by lutjanids at the mid-shelf platforms (Figure

25). The relatively low numbers of settlement-size lutjanids and serranids was not surprising

because many of these taxa have been shown to settle out in alternative, more coastal nursery

habitats, such as sea grass beds, mangroves, or low relief hard substrate (Lindeman 1989; Beets

and Hixon 1994; Szedlmayer and Conti 1998). However, juvenile Lutjanus spp. have also been

collected west of the Delta in association with rock jetties (Hernandez 2001). Although larger

postflexion, rhynchichthys-stage holocentrids were collected at MP 259 and GC 18 (Figure 27),

these individuals probably represented presettlement stages as holocentrids have extended larval

durations (pelagic specimens up to 48 mm; Leis and Rennis 2000b)
95

The relative abundance of settlement-size reef larvae at platforms may be influenced by

two disparate spawning strategies exhibited by reef fishes: broadcast spawning of pelagic eggs or

benthic spawning of demersal adhesive eggs. The most abundant settlement-size larvae at

platforms were from the benthic-spawning families Blenniidae and Pomacentridae. The other

reef families of interest in this study were from pelagic-spawning families and were either absent

(Acanthuridae and Pomacanthidae), rare (Chaetodontidae and Labridae) or predominantly

collected in presettlement stages (Holocentridae, Lutjanidae, Scaridae and Serranidae). The

differences in relative supply of potential settlers are probably related to early life history traits

that are often associated with the two spawning strategies. In general, as compared to benthic

eggs, pelagic eggs are smaller and result in less developmentally-advanced larvae that have

longer larval durations and may, therefore, be more vulnerable to predation and advective loss

(Cowen and Sponaugle 1997). Furthermore, the transition from pelagic to reef environments is

often characterized by high levels of predation by resident fishes (the “wall of mouths”, Hamner

et al. 1988). Therefore, the probability of recruitment for pelagically-spawned larvae may be

considerably low. Because they generally have shorter larval durations, benthically-spawned

larvae probably have limited dispersal and may be able to remain in areas where suitable

settlement habitat is more available (Barlow 1981). This may explain, in part, the high

abundance of postflexion and settlement-size blenniids and pomacentrids at the mid-shelf

platforms, which reside in areas of high platform concentration (Tolan 2001).

Environmental Variables and Larval and Juvenile Fish Abundances

Seasonal variations in temperature influenced the abundances of several taxa at the outer

shelf platform MP 259. Plankton net-collected Auxis spp., Euthynnus alletteratus and

Symphurus spp. were positively associated with temperature (Table 18), and represent taxa with

peak larval abundance during the spring and summer months (Ditty et al. 1988). In contrast,

plankton net-collected Bregmaceros cantori, Myctophidae and Serranidae were negatively

associated with temperature, which is consistent with their peak larval abundances during the

spring and fall in the northern Gulf (Houde 1982; McGowan 1985; Ditty et al. 1988).

Several taxa were associated with different water masses that occurred at MP 259. Two

prominent features exhibited by plots of environmental variables at MP 259 were the relatively

low salinity, high turbidity waters that occurred during late May and late August, and the

relatively high salinity, low turbidity water during early July (Figure 2). Scomberomorus

maculatus in plankton nets and Chloroscombrus chrysurus in light traps seemed to be associated

with the lower salinity waters (Tables 18 and 19). Because larvae of S. maculatus and C.

chrysurus have been found in highest densities in the shallower, coastal sections of the Gulf

(Patillo et al. 1997; Hernandez 2001), these taxa were probably advected offshore by an intrusion

of coastal water. In contrast, light trap-collected Decapterus punctatus, Holocentrus spp.,

Hypsoblennius invemar, Pomacentrus spp. and Synodus foetens were associated with the high

salinity, low turbidity water mass (Table 19). The reasons for this association are not evident,

however, as previously discussed, it is possible that some of these taxa (i.e., H. invemar and

Pomacentrus spp.) may have been part of a settlement pulse. Considering that non-clupeiform

light trap catches were 4 to 5 times greater during the early July sampling trip than other

sampling trips (Figure 17), it is also possible that some oceanographic event may have entrained,

concentrated and transported these postflexion and juvenile fishes to MP 259.
96

There was less environmental variability at the mid-shelf platform VK 203, and

consequently microzooplankton biomass was the most influential environmental variable for

larval and juvenile fish abundances. Taxa positively associated with this variable included

Bregmaceros cantori, Etropus crossotus, Saurida brasiliensis, Scomberomorus maculatus and

Symphurus spp. in plankton nets and S. maculatus in light traps (Tables 20 and 21). Serranidae

were also positively associated with microzooplankton biomass, but were only collected when

temperatures were relatively cooler (Table 20). These results are not surprising because

zooplankton are a patchy resource and fish larvae that are associated with high prey densities

often benefit from enhanced growth and survival (Hunter 1981; Grimes 2001). However, light

trap-collected Hypsoblennius invemar, Pomacentrus spp. and Lutjanidae were negatively

associated with microzooplankton biomass (Table 21). Similar to MP 259, these taxa may have

been associated with settlement pulses that occurred coincidentally with low prey biomass.

