OCS Study 

MMS 2006-037 

 

Coastal Marine Institute 

Effects of Depth, Location, and 

Habitat Type on Relative Abundance and 

Species Composition of Fishes Associated 

with Petroleum Platforms and Sonnier Bank 

in the Northern Gulf of Mexico 

Final Report 

Cooperative Agreement 

Coastal Marine Institute 

Louisiana State University 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 
U.S. Department of the Interior Cooperative Agreement 

Minerals Management Service Coastal Marine Institute 

Gulf of Mexico OCS Region Louisiana State University 

OCS Study 

 MMS 2006-037 

Coastal Marine Institute 

Effects of Depth, Location, and 

Habitat Type on Relative Abundance and 

Species Composition of Fishes Associated 

with Petroleum Platforms and Sonnier Bank 

in the Northern Gulf of Mexico 

Final Report 

Editors 

Charles A. Wilson 

Mark W. Miller 

Yvonne C. Allen 

Kevin M. Boswell 

David L. Nieland 

August 2006 

Prepared under MMS Contract 

1435-0001-30660-19947 

by 

Louisiana State University 

Coastal Fisheries Institute 

School of the Coast and Environment 

Baton Rouge, Louisiana 70803 

Published by 
iii 

DISCLAIMER 

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

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

reviewed by the MMS, LSU, 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 trade 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 

Mineral Management Service 

Gulf of Mexico OCS Region 

Attn: Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone: (504) 736-2519 or 

 1-800-200-GULF 

CITATION 

Suggested citation: 

Wilson, C.A., M.W. Miller, Y.C. Allen, K.M. Boswell, and D.L. Nieland. 2006. Effects of 

depth, location, and habitat type on relative abundance and species composition of fishes 

associated with petroleum platforms and Sonnier Bank in the northern Gulf of Mexico. 

U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, LA. OCS Study MMS 2006-037. 85 pp. 
v 

ACKNOWLEDGMENTS 

The authors wish to thank Mr. Greg Boland in the Environmental Sciences Section of the 

Minerals Management Service in New Orleans for his guidance during this project. We extend 

our deepest gratitude to James Cowan, James Geaghan, and Larry Rouse for assistance in 

research direction, data analysis, and critical review. This research was supported by the 

Minerals Management Service, U.S. Department of the Interior through the Coastal Marine 

Institute of Louisiana State University under a cooperative agreement with Louisiana State 

University. 
vii 

EXECUTIVE SUMMARY 

Scientists and managers continue to ponder both the role of artificial reefs in fisheries 

management and the effect that artificial reefs have on both healthy and stressed fish 

populations. This issue is exemplified by the northern Gulf of Mexico (GOM), which has the 

largest artificial reef complex in the world. 

We have spent over a decade studying the fish communities associated with operating oil and gas 

platforms and with several artificial reefs created from platforms. Although we are still 

uncertain of the life history consequences of this artificial habitat and its value as compared to 

the limited natural habitat in the region, we have gained a great deal of knowledge that has 

advanced this science. Platforms in the region form an extensive resource for recreational and 

commercial fishery user groups; regional fisheries managers and user groups have reasonable 

justification to believe that platforms enhance fisheries and organisms dependent on hard 

substrate. Fish abundance measured as fish biomass at platforms invariably exceeds the 

measurable fish community associated both with open water bottoms of the GOM and with 

natural reef and reef-like habitats. 

Petroleum platforms differ from traditional artificial reefs in that their vertical profile extends 

throughout the water column and provides habitat from the photic zone to the substrate. 

Historically, a major obstacle in deriving scientific data about the effect of deep water artificial 

reefs (and platforms) on fishes and other nekton was the difficulty in quantifying abundance and 

species composition associated with these reef communities. We have successfully used two 

complementary techniques, hydroacoustics and point count visual surveys, to estimate the 

abundance (biomass) and species composition of fishes at several of these structures (Stanley 

and Wilson 1995, 1996a, 1997, and 1998) and natural reefs (Wilson et al 2003). In these studies 

dual-beam hydroacoustics provided estimates of acoustic biomass, density, size distribution, and 

the near field area of influence. A remotely operated underwater vehicle (ROV) was used to 

determine species composition. 

We have found from 10,000 to 30,000 fishes associated with individual platforms; the lowest 

numbers were found at the largest and deepest structures, leading us to question the value of 

deep water structures as artificial reefs. We have also reported that density of fishes around 

platforms was ten times greater than open water and from two to three times greater than fish 

density associated with the upper portion of the West Flower Garden Bank off eastern Texas; 

species composition also differed between these two habitat types. (Wilson et al 2003). 

The purpose of this project was to extend our research to address the effect that depth and 

latitude have on the fish community associated with platforms and to compare the fish 

community at a natural hard bottom (Sonnier Bank) to that of a nearby standing platform. 

The test of these hypotheses is critical both to future Minerals Management Service policy and to 

platform dependent artificial reef programs. We proposed to describe the fish communities at 

several platforms along a north-south transect twice a year for each of three years, sampling 

along a new transect each year. Transects were located east of the Mississippi River in year one, 

in the central Gulf of Mexico off the Louisiana coast in year two, and the western Gulf of 

Mexico off the coast of Texas in year three. A contract modification allowed us to add an 
viii 

additional survey of the Sonnier Bank features to provide for comparison of the fish community 

at platforms to that of nearby natural substrate. 

Across the three regions (east, central and west), we observed a significant difference in fish 

biomass. Fish biomass per unit area was highest in the central region and lowest along both the 

eastern and western regions. Fish biomass at platforms in the central region were also dominated 

planktivores that reside high in the water column. We speculate that increased nutrients from the 

Mississippi river may allow for increased primary and secondary productivity in the central 

region. Species richness was higher in the central and western GOM compared to the eastern 

GOM. 

Average acoustic fish size also varied significantly across regions and was again highest in the 

central region and lowest in the western region (Figure 12a, 13). Targets along the western 

region averaged 30% smaller than those of the central or eastern regions. Targets along the 

eastern region were also slightly smaller than target size along the central region. 

A comparison of the high-relief pinnacles at Sonnier Bank and the standing platform VR279 

again demonstrated that based on our acoustic fish biomass estimates, fish biomass, abundance 

and size were all substantially greater around the standing platform, particularly in the upper 

water column. Fish biomass over the low-relief feature at Sonnier Bank showed a greater fish 

biomass compared to the pinnacles 

This research continues to support the premise that standing oil and gas platforms make useful 

artificial reefs because they support fish densities that can be from 10 to 1000 times greater than 

the densities found over adjacent sand and mud bottom habitats. Also, fish densities at standing 

platforms almost always exceed those found both at artificial reefs (both partially removed and 

toppled) and at natural habitats such as Sonnier Bank. However, the fish species associated with 

artificial reefs (including standing platforms) differ from those found on natural habitats. Future 

research efforts might be directed toward determining the reasons for this difference. 
ix 

TABLE OF CONTENTS 

Page

LIST OF FIGURES ....................................................................................................................... xi 

LIST OF TABLES.........................................................................................................................xv 

1.0 INTRODUCTION ....................................................................................................................1 

1.1 Past Research..................................................................................................................... 2 

1.2 Goals of This Study ........................................................................................................... 3 

2.0 METHODS AND MATERIALS .............................................................................................5 

2.1 Site Descriptions................................................................................................................ 5 

2.1.1 Eastern Region .........................................................................................................5 

2.1.2 Central Region..........................................................................................................5

2.1.3 Western Region ........................................................................................................8 

2.1.4 Sonnier Bank ............................................................................................................8

2.2 Survey Designs .................................................................................................................. 8 

2.2.1 Standing Platforms ...................................................................................................8 

2.2.2 Sonnier Bank ..........................................................................................................12 

2.3 Acoustic Data Collection and Processing........................................................................ 14 

2.4 Data Analysis................................................................................................................... 17 

2.4.1 Statistical Analysis .................................................................................................17 

2.4.2 Fish Abundance Estimation....................................................................................18 

2.5 Visual Surveys................................................................................................................. 18 

2.6 Environmental Data......................................................................................................... 18 

3.0 RESULTS ...............................................................................................................................21 

3.1 Field Operations.............................................................................................................. 21 

3.2 Platform Results............................................................................................................... 22 

3.2.1 Acoustic Volume Backscatter (Fish Energy) .........................................................22 

3.2.2 Density Estimates...................................................................................................26 

3.2.3 Target Strength (TS)...............................................................................................29 

3.3 Platform Results - Horizontal .......................................................................................... 33 

3.3.1 Volume Backscatter ..............................................................................................33 
TABLE OF CONTENTS (continued) 

Page

x 

3.4 Platform Species Composition and Abundance .............................................................. 35 

3.5 Sonnier Bank .................................................................................................................. 57 

3.5.1 Broad-Scale Survey Results of Sonnier Bank.......................................................57 

3.5.2 Low-Relief Pinnacle Area of Sonnier Bank..........................................................57 

3.5.3 High-Relief Pinnacle Area of Sonnier Bank .........................................................60 

3.5.4 Species Composition .............................................................................................60 

3.6 Comparison of Sonnier Bank High-Relief Pinnacle Area and VR 279.......................... 67 

4.0 DISCUSSION.........................................................................................................................73 

4.1 Technical and Analysis Logistics ................................................................................... 73 

4.2 Patterns in Acoustic Variables........................................................................................ 74 

4.3 Patterns in Species Composition .................................................................................... 77 

4.4 Comparison of Sonnier Bank and a Standing Platform.................................................. 78 

BIBLIOGRAPHY..........................................................................................................................81 
xi 

LIST OF FIGURES 

Figure Page

Figure 1. Overview map of study platforms, regions, and location of Sonnier Bank. ................ 6 

Figure 2. Bathymetry of Sonnier Bank. ....................................................................................... 9 

Figure 3. Figure shows the footprint of the Sonnier Bank pinnacles (black areas) 

compared to the footprints of a partially removed platform (top grey rectangle) 

and toppled platform (bottom grey rectangle) at the same depth below surface 

(30m). ........................................................................................................................... 10 

Figure 4. Configuration of hydroacoustic beams. ...................................................................... 11 

Figure 5. Overview of Sonnier Bank bathymetry intended track lines of the 

hydroacoustic survey conducted in May 2002............................................................. 13 

Figure 6. LS mean volume backscatter (Sv in dB ± 1 SE) by a) region, b) depth, c) side 

and d) time of day for platforms in the northern Gulf of Mexico. ............................. 24 

Figure 7. Least squares mean volume backscatter (Sv) at all study locations. ........................... 25 

Figure 8. Range of variation in acoustic biomass at all study platforms due to depth, 

location, time of day, and side. .................................................................................... 25 

Figure 9. LS mean fish per cubic meter (± 1 SE) by a) region, b) depth, c) side and d) 

time of day for platforms in the northern Gulf of Mexico. ........................................ 27 

Figure 10. Least squares mean fish density (fish m-3) at all study locations. ............................. 28 

Figure 11. Range of variation in fish density at all study platforms due to depth, location, 

time of day, and side. ................................................................................................... 28 

Figure 12. LS mean target strength (dB ± 1 SE) by a) region, b) depth, c) side and d) 

time of day for platforms in the northern Gulf of Mexico. ........................................ 30 

Figure 13. Least squares mean target strength at all study locations. ........................................ 31 

Figure 14. Range of variation in target strength at all study platforms due to depth, 

location, time of day, and side. ................................................................................... 31 

Figure 15. Alternate method to examine the spatial distribution of target strength and fish 

at HIA 350. ................................................................................................................. 32 

Figure 16. Horizontal LS mean volume backscatter (Sv in dB ± 1 SE) by a) region, b) 

depth, c) side and d) time of day for platforms in the northern Gulf of Mexico. ....... 34 
LIST OF FIGURES (continued) 

Figure Page

xii 

Figure 17. Map of species richness observed by ROV surveys at all study locations and 

visits.............................................................................................................................. 36 

Figure 18. Map of Shannon-Wiener Diversity Index observed by ROV surveys at all 

study locations and visits.............................................................................................. 36 

Figure 19. Map of the proportion of planktivores at all study locations and visits. .................... 38 

Figure 20. Map of the proportion of grazers at all study locations and visits............................. 38 

Figure 21. Map of the proportion of carnivores at all study locations and visits. ....................... 39 

Figure 22. Map of the proportion of top predators at all study locations and visits. ................... 39 

Figure 23. Actual survey track lines for hydroacoustic survey conducted in May 2003 of 

Sonnier Bank projected over bathymetry..................................................................... 58 

Figure 24. LS mean volume backscatter (Sv in dB ± 1 SE) at depth (m) for the small 

feature at Sonnier Bank, northern Gulf of Mexico. .................................................... 61 

Figure 25. LS mean volume backscatter (Sv in dB ± 1 SE) at directional quadrant for the 

small feature at Sonnier Bank, northern Gulf of Mexico. .......................................... 61 

Figure 26. LS mean target strength (dB ± 1 SE) at depth (m) for the small feature at 

Sonnier Bank, northern Gulf of Mexico. .................................................................... 62 

Figure 27. LS mean density (fish per cubic meter) at depth (m) for the small feature at 

Sonnier Bank, northern Gulf of Mexico. .................................................................... 62 

Figure 28. LS mean volume backscatter (Sv in dB ± 1 SE) at depth (m) for the pinnacle 

feature at Sonnier Bank, northern Gulf of Mexico. .................................................... 63 

Figure 29. LS mean target strength (dB ± 1 SE) at depth (m) for the pinnacle feature at 

Sonnier Bank, northern Gulf of Mexico. .................................................................... 63 

Figure 30. LS mean density (fish per cubic meter) at depth (m) for the pinnacle feature at 

Sonnier Bank, northern Gulf of Mexico. .................................................................... 64 
LIST OF FIGURES (continued) 

Figure Page

xiii 

Figure 31. LS mean volume backscatter (SV in dB ± 1 SE) at depth (m) at VR 279, 

northern Gulf of Mexico. ............................................................................................ 68 

Figure 32. LS mean target strength (dB ± 1 SE) at depth (m) at VR 279, northern Gulf of 

Mexico. Means with same letters are not significantly different at α= 0.005............. 68 

Figure 33. LS mean density (fish per cubic meter) at depth (m) at VR 279, northern Gulf 

of Mexico. .................................................................................................................. 69 

Figure 34. LS mean volume backscatter (Sv) at the standing platform (VR279) 

compared to Sv at the small feature and pinnacle feature at Sonnier Bank. ................ 70 

Figure 35. LS mean target strength (TS) at the standing platform (VR279) compared to 

TS at the small feature and pinnacle feature at Sonnier Bank. .................................... 70 

Figure 36. Fish energy at the standing platform (VR279) compared to Sv at the small 

feature and pinnacle feature at Sonnier Bank. ............................................................. 71 

Figure 37. Fish density at the standing platform (VR279) compared to Sv at the small 

feature and pinnacle feature at Sonnier Bank............................................................... 71 

Figure 38. Comparison of the proportion of each fish functional group found at Sonnier 

Pinnacle, Sonnier low-relief feature, and VR279. ....................................................... 72 

Figure 39. Hydroacoustic echogram of Sonnier Bank pinnacle feature illustrating the 

high density of fish associated with the two pinnacle features. ................................... 76 

Figure 40. Hydroacoustic echogram of Sonnier Bank small feature illustrating a few 

acoustic targets associated with the small feature and a natural gas plume. .............. 76 

Figure 41. Proportion of red snapper observed at each study platform based on ROV 

surveys. ....................................................................................................................... 79 
xv 

LIST OF TABLES 

Table Page

Table 1. General information about the oil and gas structures surveyed with stationary 

dual beam hydroacoustics. ............................................................................................. 7 

Table 2. Parameter settings for Echoview analysis. .................................................................... 15 

Table 3. Results of split plot design analysis of variance showing the effect of region, 

time of day, platform side, and depth stratum on the dependent variable fish 

energy. .......................................................................................................................... 23 

Table 4 Results of split plot design analysis of variance showing the effect of location, 

time of day, platform side, and depth stratum on the dependent variable fish 

density. ......................................................................................................................... 26 

Table 5. Results of split plot design analysis of variance showing the effect of region, 

time of day, platform side, and depth stratum on the dependent variable target 

strength.. ....................................................................................................................... 29 

Table 6. Results of split plot design analysis of variance showing the effect of region, 

time of day, platform side, and depth stratum on the dependent variable Sv for 

horizontal hydroacoustic data....................................................................................... 33 

Table 7. Fish functional group assignments based on ROV survey observations....................... 37 

Table 8. Number of fishes by species observed during ROV video surveys by depth at 

VK 203, Visit 2, 1/1999. .............................................................................................. 40 

Table 9. Number of fishes by species observed during ROV video surveys by depth at 

MP 259, Visit 1, 6/1998. .............................................................................................. 41 

Table 10. Number of fishes by species observed during ROV video surveys by depth at 

MP 259, Visit 2, 12/1998. ............................................................................................ 42 

Table 11. Number of fishes by species observed during ROV video surveys by depth at 

MP 289, Visit 1, 6/1998. .............................................................................................. 43 

Table 12. Number of fishes by species observed during ROV video surveys by depth at 

MP 289, Visit 2, 12/1998. ............................................................................................ 44 

Table 13. Number of fishes by species observed during ROV video surveys by depth at 

MP 142, Visit 1, 7/1998. .............................................................................................. 45 
LIST OF TABLES (continued) 

Table Page

xvi 

Table 14. Number of fishes by species observed during ROV video surveys by depth at 

SP 89, Visit 2, 10/1998................................................................................................. 46 

Table 15. Number of fishes by species observed during ROV video surveys by depth at 

VR 214, Visit 2, 4/2000. .............................................................................................. 47 

Table 16. Number of fishes by species observed during ROV video surveys by depth at 

WC 564, Visit 1, 5/1999............................................................................................... 48 

Table 17. Number of fishes by species observed during ROV video surveys by depth at 

HIA 350, Visit 1, 6/1999.............................................................................................. 49 

Table 18. Number of fishes by species observed during ROV video surveys by depth at 

WC 643, Visit 1, 3/2000............................................................................................... 50 

Table 19. Number of fishes by species observed during ROV video surveys by depth at 

WC 643, Visit 2, 2/2001............................................................................................... 51 

Table 20. Number of fishes by species observed during ROV video surveys by depth at 

GB 236, Visit 1, 5/1999. .............................................................................................. 52 

Table 21. Number of fishes by species observed during ROV Video Surveys by depth at 

GB 236, Visit 2, 2/2000. .............................................................................................. 53 

Table 22. Number of fishes by species observed during ROV video surveys by depth at 

VR 279, Visit 1, 6/2003. .............................................................................................. 54 

Table 23. Number of fishes by species observed during ROV video surveys by depth at 

MI 703, Visit 1, 10/2002. ............................................................................................. 55 

Table 24. Number of fishes by species observed during ROV video surveys by depth at 

MU 787, Visit 1, 10/2002............................................................................................. 56 

Table 25. Results of Split plot design analysis of variance showing the effect of depth, 

feature, and region on the dependent variable Sv for horizontal hydroacoustic 

data. .............................................................................................................................. 59 

Table 26. Number of fishes by species observed during ROV Video Surveys by depth at 

Sonnier Bank Low-Relief Feature, 5/2002................................................................... 65 

Table 27. Number of fishes by species observed during ROV video surveys by depth at 

Sonnier Bank Pinnacle Feature, 5/2002.. ..................................................................... 66 
1

1.0 INTRODUCTION 

The benthic environment of the northern Gulf of Mexico (GOM) from Destin, FL to 

Brownsville, TX is characterized by silt, mud, and sand with very little hard substrate. Parker et 

al. (1983) estimated that there was approximately 2800 km2 of naturally occurring hard bottom in 

the region. Over the last 60 years, the area of hard substrate in this region has increased through 

the development of infrastructure for a thriving oil and gas industry. Currently, there are some 

4000 oil and gas platforms operating in state and federal waters of the northern GOM (MMS 

2005), creating the largest de facto artificial reef system in the world. Combined, these platforms 

only increase the surface area of hard substrate available in the northern GOM by 4% (Stanley 

and Wilson 1990, 1991, 1997), but they have altered the available fish habitat and possibly 

affected regional fisheries by providing hard substrate throughout the water column. Off the 

coast of Louisiana in particular, the Mississippi River deposits vast amounts of clay and other 

fine-grained sediment; oil and gas platforms thus provide a large percentage of hard-bottom area 

for reef-dwelling fishes. For example, Stanley and Wilson (1997, 1998, 2000), determined that a 

standing platform in about 30 m water depth seasonally provided habitat for 10,000-20,000 

fishes, many of which are of recreational and commercial importance. By adding even 

minimally to the amount of reef available, petroleum platforms have doubtless affected many 

regional ecosystem processes such as energy (food) availability, habitat, recruitment, 

competition, and predation (Menge and Sutherland 1987; Doherty and Williams 1988; Bohnsack 

et al. 1991; Stanley and Wilson 2000). 

In response to both the federally mandated removal of obsolete platforms and to the popularity of 

petroleum platforms as fishing destinations, Louisiana, Texas, and other states along the northern 

GOM now convert retired oil and gas platforms into artificial reefs (NRC 1996). The Louisiana 

Artificial Reef Program (LARP) was established in 1986 and is currently administered by the 

Louisiana Department of Wildlife and Fisheries. The Texas Parks and Wildlife Department 

established a rigs-to-reef program in 1988. To date, these states have emplaced over 200 

platforms as artificial reefs. 

The concept of using oil and gas platforms as artificial reefs, i.e. “Rigs to Reefs” (RTR), is 

strongly supported by recreational and commercial fishers and by supporting organizations such 

as the Coastal Conservation Association. The scientific community, however, still ponders the 

ecological impact of artificial reefs in general, and more specifically the effectiveness of 

petroleum platforms as artificial reefs. How do species composition and density of artificial reef 

fish communities compare to those found in association with natural substrates? The current 

paradigm with artificial reefs and platforms is that they improve and/or diversify habitat, increase 

resources, and modify assemblages of organisms (Seaman and Jensen 2001). 

Beyond purely scientific concerns, there are also many management questions concerning the 

effect of water depth, geographic location, and general reef configuration on fish production at 

both standing platforms and artificial reefs. For example, for past RTR projects, oil and gas 

platforms were typically converted into reefs by laying a platform on its side either by toppling 

in place or by moving a toppled platform to a new location. More recently, the Louisiana 

Artificial Reef Program has followed the Texas Artificial Reef Program by removing the top 

portion of a standing platform to a depth (typically 90 feet below surface) sufficient to comply 

with Coast Guard regulations. This practice is referred to as partial removal. Stanley and 
2

Wilson (1997) proposed that vertical profile is important in maintaining the fish density resident 

at standing platforms. Wilson et al (2003) later demonstrated that a partial removal did, in fact, 

retain a larger portion of the population reported at an operating platforms than did a platform 

toppled in place. 

1.1 Past Research 

There have been several surveys of fish communities conducted around oil and gas platforms; 

the extent of this work was reviewed by Stanley and Wilson (2000) and is summarized below. 

Scientific investigations of fish assemblages at platforms did not start until the mid 1970's. They 

consisted of visual surveys performed with SCUBA divers, with remotely operated underwater 

vehicles (ROV), and with stationary cameras. The majority of these projects were short term 

(Sonnier et al. 1976; Gallaway et al. 1981; Continental Shelf Associates 1982; Gallaway and 

Lewbel 1982; Putt 1982). The results of this early research described the fish density and 

species composition found at each platform, and how they varied among platforms, water depths, 

and times of year. The results however are difficult to compare due to sampling problems 

associated with limited visibility, gear bias, diver avoidance by fish, and a lack of standardized 

survey methodology. Although visual surveys are the method of choice to survey natural and 

artificial reefs (Bortone and Kimmel 1991), the presence of SCUBA divers affects the density 

and possibly the species composition of fishes at a site (Sale and Douglas 1981; Brock 1982; 

Bohnsack and Bannerot 1986; Stanley and Wilson 1995). 

The composition of the platform fish community has also been shown to change with location 

and water depth. Previous researchers have noted a zonation of the assemblage of fishes 

associated with platforms and have divided the shelf waters of the northern GOM into three 

zones: coastal (water depth < 27 m), offshore (water depth 27 to 64 m) and bluewater (water 

depth > 64 m) (Gallaway 1980; Gallaway et al.1981; Gallaway and Lewbel 1982). This was 

more recently confirmed by Stanley and Wilson (2000), who reported that, at a platform in 219 

m of water, pelagic bluewater fishes were found principally in the upper 100 m of the water 

column and fish density was essentially zero below 100 m depth. At a platform situated at a 

water depth of 60m, they found fishes throughout the water column, but fish density was highest 

near the surface and adjacent to the bottom; the fish community was characterized as reefassociated shelf species. At a nearshore platform at 20 m depth, fishes were found throughout 

the water column and the population was characterized as estuarine. 