Effects of Water Current Speed on Light Trap Catches

Light trap catch efficiency seemed to be affected by high current speeds at MP 259 and

VK 203. At current speeds greater than 30 cm s-1 light trap CPUEs, with or without clupeiforms

included, were <10 fish/10 min., while at lower current speeds catch rates were often above 20

fish/10 min. (Figure 30). Doherty (1987) speculated that for fixed light traps (light traps that are

not allowed to drift freely with the current) catches should increase initially with faster currents

as more water is sampled, but then decrease as current speed interferes with the catchability of

larval fish. At this point, catchability would be highly dependent on the ability of a larval fish to

swim to the light trap. While a dome-shaped relationship was not evident from the results of this

study, a change in fish catchability was observed. At higher current speeds (>40 cm s-1) the

median length of fish collected by light traps, with or without clupeiforms, was less than 10 mm,

while at lower current speeds median lengths were above 10 mm and a wide range of sizes were

collected (Figure 31 and 32). The larvae <10 mm in length collected at the higher current speeds

were undoubtedly passively entrained in the light trap gear. Larger postlarvae and juveniles

were seemingly relatively unavailable to light traps at high current speeds. Swimming speeds of

fish larvae, in general, have been measured at 1-5 body lengths sec-1 for sustained swimming and

10-20 body lengths sec-1 for burst-swimming speeds (Blaxter 1986). Therefore, for the average

20 mm fish burst-swimming speeds would be 20-40 cm s-1. At these swimming speeds it would

be difficult for a fish to maneuver to and enter a light trap at the highest current speeds observed.

Conclusions

This study represents an important step towards understanding the ecological significance

of oil and gas platforms to the early life history stages of fishes. Given the extensive network of

platforms in the northern Gulf, it is important to determine what species are associated with

platforms and how they affect larval and juvenile fish assemblages, including whether they

provide nursery/recruitment habitat for postlarval and juvenile fishes. Such information is

necessary to determine whether platforms, and artificial reefs in general, contribute to the new

production of fish.

The synthesis of data from platforms east of the Mississippi River Delta with data from

platforms west of the Delta allowed us to characterize the larval and juvenile fish assemblages
97

collected at platforms across longitudinal and latitudinal gradients. Similar to adult fish

distributions (Hoese and Moore 1998; Stanley and Wilson 2000), the larval and juvenile fish

assemblages seemed to be influenced by across-shelf gradients of increasing depth. High larval

densities of coastal taxa such as sciaenids, synodontids and Scomberomorus maculatus

characterized inner shelf non-clupeiform assemblages. Mid-shelf assemblages were comprised

of a high diversity of larval and juvenile blenniids, synodontids, bregmacerotids and scombrids.

Outer shelf catches were generally poor, and primarily composed of scombrids, carangids, and

other oceanic taxa. Reef taxa were most abundant and diverse at the mid-shelf platforms, due to

large numbers of larval and juvenile blenniids, pomacentrids and lutjanids. This high abundance

and diversity at mid-shelf could be attributed to the high concentration of platforms (i.e., more

potential sources of larvae) and the favorable environmental conditions at mid-shelf (Parker

1960; Gallaway 1981; Tolan 2001). The only differences observed in the larval and juvenile fish

assemblages across longitudinal gradients (i.e., east or west of the Delta) were differences in the

abundance of certain taxa. Higher abundance of these taxa east or west of the Delta may, in turn,

reflect differences in the hydrographic conditions and/or habitat availability. Despite the higher

concentration of natural reef-type habitats east of the Delta, reef larvae were not more abundant

at platforms in these areas. The extensive network of platforms may have allowed for the range

extension of reef taxa that would otherwise be limited by the availability of hard-substrate in the

northern Gulf (Dennis and Bright 1988; Tolan 2001).

The higher abundance and diversity of postlarval and juvenile fishes within the platform

structure, as compared with adjacent down-current open waters, indicates that platforms have a

significant impact on the distribution of postlarval and juvenile fishes in the northern Gulf.

Platforms represent structure in the pelagic habitat, and concentrations of larval and juvenile

fishes have often been found associated with biotic or abiotic structure (Kingsford 1993). Light

trap collections from within the platform were dominated by clupeids, engraulids and

presettlement synodontids. These typically pelagic fishes may have been attracted by the

nighttime light-fields of the platforms and/or concentrations of prey that may be found in the

waters around platforms. Although generally more abundant away from the platform, highly

predatory scombrids and carangids also may be opportunistically utilizing platform areas for

increased feeding opportunities. Therefore, populations of clupeids, engraulids and scombrids,

which would otherwise spend considerable time foraging for prey in the pelagic environment,

may benefit from the presence of platforms, particularly in more oligotrophic offshore waters.

However, increased prey availability may be offset by increased predation from the large

populations of adult fishes at platforms.

Platforms may provide nursery/recruitment habitat for certain reef taxa. Blenniids and

pomacentrids, the most common reef taxa collected at platforms, were predominantly collected

as settlement-size individuals. These settlement-size fishes were most often collected in discrete

pulses during the darker lunar phases (i.e., new and first quarter moons), which is typical of other

settling reef fishes. The fact that blenniids and pomacentrids generally have short larval

durations, and, therefore, limited dispersal and advective loss, further increases the probability

that these taxa may use platforms as nursery/recruitment habitat. Other reef taxa collected at

platforms (e.g., labrids, lutjanids and serranids) were rare or predominantly collected as

presettlement larvae. The lack of settlement-size larvae from these taxa does not preclude the

possibility that these taxa use platforms as nursery/recruitment habitat. It is possible that pulses
98

of settling reef larvae could have been missed, or larvae may have settled to benthic habitats at

the base of the platform, areas that were beyond the reach of the sampling gear utilized. During

this study, the areas sampled at platforms represented shallow water hard-substrate, habitat that

is obviously limited on the continental shelf of the northern Gulf. Because artificial reefs most

likely benefit species that are habitat-limited (Bohnsack 1989), platforms should, therefore,

enhance the production of typical shallow water reef taxa (e.g., blenniids and pomacentrids).
99

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