Several investigators evaluated and established guidelines for using acoustics to survey platform 

communities. Gerlotto et al. (1989) demonstrated that towed hydroacoustics could be used to 

measure fish densities near petroleum platforms off Cameroon. Scientists at Louisiana State 

University’s Coastal Fisheries Institute combined visual surveys and quantitative dual beam 

hydroacoustics to document the assemblage of fishes associated with four petroleum platforms in 

the northern Gulf of Mexico (Wilson and Stanley 1991; Stanley and Wilson 1995, 1996a, 1997, 

2000; Wilson et al 2003). 

Regardless of methodology used, investigators have consistently reported that platforms harbor 

large numbers of fishes, and that abundances and species compositions vary greatly temporally 

and with proximity to structure (Sonnier et al. 1976; Continental Shelf Associates 1982; 

Gallaway and Lewbel 1982; Putt 1982; Stanley and Wilson 1996b, 1997, 2000). Gerlotto et al. 
3

(1989) reported that fish densities were 5 - 50 times higher immediately adjacent to a platform 

than 50 m away. Stanley and Wilson (1996a, 1997) reported that fish densities were 3-25 times 

higher within 16m of a standing platform on the continental shelf. Stanley and Wilson (1997) 

reported that at 30 m away from a platform, fish densities were comparable to the open waters of 

the northern GOM. Long-term studies have also reported that fish populations at platforms are 

highly variable through time. Putt (1982) reported that fish density varied by a factor of two 

from month to month, while Stanley and Wilson (1996a, 1997, 2000) reported that monthly and 

seasonal abundances varied by up to a factor of five. 

Wilson et al (2003) conducted hydroacoustic and ROV surveys of the West Flower Garden Bank 

(WFGB) and compared fish abundance and composition at this natural reef to that found at 

nearby artificial reefs and a platform. They reported three distinct reef fish assemblages at the 

WFGB in association with three major biotic zones (coral reef, algal-sponge, and drowned reef) 

(Dennis and Bright 1988). Based on hydroacoustics, fish biomass (biomass), was highest over 

the shallowest portion of the WFGB (upper terrace) where biomass was similar to that found at a 

nearby artificial reef. Biomass was, however, not as high as that found at a nearby operating oil 

and gas platform . Species composition associated with the upper terrace of the WFGB was 

similar to that observed on the outer slope of Caribbean coral reefs, but the WFGB exhibited a 

much lower diversity (253 primary species reported at Caribbean reefs versus 84 found at the 

WFGB). The fish community along the upper terrace was also different than that at nearby 

artificial reefs or operating oil and gas platforms. Wilson et al (2003) pointed out that the low 

vertical structure of fish habitat found at the WFGB is substantially different from the extreme 

high-relief vertical profile found at a platform. 

1.2 Goals of This Study 

The importance of standing platforms in holding significant reef fish populations has been well 

established. However, we lack an understanding of how the fish community varies along the 

longitudinal and latitudinal range of platforms in the northern GOM. Additionally, there is little 

research comparing the fish communities at artificial and natural reefs, offering similar vertical 

relief. This research project was designed to assist MMS, the National Marine Fisheries Service, 

and the artificial reef programs of Louisiana and Texas in making decisions regarding the 

deployment and placement of platforms as artificial reefs, especially with respect to deep water 

artificial reefs. 

We proposed to examine the affects of geographic location and water depth on the abundance 

and species composition of fishes associated with petroleum platforms in the northern GOM. In 

particular, we sought to determine how the fish community around platforms changes along a 

longitudinal gradient traversing the northern GOM continental shelf and slope. We also wanted 

to pinpoint the depth at which the fish community became uncoupled from the benthic substrate 

and fishes associated with the structure in the upper water column. Our null hypothesis was that 

fish communities do not vary significantly along a longitudinal gradient in the northern GOM. 

Our plan was to continue to build upon our platform based hydroacoustic research in the 

northern GOM by describing the fish community associated with a total of 15 platforms of 

similar size and age. The study sites included five sites east of the Mississippi River (Eastern), 

five in the north central GOM (Central), and five in the northwestern GOM (Western). Each of 
4

five platforms were to be sampled twice annually; platforms were to range in water depths from 

30 to 200 m at intervals of approximately 40 m. The reality was that platform placement did not 

accommodate the intended design of the project. The lack of platforms off Texas forced us to 

reduce the number of platforms visited on the Western GOM to two, but afforded us the 

opportunity to add mobile hydroacoustic and ROV surveys of the Sonnier Bank pinnacle 

complex to our study. 

The goals of this study were therefore to: 1) identify species composition at each site, 2) estimate 

(acoustic) biomass and target size distribution associated with each site, 3) determine the effects 

of orientation, depth, and distance away from each site on biomass, 4) determine if there is a 

pattern or trend in fish community composition and abundance related to region and/or depth, 

and 5) to determine whether a natural reef having an extreme vertical profile supports a fish 

community similar to that found at an artificial reef of comparable vertical relief. 

This report is presented in four sections. The following Materials and Methods (Section 2) gives 

site descriptions and explains the equipment, site specific sampling design, and acoustic and 

statistical analysis. Results (Section 3), are separated into the results for the standing platforms 

and Sonnier Bank. A collective Discussion (Section 4) is used to compare and synthesize the 

results. 
5

2.0 METHODS AND MATERIALS 

2.1 Site Descriptions 

This research project was designed to effectively sample and compare the fish populations 

associated with standing oil and gas platforms located within each of three regions of the 

northern Gulf of Mexico: an eastern region including platforms east of the Mississippi River, a 

central region including platforms off Louisiana, and a western region including platforms off 

southeastern Texas (Figure 1). The platforms in each of these regions were located at water 

depths ranging from 30 m to over 200 m with the objective of selecting one platform in each of 

five increasing depth zones. Once the study had begun, it was decided to examine two additional 

sites: a natural reef with vertical characteristics similar to a platform (Sonnier Bank) and an 

nearby oil platform (VR 279) at the same water depth. All sampling was variously conducted 

from 1998 to 2003 beginning with the eastern region (Table1). 

2.1.1 Eastern Region 

Platforms selected in the eastern region included; Viosca Knoll 203 (VK 203) (located 

29.7815°N and 88.3331°W, water depth 37.2 m, operated by Murphy Oil Inc.), Main Pass 142 

(MP 142) (located 29.2872°N and 88.2872°W, water depth 61.1 m, operated by Chevron), Main 

Pass 289 (MP 289) (located 29.2585°N and 88.4415°W, water depth 97.5 m, operated by Shell 

Oil), Main Pass 259 (MP 259) (located 29.3257°N and 88.0201°W, water depth 119.5 m, 

operated by SOCO Inc.), South Pass 89 (SP 89) (located 28.7031°N and 89.3910°W, water depth 

139.0 m, operated by Exxon Inc.) (Table 1, Figure 1). 

2.1.2 Central Region 

Platforms selected in the central region included five, according to original survey design plus 

one (VR 279) that was added because of its proximity to Sonnier Bank. The VR 279 platform 

allowed the comparison of fish population composition between a platform and nearby natural 

reef system. The platforms in this region were: Vermillion 214 (VR 214) (located 28.6963°N 

and 92.2623°W, water depth 38.7 m, operated by Chevron), Vermillion 279 (VR 279) (located 

28.4495°N and 92.5099°W, water depth 54.9 m, operated by W&T Offshore, Inc.), West 

Cameron 564 (WC 564) (located 28.2699°N and 93.3433°W, water depth 57.3 m, operated by 

Chevron), High Island A350 (HI A350) (located 28.0188°N and 93.4585°W, water depth 93.0 m, 

operated by Shell Oil), West Cameron 643 (WC 643) (located 27.9814°N and 93.0342°W, water 

depth 114.3 m, operated by Texaco), and Garden Banks 236 (GB 236) (located 27.7611°N and 

93.1377°W, water depth 208.8 m, operated by Chevron) (Table 1, Figure 1). 
6

Figure 1. Overview map of study platforms, regions, and location of Sonnier Bank. Graded color background indicate

elevation. 
7

Table 1 

General Information about the Oil and Gas Structures Surveyed with Stationary Dual Beam 

Hydroacoustics. 

Site 

Visit 

(mm/yy) 

Region Depth 

(m) Installed Piles Conductors Latitude(N) Longitude(W) 

VK 203 

05/98, 

01/99 

Eastern 37.2 1993 6 3 29.7815 88.3331 

MP 142 07/98 Eastern 61.3 1969 4 0 29.2872 88.2872 

MP 289 

07/98, 

12/98 

Eastern 97.5 1968 6 17 29.2585 88.4415 

MP 259 

06/98, 

12/98 

Eastern 119.5 1994 6 10 29.3257 88.0201 

SP 89 

05/98, 

10/98 

Eastern 139.0 1978 8 8 28.7031 89.3910 

VR 214 

01/00, 

04/00 

Central 38.7 1971 4 15 28.6963 92.2623 

VR 279 06/03 Central 54.9 1996 4 10 28.4495 92.5099 

WC 564 05/99 Central 57.3 1976 4 5 28.2699 93.3433 

HI A350 06/99 Central 93.0 1976 8 20 28.0188 93.4585 

WC 643 

03/00, 

02/01 

Central 114.3 1975 8 16 27.9814 93.0342 

GB 236 

05/99, 

02/99 

Central 208.8 1980 8 13 27.7611 93.1377 

MI 703 2002 Western 39.0 1981 8 5 27.8956 96.4280 

MU 787 2002 Western 46.0 1993 4 8 27.5772 96.7080 
8

2.1.3 Western Region 

We had intended to select five platforms in the western region, but a lack of suitable platforms in 

the desired depth ranges limited the western region to only two sites. This was when the 

decision to survey Sonnier Bank and an adjacent platform (VR 279) was made. The platforms in 

this region are Matagorda Island 703 (MI 703) (located 27.8956°N and 96.4280°W, water depth 

39 m, operated by British Petroleum), and Mustang Island 787 (MU 787) (located 27.5772°N 

and 96.7080°W, water depth 46 m, operated by British Petroleum) (Table 1). 

2.1.4 Sonnier Bank 

Sonnier Bank is located approximately 250 km southeast of Galveston, TX, near the edge of the 

continental shelf at 28.3305°N and 92.4659°W (Figure 2). The physical structure of the reef 

complex is unique for this region of the northern Gulf of Mexico. The main body of the reef 

system is comprised of five steeply sloped pinnacle features whose profile and scale resemble a 

standing platform that comes to within 20 m of the ocean surface. At 30m below the water 

surface, the footprints of artificial reefs created by both partially removed, and toppled platforms, 

are similar to that found at Sonnier (Figure 3). Below this depth, the Sonnier Bank pinnacles 

spread out more toward the bottom. 

2.2 Survey Designs 

Dual beam hydroacoustic surveys were conducted with three stationary arrays of four 

transducers at the standing platforms. At Sonnier Bank, a mobile acoustics survey was 

conducted with a single transducer mounted on a v-fin tow body (towfish). The arrays at the 

standing platforms were composed of upward-oriented, downward-oriented, and outwardoriented (horizontal) transducers at each site after Stanley and Wilson (1997). 

2.2.1 Standing Platforms 

The standing platforms were surveyed with stationary dual beam hydroacoustic equipment 

developed during past research (Wilson and Stanley 1991; Stanley and Wilson 1995, 1996b, 

1997, 1998). Array 1 (Figure 4a) was designed to measure in situ target strength distribution and 

density of fishes immediately adjacent to each side of the platform. It consisted of four 

downward-oriented transducers (120 kHz) placed approximately 5 m below the surface, one on 

each side of the platform. Array 2 (Figure 4b) was designed to measure the density of the fish 

within the upper portion of the water column. This array utilized four upward-oriented 

transducers suspended on heavy monofilament line 20 m below the waterline. Array 3 (Figure 

4c) consisted of four transducers oriented horizontally at 20 m water depth with each transducer 

facing away from the platform. The downward and upward oriented transducers provided 

acoustic coverage from a depth of 1 m to within 0.5 m of the substrate. The outward-oriented 

transducers allowed the simultaneous measurement of fish biomass and abundance out to 80 m 

away from the platform. Stationary hydroacoustics allows for the collection of many samples or 

"snapshots" of density and size distribution over time; each “snapshot” consisted of 5 minute 

blocks of acoustic data. Previous analysis of acoustic data in segments from 1 to 15 minutes has 

shown that variances of density estimates are not significantly different after a five-minute 

duration. 
9

Figure 2. Bathymetry of Sonnier Bank. Top inset shows the overview location of Sonnier 

Bank. The bottom inset shows the proximity of the nearest study platform (VR 

279) with respect to Sonnier Bank. 
10

Figure 3. Figure shows the footprint of the Sonnier Bank pinnacles (black areas) compared 

to the footprints of a partially removed platform (top grey rectangle) and toppled 

platform (bottom grey rectangle) at the same depth below surface (30m). Also 

pictured is the low-relief feature in the southwest corner. 
11

Figure 4. Configuration of hydroacoustic beams. Note that platform and beam dimensions 

are to scale. Water surface is indicated with a semi transparent green plane. 
12

Acoustic sampling was accomplished during four two-hour intervals distributed over the solar 

day (dawn, noon, dusk, and midnight). Within each interval, data were collected in 15 minutes 

segments with a twice repeated progression through the transducers on each side (north, west, 

south, and east) of the platform. Each 15 minute segment was further divided into three subsegments of five minutes each resulting in six observation periods per side per sampling interval. 

Up and down vertical data were pooled in 10 m depth strata from the surface to the substrate; 

horizontal data were pooled in 10 m distance strata from the platform out to 70 m. We made no 

attempt to acoustically sample within the underwater confines of the platform itself due to the 

interference of the metal piles and cross members. 

2.2.2 Sonnier Bank 

A towed transducer (Wilson et al. 2003) was used to estimate the acoustic biomass,density and 

size frequency distribution of fishes associated with the natural reef site during the summer of 

2002. A 120 kHz downward-oriented transducer was towed from the stern of the research vessel 

M/V Spree. The towfish was towed at approximately 2 m/sec, 5 m out from the stern and 3 m 

below the surface to keep the transducer out of the prop wash. The use of a towed downwardoriented transducer enabled the calculation of density and size distribution of fishes from a depth 

of 5 m to 1 m above the bottom. Navigational data were collected with a Garmin V Plus global 

positioning system (GPS); the GPS antenna was held above the towfish with a telescoping mast. 

The navigation data stream, which updated once per second, was incorporated into the acoustic 

data string, and saved on a laptop computer. 

The survey design for the Sonnier Bank reef complex was divided into three separate sections 

that were completed over a four day period (Figure 5). The entire Sonnier Bank complex was 

first surveyed with 22 north-south transects spaced 200 m apart. A small low-relief feature in the 

southwest region of the complex was surveyed with 24 north-south transects spaced 40 m apart. 

The high-relief pinnacle series on the east side of the complex was surveyed with 28 north-south 

transects spaced 40 m apart. The tighter line spacing over the two special interest regions of the 

reef provided much finer detail for resolving bottom relief and for providing better estimates of 

acoustic biomass over each targeted habitat. 
13

Figure 5. Overview of Sonnier Banks bathymetry intended track lines of the hydroacoustic 

survey conducted in May 2002. 
14

2.3 Acoustic Data Collection and Processing 

Acoustic data were collected with a Biosonics model DT5000 scientific echosounder following 

the methods outlined in Wilson et al (2003). All data were collected with 120 kHz transducers 

factory calibrated with a - 42 dB tungsten sphere. The source level of the acoustic system was 

223 dB/Pa at 1 m; acoustic sampling rate was five pings/sec with a pulse duration of 0.4 msec. 

Received signals, adjusted for spreading loss by applying a 40 log R time varied gain, were 

digitized and recorded on a computer hard drive and later transferred to CD digital media. The 

data collection minimum and maximum thresholds were set to -55 dB and -20 dB, respectively, 

corresponding to a 2.9 cm minimum and a 1.9 m maximum length, based on a dorsal perspective, 

for target detection (Love 1971). 

Perhaps the most important development, other than improvements in the data collection 

equipment, has been the evolution of sophisticated hydroacoustic analysis packages that enable 

the user to manipulate many variables during post-processing. In the late 1990’s an innovative 

advance in analysis software, Echoview, which has been described as revolutionizing 

hydroacoustic data analysis (pers. comm., I. Higginbottom, Biosonics, Inc., 2003), became 

available. The most recent version of this analysis package, Echoview 3.10, allows one to extract 

a variety of target specific variables resulting in improved acoustic biomass and single target size 

estimates. Echoview also permits removal of acoustic noise, clear separation of the acoustic 

bottom from fish targets, and access to powerful visualization techniques. 

Digitized hydroacoustic data collected at each survey site were processed with Echoview 3.10 

(Sonar Data, Pty Ltd.). The factory settings for Echoview are intended for use with data 

generated with a 38 kHz single beam transducer, not the 120 kHz transducers that we employed. 

Several adjustments in the Echoview programming were necessary for proper handling and 

analysis of our data. The first step in processing the data files was to enter both the correct 

transducer frequency (120 kHz for this study) and the environmental parameters, such as water 

temperature and salinity, at each site into the speed of sound calculator to produce the corrected 

speed of sound and absorption coefficient. The appropriate calibration offset, transmitted pulse 

length, frequency, and two-way beam angle (Table 2) were input for calibrating echo positions. 

The proper values for these variables are critical for correct placement of echo returns in the 

beam; changes in speed of sound, for example, will shift the position of a target returning an 

echo nearer to or farther from the transducer, thus affecting either vertical or horizontal spacing 

of the targets. 

A grid was applied to the data establishing 5 min by 10 m bins (or strata) that were used for all 

subsequent analyses. Parameters for color display minimum and for color display range were set 

to match the data collection threshold (–55 dB for this study). The echogram display limits, 

corresponding to the water depth at each site for the downward-oriented transducers and the 

maximum range desired for the horizontal and upward-oriented transducers were specified The 

minimum analysis threshold was set to the minimum collection threshold (–55dB), the maximum 

analysis threshold was set to the maximum collection threshold (–20dB), and the time-varied 

threshold (TVT) was adjusted to reduce as much noise from the far edge of the data record as 

was possible (Table 2). The time-varied threshold is specific for each study site as the amount of 

noise affecting data quality is directly influenced by the varying environmental conditions among 

sites. 
15

Table 2 

Parameter Settings for Echoview Analysis. 

Site Visit 

Temp 

(

o

C) 

Salinity 

(ppt) 

Absorption 

Coefficient 

Speed of 

Sound 

(m sec -1) 

TimeVaried 

Threshold 

(dB) 

VK 203 1 29 34 0.046545 1523.05 -87 

VK 203 2 18 28 0.040584 1510.52 -90 

MP 259 1 28 34 0.046151 1535.57 -84 

MP 259 2 22 34 0.046692 1525.7 0 

MP 289 1 28 34 0.045288 1540.08 -100 

MP 289 2 22 34 0.046692 1525.7 0 

MP 142 1 28 34 0.045288 1540.08 -95 

SP 89 1 26 34 0.047124 1518.67 -90 

SP 89 2 28 34 0.045288 1540.08 -100 

GB 236 1 26 36 0.047427 1536.64 0 

GB 236 2 22 36 0.045199 1509.81 0 

WC 564 1 27 35 0.047033 1538.92 -90 

WC 643 1 19 36 0.047124 1518.67 0 

WC 643 2 18 34 0.045031 1525.3 0 

VR 214 1 19 36 0.047124 1518.67 0 

VR 214 2 21 36 0.0478 1526 0 

HI A350 1 27 35 0.047033 1538.92 0 

MU 787 1 19 36 0.047124 1518.67 -100 

MI 703 1 19 36 0.047124 1518.67 -95 

VR 279 1 18 34 0.045031 1514.64 -91 
16

Data files were first played through Echoview with the bottom tracking feature turned on; the 

bottom exclusions were then visually inspected and adjusted to ensure the bottom and platform 

legs were not included in the analyzer window. After the bottom had been edited, each echogram 

was scrutinized for “bad data” such as mid-water platform inclusions, air entrainment, and any 

other non-biological returns (e.g. gas seeps). All of these areas were manually identified, 

designated as “bad data” regions, and excluded from data analysis and echo integration. 

Software updates and new features for Echoview are released frequently, allowing users to 

remain on the cutting edge of hydroacoustic data analysis and presentation. Echoview has been 

used previously to estimate single target sizes (Higginbottom et al. 2000, Lawson et al 2001, Abe 

et al. 2002, Sellers 2003, Goss et al. 2004), to discriminate aggregated fish species and single 

targets (Lawson et al. 2001, Logerwell and Wilson 2002, Woodd-Walkera et al. 2002, Churnside 

et al. 2003, Gerlotto et al. 2004,), and to visualize fish movement (Higginbottom et al. 2000, Abe 

et al. 2002, Buelens et al. 2003). 

The many output variables generated in Echoview make it one of the most versatile and powerful 

software packages available for analysis of hydroacoustic data. Two primary outputs are 

routinely used for statistical analysis: 1) mean volume backscatter (Sv), reported herein as 

dB/m3, commonly used as an approximation of fish biomass and 2) target strength (TS), an 

acoustic measure (in dB) of target size, generally used to derive estimates of fish length. Target 

strength values are generated for targets that are identified and tracked for at least six 

consecutive pings with the single target detection algorithm. Both Sv and TS are logarithmic in 

their scales and as such are not directly amenable to the calculation of mean values; thus each is 

converted to an arithmetic, or linear, form prior to such manipulations. Fish energy (FE), the 

linear counterpart of Sv, is converted with the equation 

 FE = 10 Sv/10 (Eqn 1). 

Similarly, sigma, the linear equivalent of TS, is calculated with the equation 

 sigma = 10 TS/10 (Eqn 2). 

Once either a mean sigma or a mean FE has been calculated, this number can then be converted 

back to yield mean TS and mean Sv, respectively. 

Fish density was calculated as the mean Sv (from mean FE) of a known volume (m3

) of water 

divided by the mean TS (from mean sigma) of that same volume of water. Fish density is 

calculated with the equation: 

 Fish density (# m-3) = 10 Sv/10 /10 TS/10 (Eqn 3). 

Density estimates are dependent on accurate estimates of the mean TS of the targets. Analyses 

with Echoview version 3.10 of data from fixed transducers at standing platforms produced useful 

density estimates; however, analyses of the towed transducer data from Sonnier Bank required 

modification for accurate density comparisons. The accuracy of mean TS increases with repeated 

acoustic samples of individual targets (pers. Comm.., J. Dawson, Biosonics Inc., 2003). Given 

that the Sonnier Bank surveys were based on a vessel moving at 2 m/sec and transmitting 

acoustic pulses (pings) at a rate of five pings/sec, the probability of hitting a target multiple times 
17

was very low. In many cases data collection and subsequent processing produced a Sv, but no 

TS, for the same volume of water. In such cases a mean sigma was calculated for a specific 

linear distance within a stratum and used as a proxy for TS to calculate densities in those cells 

where TS was lacking. The assumption was that mean fish sizes within several adjacent cells 

within the same stratum would be roughly equivalent. 

2.4 Data Analysis 

2.4.1 Statistical Analysis 

Split-plot randomized block design ANOVA (SAS Institute Inc. 2002) were performed to test for 

differences among acoustic variables including Sv (fish biomass), TS (fish numbers), and fish 

density. We used the SAS software procedure Proc Mix, which has several advantages over 

earlier software, to run the analyses. In addition to providing better estimates for random 

components, it provides the investigator with other options like the ability to test for, and to fit, 

non-homogeneous variances and to specify covariance structures. 

The dependent variable, FE (converted from Sv as above), was log transformed (ln (fish energy x 

109

+ 1)) to allow for the high number of observations in which Sv was zero. Data from the 

standing platforms were initially analyzed with the following class variables: year of survey, 

platform, number of conductors on platform, region (East, Central, and West), time of day 

(TOD; dawn, noon, dusk, and midnight), platform side (north, south, east, and west) ten meter 

depth or distance stratum (strata), dissolved oxygen, and logical interactions. After several trials 

year, region, platform, and dissolved oxygen were dropped from the test model as they reduced 

the degrees of freedom, but did not improve the model. For the Sonnier Bank data class variables 

included depth stratum and a bottom depth threshold (-50 m) indicating over or away from a 

pinnacle. Tukey's standardized range tests (Ott 1982) were used to compare the means of 

significant variables. Statistical tests were reported as significant at α ≤ 0.05. 

In order to compare the effects of class variables on fish biomass, TS, and Sv, we applied 

statistics that generated least square means rather than elementary arithmetic means. In our 

earlier studies we used arithmetic means; however, in unbalanced designs with more than one 

effect (characteristic of acoustic surveys), the arithmetic mean for a group will not accurately 

reflect the "typical" response for that class variable since it does not take other effects into 

account. Given the large number of zero values and the various “effects” included in our final 

models, we used least square means in Proc Mixed of SAS (SAS Institute Inc. 2002) to illustrate 

the responses observed in our analysis. Thus those statistical comparisons of biomass, TS, and 

Sv illustrated in the various figures herein are based on least square means generated with the 

above procedure. This is confounding as it constrains our ability to compare and illustrate 

differences among our earlier studies (Stanley and Wilson 2000, Wilson et al. 2003) and this 

study. We therefore confine comparative conclusions to habitats considered in this study. 

ANOVA, as above, was also used to analyze TS (as sigma) at all sites. Class variables used in 

the analysis of standing platforms were the same as those stated above and they too were reduced 

down to region, time of day, platform side, ten meter depth or distance stratum (strata), and 

logical interactions. Sonnier Bank class variables included depth stratum and a bottom depth 

threshold (-50m) indicating over or away from a pinnacle (bump). Tukey's standardized range 
18

tests (Ott 1982) were used to compare the means of significant variables. Statistical tests were 

reported as significant at α ≤ 0.05. 

2.4.2 Fish Abundance Estimation 

The total fish abundance estimates were calculated for comparison to previous studies. Total 

abundance at each platform was calculated by determining a 20 m near-field area of influence of 

each reef site or platform, then multiplying mean density values by stratum and side by the 

volume of water on each side of or over the platform (Stanley and Wilson 1998). Fish density 

within the center of each platform was not measured with acoustics due to interference by 

structural members and was assumed to be the average of the density estimates of the four sides 

of the platform. Fish abundance in the center of the platform was calculated by multiplying the 

estimated fish density of the center by the volume of water in the center of the platform. Fish 

abundance estimates at Sonnier Bank were based on the average density by stratum over the 

whole area and separately at each pinnacle assuming a 20 m area of influence, although that is an 

arbitrary area of influence used here solely for comparative purposes. 

2.5 Visual Surveys 

Visual surveys were conducted with a Deep Ocean Engineering Phantom HD2 ROV or a 

VideoRay Pro II ROV with standard visual census techniques and recording video on S-VHS or 

digital tape with the method described by Bohnsack and Bannerot (1986) and previously 

reported by Stanley and Wilson (1998) and Wilson et al. (2003). Cryptic fishes (fish length < 

10 cm) that stayed within half a meter of either the bottom or the platform were not included in 

the video surveys results as could not be accurately assessed in the acoustic surveys. Point 

counts were conducted to identify individual fish to the lowest taxonomic level. Results and data 

were expressed as percent composition by species and stratum at each site. At standing 

platforms, the ROV was deployed from the water surface to the bottom, stopping every 10 m for 

approximately 5 minutes. 

During the Sonnier Bank survey, the ROV was deployed from the M/V Spree. The ROV was 

deployed to dive and capture video of the species composition associated with the two most 

distinctive habitat types found in the Sonnier Bank system. The vessel was moored to the highrelief pinnacle feature during ROV deployment at this site. At the deeper, low-relief feature, the 

vessel could not be moored and was allowed to drift during ROV deployment. During the 

survey, the ROV would descend to the bottom, stopping every 10m for approximately 1 minute. 

Once on the bottom the ROV was flown approximately 40m along random transects along these 

features and the surrounding natural bottom. 

2.6 Environmental Data 

Environmental data collected in the East and part of the Central regions consisted of temperature 

(+/- 0.1 oC), salinity (+/- 0.1 ppt) and dissolved oxygen (+/- 0.5 ppm); parameters were measured 

with a Seabird SBE 19 meter. Current speed (+/- 0.1 m/s) and direction (+/- 1 degree) were 

measured an InterOcean S4 meter during each hydroacoustic data collection period for the East 

region only. Subsequent discussions with other users of the S4 current meters and review of 

InterOcean literature concerning deployment of the S4 in environments with large amounts of 
19

ferrous metal or magnetically impeded regions led us to stop using it due to reported inaccuracies 

caused by metal in the data reception field. The recommended deployment technique for this 

type of environment suggests that the S4 should be rigidly mounted to or near the structure and 

calibrated for the magnetic interference at that particular site. We did not have the ability to 

provide this type of mounting system for such a dynamic survey. We deployed the S4 from a 

temporary winch, lowering the unit through the water column where it was allowed to swing and 

spin freely adjacent to the steel platform. These data were not addressed in this analysis.
21 

3.0 RESULTS 

3.1 Field Operations 

During the course of this study, three or four scientists participated in 23 excursions to platforms 

off the Louisiana and Texas coastlines and spent up to five days at each location. Technology 

advancements in the acoustic equipment and improvements in ROV technology allowed us to 

reduce both the amount of gear required for sampling and the effort required for deploying the 

sampling equipment. By the end of this project our four transducer hydroacoustic system and 

ROV could be housed in a single 6 x 6 x 6 foot aluminum container designed for lifting by crane 

or forklift. 

Transducer deployment at platforms has also become easier over time. As the project progressed 

the technique of deploying the transducer system evolved to improve safety and data quality as 

well as to reduce the manpower required for data collection. The original method of deployment 

involved large aluminum frames suspended from the platform that held the transducers in such a 

fashion that allowed the transducer to fire upward through the center of the frame without 

interference. Though effective, this method was very cumbersome and potentially hazardous to 

deploy, especially in rough weather. This evolved to a heavy, single point monofilament bridle 

that allowed the transducer to be lowered easily, accurately, and safely. With the use of the 

acoustically invisible monofilament, the technique evolved further to use a small hand winch to 

raise and lower each transducer. 

The original method to deploy the downward and horizontal transducers involved the use of as 

many as 16 four-foot sections of aluminum pipe for each of the four transducers. The gear had 

to be assembled and moved manually with each deployment. This method required no less than 

four people to deploy and was often plagued by poor data quality due to excessive transducer 

motion when sea conditions got rough. This led to a modification of the upward-oriented 

deployment method, hand winches and single stainless steel cable suspension systems. The 

transducers were attached to lead ballasted base plates that were attached to four point cable 

bridles. These bridles proved to be crucial as they allowed the angle of the transducers to be 

adjusted to match the angle of the legs of the platform before they were lowered into the water. 

This provided an optimal acoustic view throughout the entire water column. The advantage of 

the system was that it provided a much smaller cross-section in the wave zone, so it had the 

effect of de-coupling the wave and surge motion from the transducer. This was evident 

immediately when viewing the echograms: the return from the bottom was now a smooth line 

instead of a wavy line caused by the sea surface motion. The use of the hand winch system also 

reduced the required number of persons for safe deployment from four to three. The deployment 

and retrieval in an emergency could be handled by as few as two people. 
22 

One of our original objectives was to collect current data during acoustic sampling. During the 

first year of the project we had intermittent success with the S4 current meter. Although 

frequently collected, the data were influenced by the large metal legs and cross members of the 

platform; subsequent discussions with a physical oceanographer (Dr. Bjorn Kjerve, Texas A&M 

University) confirmed this type of current meter is subject to the influence of large metal objects 

and likely would not be dependable for data collection around platforms. We therefore 

discontinued data collection using the S4 current meter and decided not to incorporate the current 

data collected during the first year of the project. 

3.2 Platform Results 

The long-term data set, which included acoustic data from up to two trips to each of 12 different 

platforms, was pooled for statistical analysis as our primary objective was to determine if there 

were effects of latitude and longitude on the fish communities around platforms. Variables 

included region, water depth of platform, time of day, month, year, platform side, conductor 

number, piles, and depth stratum in 10 m intervals and their logical interactions. Analyses of the 

data using a split plot analysis of variance (SAS Institute Inc. 2002) were conducted iteratively 

using Proc Mixed by removing insignificant variables. Runs were continued until the Akaike 

Information Criteria (AIC) value was lowest. The dependent variables in the data analysis 

included volume backscatter (Sv), density, and target strength (TS). 

The vertical and horizontal data sets were analyzed separately. The vertical data set was used to 

analyze the effect of various parameters on acoustic fish density and biomass and acoustic target 

size adjacent to the platform. The horizontal data set was used to analyze the effect of distance 

from a platform on the resident fish communities. 

3.2.1 Acoustic Volume Backscatter (Fish Energy) 

Volume backscatter (Sv) is a measurement of the amount of reflected acoustic energy that is 

returned to a transducer. It is a proxy of fish biomass m-3. For statistical analysis and 

comparison to previous studies Sv was converted to fish energy using Equation 2 in Materials 

and Methods. For graphical purposes, fish energy means were calculated and then converted 

back to Sv for display. 

Volume backscatter ranged from -100 (default for 0) to -32 dB over the course of study. 

Following several iterations of analysis of variance, we determined that location off the coast 

(East, Central, or West), depth stratum, platform side, and time of day were significant at 

influencing the changes observed in Sv. Platform age was not significant; although number of 

conductors and platform depth were significant, removing them from the model reduced the AIC 

coefficient (Table 3). We surmised that this was likely due to the fact that they were confounded 

by platform location and depth stratum. 

Throughout the study, we found a general trend of lower acoustic biomass east of the Mississippi 

River and off of Texas. The highest biomass was found in the central GOM (Figure 6a, 7). Due 

to large variability in the data set the central and western area estimates of acoustic biomass 

were, however, not significantly different. We found a decrease in fish biomass with increasing 

water depths. When examining all platforms together, the highest biomass was in the top 50 m 
23 

of the water column with the lowest biomass occurring below 50 m (Figure 6b). Fish biomass 

also varies significantly with platform side and time of day. Acoustic fish biomass was highest 

on the west side of the platforms and lowest on the north side (Figure 6c). Fish biomass was 

lowest at noon, increasing through midnight when it was highest, and then dropped off at dawn 

(Figure 6d). 

Depth elicited the largest range of variation in acoustic biomass. The range of variation for 

acoustic biomass due to variation in location and time of day were approximately equal, and the 

range of variation was lowest due to side of platform (Figure 8). 

Table 3 

Results of Split Plot Design Analysis of Variance Showing the Effect of Region (Location), 

Time of Day (TOD), Platform Side (Side), and Depth Stratum (Stratum) on the Dependent 

Variable Fish Energy 

Effect Num DF Den DF F Value Pr > F

Location 2 36 5.39 0.0089

TOD 3 27000 205.25 <.0001

Side 3 27000 15.51 <.0001

Stratum 10 36 8.66 <.0001

Type 3 Tests of Fixed Effects
Figure 6. LS mean volume backscatter (Sv in dB ± 1 SE) by a) region, b) depth, c) side and d) time of day for platform

northern Gulf of Mexico. Means with same letters are not significantly different at α= 0.005. 

c) d) 

a) b) 

24 
25 

Figure 7. Least squares mean volume backscatter (Sv) at all study locations. Note that data 

from Sonnier Bank are included. 

Figure 8. Range of variation in acoustic biomass at all study platforms due to depth, 

location, time of day, and side. 
26 

3.2.2 Density Estimates 

Fish density is a gross approximation of the number of fish per cubic meter based on the average 

reflected acoustic energy (Sv) over a sampling period divided by the average target strength (TS) 

encountered over that same five minute block of time. Throughout the study, fish density ranged 

from zero to over eight fish per cubic meter. Fish density varied with time of day, platform side 

and depth stratum, but was not influenced by location (Figure 9a), platform age, number of 

conductors, or number of legs (Table 4), but some shallow water platforms appeared to hold 

larger numbers of fish (Figure10). 

Fish density data followed the general pattern of acoustic biomass and was highest near the 

surface; the overall project average was one fish per 10 cubic meters and approached zero near 

the bottom. Below 50 m fish densities were not significantly different (Figure 9b). Fish density 

also varied with platform side and time of day. Fish density was significantly lower on the north 

side of the platforms than the west, south or east sides (Figure 9c). Fish density was lowest at 

noon and dusk and highest at dawn and midnight (Figure 9d). 

Again, depth revealed the largest range of variation in acoustic biomass. The range of variation 

for acoustic biomass due to variation in time of day and side were approximately equal, and the 

range of variation was lowest due to location (Figure 11). 

Table 4. 

Results of Split Plot Design Analysis of Variance Showing the Effect of Region (Location), 

Time of Day (TOD), Platform Side (Side), and Depth Stratum (Stratum) on the Dependent 

Variable Fish Density. 

Effect Num DF Den DF F Value Pr > F

Location 2 15 0.16 0.8565

TOD 3 27000 7.71 <.0001

Side 3 27000 8.16 <.0001

Strata 10 15 4.62 0.0041

Type 3 Tests of Fixed Effects
Figure 9. LS mean fish per cubic meter (± 1 SE) by a) region, b) depth, c) side and d) time of day for platforms in the n

Mexico. Means with same letters are not significantly different at α= 0.005. 

c) d) 

a) b) 

27
28 

Figure 10. Least squares mean fish density (fish m-3) at all study locations. Note that data 

from Sonnier Bank are included. 

Figure 11. Range of variation in fish density at all study platforms due to depth, location, 

time of day, and side. 
29 

3.2.3 Target Strength (TS) 

Target strength is an acoustic measure of fish size. Individual targets are identified in the 

reflection of an acoustic ping and tracked over consecutive pings. Target strengths can vary 

depending on target orientation and are therefore considered to be a rough approximation of 

target size. However, by tracking, measuring, and averaging the acoustic reflection of a target 

over multiple pings we can achieve a reasonable estimate of target size. We used target strength 

as a dependent variable to test for relationships between trends and target size for the 

independent variables outlined above. Throughout the study, target strength ranged from –20 dB 

to –53 dB. Target strength was significantly affected by region, time of day, platform side, and 

depth stratum. 

Mean acoustic target size varied with region, water depth, platform side, and time of day (Table 

5). Fish were smallest along the western region, largest along the central region, and the central 

and eastern regions were very similar (Figure 12a, 13). The few fish encountered at the deeper 

strata were much larger than fish near the surface and there was a general trend of increasing fish 

size with increased water depth although smallest fish size was between 10 and 20 m water depth 

(Figure 12b). All platforms sides exhibited statistically different target strength with the largest 

size occurring on the east side of platforms and the smallest sizes occurring on the north side 

(Figure 12c). Fish size varied slightly with time of day and was largest during the daylight hours 

and smallest at midnight (Figure 12d). Target strength showed the greatest variation with depth 

and location. Time of day and side showed smaller ranges in variability, respectively (Figure 

14). 

An alternate method to examine the spatial distribution of target strength and fish is illustrated in 

Figure 15. Figure 15 is a plot of the target strength data from HI A350 that shows high 

concentration of single targets at 30 and 18 meters water depth. Targets near the surface showed 

a large variation in sizes. Targets at 30 m were dominated by smaller size classes; and targets 

occurring at deeper depths tended to be exclusively larger in size. 

Table 5 

Results of Split Plot Design Analysis of Variance Showing the Effect of Region (Location), 

Time of Day (TOD), Platform Side (Side), and Depth Stratum (Stratum) on the Dependent 

Variable Target Strength. 

Effect Num DF Den DF F Value Pr > F

Location 2 72 3.44 0.0375

TOD 3 500000 2260.42 <.0001

Side 3 500000 175.08 <.0001

Stratum 11 44 26.37 <.0001

Type 3 Tests of Fixed Effects
Figure 12. LS mean target strength (dB ± 1 SE) by a) region, b) depth, c) side and d) time of day for platforms in the n

Mexico. Means with same letters are not significantly different at α= 0.005.

c) b) 

a) b) 

30 
31 

Figure 13. Least squares mean target strength at all study locations. Note that data from 

Sonnier Bank are included. 

Figure 14. Range of variation in target strength at all study platforms due to depth, location, 

time of day, and side. 
32 

Figure 15. Alternate method to examine the spatial distribution of target strength and fish 

at HIA 350. Blue indicates lower occurrence of a given target strength and red 

indicates a higher occurrence of a given target strength. 
33 

3.3 Platform Results - Horizontal 

3.3.1 Volume Backscatter 

 Acoustic reflectivity of targets such as fish can vary greatly depending upon target orientation. 

A vertical aspect, either upward or downward, provides the most meaningful perspective from 

which to infer size and biomass data. The data collected with the horizontal oriented transducers 

are useful for comparative purposes to determine area of influence, relative biomass changes 

with distance from the platform, as well as the effect of side and time of day on the same 

parameters. Acoustic biomass was significantly affected by distance from the platform, transect 

location, side of the platform, and time of day (Table 6). 

Acoustic biomass varied among platforms depending on region: acoustic biomass was highest in 

the central region and lower in the eastern and western regions (Figure 16a). Acoustic biomass 

was highest from 10 to 20 m away from the platforms studied and then dropped off to very low 

levels beyond 70 m (Figure 16b). Although there were no statistical differences, acoustic 

biomass gradually declined from 20 m out to 80 m, which was the limit of our ability to collect 

acoustic data. Acoustic biomass also varied significantly with platform side and was highest on 

the west side and lowest on the south side (Figure 16c). Acoustic biomass varied with time of 

day and was highest at midnight and lowest at noon (Figure 16d). 

Table 6 

Results of Split Plot Design Analysis of Variance Showing the Effect of Region (Location), 

Time of Day (TOD), Platform Side (Side), and Depth Stratum (Stratum) on the Dependent 

Variable Sv for Horizontal Hydroacoustic Data. 

Effect Num DF Den DF F Value Pr > F

Location 2 32 5.23 0.0109

TOD 3 17000 130.77 <.0001

Side 3 17000 70.9 <.0001

Stratum 7 32 6.02 0.0002

Type 3 Tests of Fixed Effects
Figure 16. Horizontal LS mean volume backscatter (Sv in dB ± 1 SE) by a) region, b) depth, c) side and d) time of day 

the northern Gulf of Mexico. Means with same letters are not significantly different at α= 0.005.

c) d) 

a) b) 

34 
35 

3.4 Platform Species Composition and Abundance 

Examination of the ROV video surveys from most sites provides some patterns that can be 

recognized across the GOM and may lend insight into the population structure at platforms in 

various water depths throughout the three major regions of the study area: eastern, central, and 

western. 

Gulf wide, 39 different species were observed by ROV survey. Species richness appeared to be 

higher in the central and western GOM (Figure 17). Shannon-Wiener Diversity Index did not 

show a remarkable pattern with either longitude or distance from shore (Figure 18). 

Consistent with past studies by Stanley and Wilson (2000), we found fish populations to be 

generally dominated by planktivores such as blue runner, Bermuda chub, and Atlantic creolefish, 

These species were always most abundant in the upper water column at depths less than 30 m 

(Tables 7-23). To more carefully examine the patterns of fish functional groups through the 

GOM, we assigned each observed fish species to one of four categories based on its presumed 

role in the functioning of reef ecosystems (Table 7). We then determined the proportion of each 

of these functional groups at each platform based on ROV abundance observations. 

As already noted, planktivores dominated the observed fish abundance at most platforms. In the 

central GOM, however, planktivores appeared to compose a larger proportion of the population 

compared to fish at platforms in either the eastern or western regions (Figure 19). Grazers were 

important at several platforms in each region of the GOM, but they tended to be found at 

platforms that were closer to land or located in shallower depths (Figure 20). In the eastern 

GOM, carnivores and top predators composed a remarkably large proportion of observed fish 

populations (Figure 21, 22). 
36 

Figure 17. Map of species richness observed by ROV surveys at all study platforms and 

visits. Note that results from Sonnier Bank (SONN_SF and SONN_PINN) are 

also included. 

Figure 18. Map of Shannon-Wiener Diversity Index observed by ROV surveys at all study 

platforms and visits. Note that results from Sonnier Bank (SONN_SF and 

SONN_PINN) are also included. 
37 

Table 7 

Fish Functional Group Assignments Based on ROV Survey Observations. 

Planktivores

Carnivores 

(lesser predators) Top predators Grazers

Blue runner Bluefish Greater amberjack Florida pompano

Atlantic spadefish Red snapper Great barracuda Lookdown

Atlantic creolefish Almaco jack Scamp Sheepshead

Bermuda chub Rainbow runner sharks Blue angelfish

Round scad Bar jack Gray triggerfish

Crevalle jack Queen angelfish

Lesser amberjack Spanish hogfish

Cobia

Gray snapper

Vermilion snapper

Horse-eye jack

Rock hind

Lane snapper
38 

Figure 19. Map of the proportion of planktivores at all study platforms and visits. See 

Table 7 for planktivore definition. Note that results from Sonnier Bank 

(SONN_SF and SONN_PINN) are also included. 

Figure 20. Map of the proportion of grazers at all study platforms and visits. See Table 7 

for planktivore definition. Note that results from Sonnier Bank (SONN_SF and 

SONN_PINN) are also included. 
39 

Figure 21. Map of the proportion of carnivores at all study platforms and visits. See Table 7 

for carnivore definition. Note that results from Sonnier Bank are also included. 

Figure 22. Map of the proportion of top predators at all study platforms and visits. See 

Table 7 for top predator definition. Note that results from Sonnier Bank are 

also included. 
Table 8 

Number of Fishes By Species Observed during ROV Video Surveys by Depth at VK 203, Visit 2, 

(This platform lies in the eastern region. Max depth at the platform is 37 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 Total % CompositionBlue runner Caranx crysos 186 146 130 0 462 44.6%Atllantic spadefish Chaetodipterus faber 113 23 48 0 184 17.7%Bluefish Pomatomus saltatrix 33 122 2 0 157 15.1%

Florida pompano Trachinotus carolinus 0 30 89 0 119 11.5%

Red snapper Lutjanus campechanus 0 0 99 4 103 9.9%

Other 1.2%Greater amberjack Seriola dumerili 29 1 0 12 1.2%Total 334 330 369 4 1037 100.0%

40 
Table 9 

Number of Fishes by Species Observed during ROV Video Surveys By Depth at MP 259, Visit 1, 

(This platform lies in the eastern region. Max depth at the platform is 119 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-120 Total % ComGreater amberjack Seriola dumerili 0 26 41 45 4 0 116 32

Almaco jack Seriola rivoliana 0 0 22 48 0 0 70 19

Blue runner Caranx crysos 5 45 000 0 50 13

Atlantic creolefish Paranthias furcifer 0 0 42 00 0 42 11

Bermuda chub Kyphosus sectatrix 0 35 000 0 35 9

Rainbow runner Elagatis bipinnulata 0 0 32 00 0 32 8

Other 4Gray triggerfish Balistes capriscus 2 0 3 00 0 5 1

Great barracuda Sphyraena barracuda 0 2 3 00 0 5 1

Cobia Rachycentron canadum 20 2 0 0 0 4 1Scamp Mycteroperca phenax 00 0 3 0 0 3 0Total 9 108 145 96 4 0 362 10

41 
Table 10 

Number of fishes by Species Observed during ROV Video Surveys by Depth at MP 259, Visit 2, 1

(This platform lies in the eastern region. Max depth at the platform is 119 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-120 Bar jack Caranx ruber 0 38 28 0 0 0 Greater amberjack Seriola dumerili 0 0 0 0 0 33 Great barracuda Sphyraena barracuda 1 1 5 0 4 21 Blue runner Caranx crysos 0 26 0 0 0 0 OtherAlmaco jack Seriola rivoliana 00 0 0 0 7 

Ocean triggerfish Canthidermis sufflamen 00 0 0 0 2 

Total 1 65 33 0 4 63 

42 
43 

Table 11 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at MP 289, Visit 1, 

(This platform lies in the eastern region. Max depth at the platform is 98 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-100 Tot

Bermuda chub Kyphosus sectatrix 0 60 43 0 0 0 0 0 10

Atlantic creolefish Paranthias furcifer 9 4 43 94000 69 

Scamp Mycteroperca phenax 2 2 0 15 10 16 7 5 57 Almaco jack Seriola rivoliana 00 6 8 0107 22 

Crevalle jack Caranx hippos 0 14 000000 14 

Greater amberjack Seriola dumerili 00 2 6 2310 14 

Rainbow runner Elagatis bipinnulata 0 12 000000 12 

Great barracuda Sphyraena barracuda 0 6 220000 10 Lesser amberjack Seriola fasciata 02 0 0 7 000 9 Cobia Rachycentron canadum 03 2 2 1 0 0 0 8 Other Gray snapper Lutjanus griseus 6 0010000 7 Horse-eye jack Caranx latus 0 0 3 00000 3 

Ocean triggerfish Canthidermis sufflamen 00 0 1 2 000 3 

Gray triggerfish Balistes capriscus 00 0 2 0000 2 Remoras Echeneidae 00 0 1 0000 1 Total 17 103 101 47 26 20 8 12 33
44 

Table 12 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at MP 289, Visit 2, 1

(This platform lies in the eastern region. Max depth at the platform is 98 m. Blanks = No Da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-100 Total % Composition

Bermuda chub Kyphosus sectatrix 57 0 0 000 57 30.2%

Greater amberjack Seriola dumerili 08 0 16 14 2 40 21.2%

Great barracuda Sphyraena barracuda 0 8 14 12 1 0 35 18.5%

Almaco jack Seriola rivoliana 0 1 28 000 29 15.3%

Blue runner Caranx crysos 0 0 16 040 20 10.6%

Other 4.2%Scamp Mycteroperca phenax 0 0 4 200 6 3.2%

Gray triggerfish Balistes capriscus 01 0 0 0 0 1 0.5%Sharks 01 0 0 0 0 1 0.5%Total 57 19 62 30 19 2 189 100.0%
Table 13 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at MP 142, Visit 1, 

(This platform lies in the eastern region. Max depth at the platform is 61 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-70 Total % CompoGray Snapper Lutjanus griseus 34 16 3 53 63.1% Lookdown Selene vomer 0 12 0 12 14.3% Sheepshead Archosargus probatocephalus 9 2 0 11 13.1% Blue Angelfish Holacanthus bermudensis 0 4 0 4 4.8% Other 4.8% Great Barracuda Sphyraena barracuda 0 3 0 3 3.6% Gray Triggerfish Balistes capriscus 0 0 1 1 1.2% Total 43 37 4 84 100.0

45
Table 14 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at SP 89, Visit 2, 10

(This platform lies in the eastern region. Max depth at the platform is 139 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-140 To

Atlantic creolefish Paranthias furcifer 15 251 9 629 289 320 0 15

Blue runner Caranx crysos 177 534 255 116 255 5 0 13Vermilion snapper Rhomboplites aurorubens 0 0 0 0 156 318 1 4

Horse-eye jack Caranx latus 0 26 10 115 67 3 0 2

Red snapper Lutjanus campechanus 0 0 2 0 15 107 55 1

Round scad Decapterus punctatus 123 44 0 0 0 0 0 1Other Blackfin tuna Thunnus atlanticus 00 0 0 0 61 0 6Greater amberjack Seriola dumerili 00 1 6 0 21 11 3

Bermuda chub Kyphosus sectatrix 2 33 56000 4

Great barracuda Sphyraena barracuda 2 7 01110 1Gray snapper Lutjanus griseus 16 12 0 9 0 0 0 3

Gag Mycteroperca microlepis 00 0 0 0 0 7 

Gray triggerfish Balistes capriscus 03 0 0 0 5 0 Atllantic spadefish Chaetodipterus faber 00 0 0 2 1 0 Sharks 0 2 00100 

Almaco jack Seriola rivoliana 00 0 0 0 1 1 

Rainbow runner Elagatis bipinnulata 0 2 00000 Total 335 914 282 882 786 843 75 41

46
47 

Table 15 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at VR 214, Visit 2, 

(This platform lies in the central region. Max depth at the platform is 39 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 Total % 

Blue runner Caranx crysos 124 153 22 4 303 Gray snapper Lutjanus griseus 2 20 17 19 58 Red snapper Lutjanus campechanus 009 39 48 

Atlantic spadefish Chaetodipterus faber 4 10 12 14 40 

Gray triggerfish Balistes capriscus 1 4 16 15 36 Bermuda chub Kyphosus sectatrix 28 000 28 Sheepshead Archosargus probatocephalus 055 13 23 

Almaco jack Seriola rivoliana 0 3 8 4 15 Other Lookdown Selene vomer 13 000 13 Blue angelfish Holacanthus bermudensis 000 4 4 Cobia Rachycentron canadum 0 0 4 0 4 Blue tang Acanthurus coeruleus 0 2 0 1 3 Atlantic creolefish Paranthias furcifer 000 3 3 Crevalle jack Caranx hippos 0300 3 French angelfish Pomacanthus paru 000 2 2 Squirrelfish Holocentrus ascensionis 000 1 1 Total 172 200 93 119 584 
48 

Table 16 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at WC 564, Visit 1, 

(This platform lies in the central region. Max depth at the platform is 58 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 Total % C

Blue runner Caranx crysos 68 116 217 375 0 776 

Atlantic creolefish Paranthias furcifer 0 7 22 210 8 247 Greater amberjack Seriola dumerili 2 14 14 42 12 84 

Great barracuda Sphyraena barracuda 24 62 0 0 32 

Bermuda chub Kyphosus sectatrix 20 00 0 0 20 

Other Red snapper Lutjanus campechanus 0 0 10 8 0 18 

Cobia Rachycentron canadum 5 24 0 0 11 

Blackfin tuna Thunnus atlanticus 0 9 00 0 9 

Crevalle jack Caranx hippos 0 0 7 0 0 7 

Almaco jack Seriola rivoliana 0 0 5 0 0 5 

Blue angelfish Holacanthus bermudensis 0 5 00 0 5 

Bar jack Caranx ruber 0 4 00 0 4 

Sharks 3 00 0 0 3 Horse-eye jack Caranx latus 2 00 0 0 2 

Scamp Mycteroperca phenax 01 0 0 1 2 

Warsaw grouper Epinephelus nigritus 00 0 1 0 1 

Total 124 164 281 636 21 1226 
49 

Table 17 . 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at HIA 350, Visit 1, 

(This platform lies in the central region. Max depth at the platform is 93 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-100 Total 

Blue runner Caranx crysos 92 33 182 0 0 0 0 307 

Bermuda chub Kyphosus sectatrix 280 0 0 0 0 0 0 280 Atlantic creolefish Paranthias furcifer 0 0 94 53 0 0 0 147 Scamp Mycteroperca phenax 0 0 2 17 7 37 10 73 Red snapper Lutjanus campechanus 0 0 0 0 11 50 2 63 Almaco jack Seriola rivoliana 5 1 22 9 0 0 0 37 Greater amberjack Seriola dumerili 02 4 4 2 8 1 21 Bar jack Caranx ruber 11 9 0 0 0 0 0 20 Other Great barracuda Sphyraena barracuda 6 10 2 0 0 0 0 18 Crevalle jack Caranx hippos 09 0 0 0 0 0 9 

Rainbow runner Elagatis bipinnulata 09 0 0 0 0 0 9 

Gray triggerfish Balistes capriscus 10 1 2 4 0 0 8 

Gag Mycteroperca microlepis 00 1 0 0 1 0 2 

Total 395 73 308 85 24 96 13 994 
Table 18 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at WC 643, Visit 1, 

(This platform lies in the central region. Max depth at the platform is 114 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 Blue runner Caranx crysos 7 206 332 0000000 

Atlantic creolefish Paranthias furcifer 0 12 99 0 64 15 219 5 33 1 

Vermilion snapper Rhomboplites aurorubens 0 0 200 0000000 

Horse-eye jack Caranx latus 000000 61 27 46 50 

Greater amberjack Seriola dumerili 0 0 0 0 29 7 22 1 54 25 OtherGreat barracuda Sphyraena barracuda 3 4 30000000 

Queen angelfish Holacanthus ciliaris 0010 4 32000 

Blue angelfish Holacanthus bermudensis 0000 4 0 4 000 

Scamp Mycteroperca phenax 000000 5 101 Gray triggerfish Balistes capriscus 000000 6 000 Blue tang Acanthurus coeruleus 0000 3 01000 French angelfish Pomacanthus paru 000000 2 100 Rock hind Epinephelus adscensionis 000000 1 1 0 0 

Almaco jack Seriola rivoliana 0 0 1 0000000 Squirrelfish Holocentrus ascensionis 000000 1 000 Total 10 222 636 0 104 25 324 36 133 77 

50
Table 19 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at WC 643, Visit 2, 

(This platform lies in the central region. Max depth at the platform is 114 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 100-110 

Blue runner Caranx crysos 373 205 398 000000 0 0 

Atlantic creolefish Paranthias furcifer 1 4 23 101 60 0 212 108 122 0 31 Bermuda chub Kyphosus sectatrix 109 32 243 86 0 0 0 0 0 0 0 Crevalle jack Caranx hippos 0 0 0 32 48 38 52 10 0 0 

Other Greater amberjack Seriola dumerili 0 0 13 10 5 3 9 1 0 0 1 Great barracuda Sphyraena barracuda 0 6 10 11 74110 0 0 Blue angelfish Holacanthus bermudensis 000131 4 30 0 0 

Spanish hogfish Holocentrus ascensionis 0 0 6 500000 0 0 Cobia Rachycentron canadum 000000 4 4 00 0 

French angelfish Pomacanthus paru 000 2 2 1000 0 0 

Scamp Mycteroperca phenax 000000002 0 3 

Gray triggerfish Balistes capriscus 1 0 0 1 00000 0 0 

Rock hind Epinephelus adscensionis 000 2 00000 0 0 Bar jack Caranx ruber 000000 1 00 0 0 

Filefishes Balistidae 000 1 00000 0 0 

Gag Mycteroperca microlepis 000 1 00000 0 0 

Total 484 247 693 253 125 47 283 118 124 0 35 

51
52 

Table 20 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at GB 236, Visit 1, 

(This platform lies in the central region. Max depth at the platform is 209 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-210 Tota

Atlantic creolefish Paranthias furcifer 1 10 132 234 341 136 71 0 925 

Almaco Jack Seriola rivoliana 0 0 1 21 65 11 21 3 122 

Blue Runner Caranx crysos 9 46 000000 55 

Bermuda Chub Kyphosus sectatrix 36 0000000 36 

Greater Amberjack Seriola dumerili 9 7654200 33 

Other Great Barracuda Sphyraena barracuda 21 10 1 0 0 0 0 0 32 

Bar Jack Caranx ruber 0 10 200000 12 

Gray Triggerfish Balistes capriscus 00 0 4 1000 5 Queen Angelfish Holacanthus ciliaris 2 3 000000 5 

Lesser Amberjack Seriola fasciata 00 0 0 3 000 3 Scamp Mycteroperca phenax 00 0 0 0 0 2 1 3 Total 78 86 142 264 414 149 94 4 123
53 

Table 21 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at GB 236, Visit 2, 

(This platform lies in the central region. Max depth at the platform is 209 m. Blanks = no da

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 100-110 110-210 Blue runner Caranx crysos 48 0 22 45 0 169 000 0 0 0 

Bermuda chub Kyphosus sectatrix 224 0 0 000000 0 0 0 Greater amberjack Seriola dumerili 5 4 15 2 2 19 44 23 10 15 11 13 

Almaco jack Seriola rivoliana 0 12 48 0 6 48 39 0 7 0 0 0 

Atlantic creolefish Paranthias furcifer 09 0 0 0 0 47 30 24 4 0 0 

Scamp Mycteroperca phenax 00 0 0 2 5 8 22 77 0 3 

Horse-eye jack Caranx latus 0 37 0 000000 0 0 0 

Queen angelfish Holacanthus ciliaris 02 0 3 3 8 643 4 0 0 

Blue angelfish Holacanthus bermudensis 0 0 0 1 0 0 0 3 11 9 4 0 Great barracuda Sphyraena barracuda 08 1 9 52100 0 0 0 

Gag Mycteroperca microlepis 00 0 3 9 2142 1 0 3 

Sheepshead Archosargus probatocephalus 00 0 6 15 0000 0 3 0 

Rock hind Epinephelus adscensionis 00 0 0 2 5 1 6 21 0 0 

Other 0 Black jack Caranx lugubris 2 0 11 000000 0 0 0 

French angelfish Pomacanthus paru 00 0 0 0 1 2 2 3 10 0 

Spanish hogfish Bodianus rufus 00 0 0 0 9 000 0 0 0 

Gray triggerfish Balistes capriscus 00 0 0 0 0 1 3 20 0 0 

Blue tang Acanthurus coeruleus 00 0 1 2 2 000 0 0 0 

Dog snapper Lutjanus jocu 00 0 0 2 0000 0 0 0 

Bar jack Caranx ruber 00 0 1 00000 0 0 0 

Filefishes Balistidae 00 0 0 0 0 0 1 00 0 0 

Gray snapper Lutjanus griseus 00 0 1 00000 0 0 0 

Total 279 72 97 72 48 270 150 98 71 42 18 19 
54 

Table 22 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at VR 279, Visit 1, 

(This platform lies in the central region. Max depth at the platform is 55 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 Total % Com

Blue runner Caranx crysos 28 1240 760 0 0 0 2028 9Red snapper Lutjanus campechanus 0 3 27 27 0 10 67 3

Almaco jack Seriola rivoliana 0 7 28 800 43 1

Other 4Gray triggerfish Balistes capriscus 0 11 13 700 31 1

Greater amberjack Seriola dumerili 00 8 9 0 0 17 0

Vermilion snapper Rhomboplites aurorubens 0 14 1000 15 0

Bermuda chub Kyphosus sectatrix 0 9 2000 11 0

Crevalle jack Caranx hippos 0 11 0000 11 0

Gray snapper Lutjanus griseus 0 1 5 001 7 0

Atlantic creolefish Paranthias furcifer 0 0 5 000 5 0

Black drum Pogonias cromis 00 0 0 0 1 1 0

Ocean triggerfish Canthidermis sufflamen 0 0 1 000 1 0

Snowy grouper Epinephelus niveatus 00 0 1 0 0 1 0

Total 28 1296 850 52 0 12 2238 10
55 

Table 23 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at MI 703, Visit 1, 1

(This platform lies in the western region. Max depth at the platform is 39 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 Total % ComposiBlue runner Caranx crysos 0 45 0 3 48 26.7%

Gray snapper Lutjanus griseus 00 2 40 42 23.3%

Lookdown Selene vomer 00 0 35 35 19.4%

Great barracuda Sphyraena barracuda 59 0 0 14 7.8%

Red snapper Lutjanus campechanus 00 4 5 9 5.0%

Sheepshead Archosargus probatocephalus 2 2 3 2 9 5.0%Bermuda chub Kyphosus sectatrix 8 000 8 4.4%Lane snapper Lutjanus synagris 00 0 6 6 3.3%

Other 5.0%

Atlantic spadefish Chaetodipterus faber 3 000 3 1.7%

Gag Mycteroperca microlepis 00 0 3 3 1.7%

Gray triggerfish Balistes capriscus 10 0 1 2 1.1%Rock hind Epinephelus adscensionis 0 0 1 0 1 0.6%Total 19 56 10 95 180 100.0%
56 

Table 24 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at MU 787, Visit 1, 

(This platform lies in the western region. Max depth at the platform is 46 m. Blanks = no dat

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 Total % Compostion

Blue Runner Caranx crysos 0 3 144 0 0 147 20.9%Creole-fish Paranthias furcifer 35 92 0 0 0 127 18.1%

Lookdown Selene vomer 0 0 107 0 0 107 15.2%Red Snapper Lutjanus campechanus 1 0 1 27 53 82 11.7%Gray Snapper Lutjanus griseus 02 0 31 26 59 8.4%

Great Barracuda Sphyraena barracuda 16 14 7 0 0 37 5.3%

Atlantic Spadefish Chaetodipterus faber 00 0 28 5 33 4.7%

Greater Amberjack Seriola dumerili 00 0 2 13 15 2.1%

Remoras Echeneidae 15 0 0 0 0 15 2.1%Rock Hind Epinephelus adscensionis 3 5 3 0 2 13 1.9%

Bar Jack Caranx ruber 12 0 0 0 0 12 1.7%

Spanish Hogfish Bodianus rufus 4 7 0 0 0 11 1.6%

Cobia Rachycentron canadum 9 00 0 0 9 1.3%

Others 5.0%Scamp Mycteroperca phenax 3 0 5 00 8 1.1%

Gray Triggerfish Balistes capriscus 31 0 4 0 8 1.1%

Sheepshead Archosargus probatocephalus 5 00 0 1 6 0.9%

Bermuda Chub Kyphosus sectatrix 4 00 0 0 4 0.6%

Gag Mycteroperca microlepis 00 0 0 4 4 0.6%

French Angelfish Pomacanthus paru 00 0 2 0 2 0.3%

Blue Angelfish Holacanthus bermudensis 1 00 0 0 1 0.1%

Crevalle Jack Caranx hippos 00 0 0 1 1 0.1%

Sharks 1 00 0 0 1 0.1%

Total 112 124 267 94 105 702 100%
57

3.5 Sonnier Bank 

The Sonnier Bank mobile survey was conducted during the summer of 2003 on board the R/V 

Spree. Acoustic transects closely matched the intended transects (Figure 23). Based on our 

survey design and the additional bathymetry data from the higher resolution survey of the lowrelief pinnacles and high-relief pinnacles areas, we were able to conduct an analysis of the whole 

Sonnier Bank as well as a targeted analysis of the low and high-relief features, and further 

compare these results to those for a nearby standing platform. The high-relief pinnacle feature is 

a natural geological formation that resembles the high vertical profile of platforms; allowing a 

more detailed comparison of the relative importance of natural vs. artificial reefs for fish habitat. 

The low-relief feature was characterized by one oval shaped mound 300 m by 600 m and rising 

approximately 13 m up from the bottom. Two tall pinnacles, each rising to within 20 m of the 

surface, characterized the pinnacle region and several smaller pinnacles located around the tall 

features. The tall pinnacles bear a resemblance in size and steepness to standing platforms or 

partially removed platforms used as artificial reefs. The low-relief mound is similar in extent 

and vertical profile to platforms that have been toppled onto the side for artificial reefs (Figure 

3). 

For data analysis, we examined the effects of water depth, quadrant (relative region within each 

sample area), and proximity to areas of vertical relief on acoustic fish biomass and acoustic size 

of targets. 

3.5.1 Broad-Scale Survey Results of Sonnier Bank 

As found in the platform surveys, acoustic biomass measured during the broad-scale survey of 

Sonnier Bank varied significantly with depth, proximity to areas of vertical relief, and the 

interaction of the two variables above (Split plot ANOVA P<0.0001). Low acoustic biomass 

was found in deep waters and also in waters distant from any features with vertical relief. Much 

higher biomass was found over both low and high-relief features. Detailed sampling of these 

areas was able to provide more accurate biomass estimates over these features compared to the 

broad scale survey results. 

For the purpose of analysis of the low and high-relief pinnacle areas, a sample unit consisted of 

two linear meters of hydroacoustic data broken out into 10 m depth strata from 5 m below the 

surface to within 0.5m of the bottom. 

3.5.2 Low-Relief Pinnacle Area of Sonnier Bank 

Based on visual observations of the acoustic data, we noticed that there were more numerous 

acoustic targets over and around the low-relief feature as well as in the Northwest region of the 

sampling area. Consequently, we examined the effects of water depth, quadrant (relative region 

within each sample area), and proximity to the low-relief feature on acoustic fish biomass and 

acoustic size of targets. Water depth, quadrant, and proximity to the low-relief feature were all 

significant in the volume backscatter model as was the interaction of feature within region (Table 

24). 
58

Figure 23. Actual survey track lines for hydroacoustic survey conducted in May 2003 of 

Sonnier Bank projected over bathymetry. 
59

Table 25 

Results of Split Plot Design Analysis of Variance Showing the Effect of Depth (in 10 m Strata), 

Feature (Low or High-Relief Feature), and Quadrant (General Quadant in Sample Area) on the 

Dependent Variable Sv for Horizontal Hydroacoustic Data. 

Type 3 Tests of Fixed Effects 

Effect Num DF Den DF F Value Pr > F 

Depth 5 15 14.51 <.0001 

feature 1 42E3 64.01 <.0001 

Quadrant 3 15 377.85 <.0001 

region(feature) 3 42E3 213.14 <.0001 
60

Acoustic fish biomass varied with depth in the low-relief survey area; biomass was highest in 

waters greater than 30 m and lowest at the surface (Figure 24). Fish biomass was significantly 

higher in the northwest and northeast quadrants and lower in the southwest and southeast 

quadrants (Figure 25). Mean volume backscatter was higher immediately over the low-relief 

feature (-72 dB) than away from it (-74 dB). 

Target strength varied with water depth and quadrant (Split plot ANOVA p<0.0001), but there 

was no difference in target strength associated with the low-relief feature. Target strength was 

higher near the bottom and lowest near the surface (Figure 26). 

Fish density was highly variable over the survey area, but in general followed the pattern 

observed in volume backscatter. Fish density varied significantly with water depth, quadrant, 

and proximity to the low-relief feature. Fish density ranged from zero to over two fish per cubic 

meter. Fish density was highest in the 20 m stratum and dropped off to less than 0.001 fish per 

cubic meter near the bottom (Figure 27). 

3.5.3 High-Relief Pinnacle Area of Sonnier Bank 

The acoustic survey of the high-relief features in the Sonnier Bank area produced results that 

were similar to the low-relief area; however, the general magnitude of acoustic biomass 

estimates and densities were lower. Water depth, quadrant, and proximity to the high-relief 

features were all significant in the volume backscatter model as was the interaction of feature 

within quadrant. Mean volume backscatter was higher near the bottom and lowest at the surface 

in the high-relief pinnacle survey area (Figure 28). Mean volume backscatter over the high-relief 

pinnacles was -80.5 dB compared to -83 dB away from the pinnacle. Target strength data from 

the high-relief pinnacle area were fairly consistent averaging from -40 to -43 dB and exhibited a 

positive trend with increasing depth where targets were smaller near the surface and largest near 

the bottom (Figure 29). 

Fish density data varied significantly with water depth and region. Estimated fish densities were 

highest in the 20 m strata and lowest near the bottom (Figure 30). 

3.5.4 Species Composition 

An ROV survey was conducted at both the small relief and high-relief pinnacle areas. The ROV 

was flown around, over, and away from geological features in each of the sampling areas. The 

resultant species composition data are shown in Tables 25 and 26. The high-relief pinnacle 

region was clearly dominated by Atlantic creolefish and Bermuda chub (planktivores), whereas 

Atlantic creolefish, vermillion snapper, and red snapper (planktivores and carnivores) were 

dominant and equally abundant at the low-relief feature. 
61

Figure 24. LS mean volume backscatter (Sv in dB ± 1 SE) at depth (m) for the small feature 

at Sonnier Bank, northern Gulf of Mexico. Means with same letters are not 

significantly different at α= 0.005. 

Figure 25. LS mean volume backscatter (Sv in dB ± 1 SE) at directional quadrant for the 

small feature at Sonnier Bank, northern Gulf of Mexico. Means with same 

letters are not significantly different at α= 0.005. 
62

Figure 26. LS mean target strength (dB ± 1 SE) at depth (m) for the small feature at Sonnier 

Bank, northern Gulf of Mexico. Means with same letters are not significantly 

different at α= 0.005. 

2

Figure 27. LS mean density (fish per cubic meter) at depth (m) for the small feature at 

Sonnier Bank, northern Gulf of Mexico. Means with same letters are not 

significantly different at α= 0.005. 
63

Figure 28. LS mean volume backscatter (Sv in dB ± 1 SE) at depth (m) for the pinnacle 

feature at Sonnier Bank, northern Gulf of Mexico. Means with same letters are 

not significantly different at α= 0.005. 

Figure 29. LS mean target strength (dB ± 1 SE) at depth (m) for the pinnacle feature at 

Sonnier Bank, northern Gulf of Mexico. Means with same letters are not 

significantly different at α= 0.005. 
64

Figure 30. LS mean density (fish per cubic meter) at depth (m) for the pinnacle feature at 

Sonnier Bank, northern Gulf of Mexico. Means with same letters are not 

significantly different at α= 0.005.
65 

Table 26 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at Sonnier Bank Low-Relief 

(Blanks = no data.) 

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 Total % Com

Atlantic creolefish Paranthias furcifer 00 0 0 0 57 57 3

Red snapper Lutjanus campechanus 00 0 0 0 38 38 2Vermilion snapper Rhomboplites aurorubens 00 0 0 0 37 37 2Little tunny Euthynnus alletteratus 0 0 23 000 23 1

Gray triggerfish Balistes capriscus 00 2 2 0 6 10 5

Spanish hogfish Bodianus rufus 00 0 0 0 7 7 3

Other 5Scamp Mycteroperca phenax 00 0 0 0 3 3 1

Queen angelfish Holacanthus ciliaris 00 0 0 0 3 3 1

Gray snapper Lutjanus griseus 00 0 0 2 0 2 1Almaco jack Seriola rivoliana 00 0 0 0 1 1 0

Total 0 0 25 2 2 152 181 10
66 

Table 27 

Number of Fishes by Species Observed during ROV Video Surveys by Depth at Sonnier Bank Pinnacle F

(Blanks = no data.) 

Common Name Scientific Name 0-10 10-20 20-30 30-40 40-50 50-60 Total % Com

Atlantic creolefish Paranthias furcifer 0 618 1291 66 0 0 1975 7

Bermuda chub Kyphosus sectatrix 295 00000 295 1Vermilion snapper Rhomboplites aurorubens 00 0 0 215 0 215 8Other 9Blue angelfish Holacanthus bermudensis 00 2 0 0 6 8 0

Gray triggerfish Balistes capriscus 0 3 1 0 3 0 7 0Greater amberjack Seriola dumerili 0 2 3 200 7 0

French angelfish Pomacanthus paru 0 0 4 020 6 0

Queen angelfish Holacanthus ciliaris 0 0 3 000 3 0

Spanish hogfish Bodianus rufus 00 0 0 0 3 3 0

Almaco jack Seriola rivoliana 00 0 0 0 1 1 0

Cobia Rachycentron canadum 0 0 1 000 1 0

Total 295 623 1304 68 220 10 2520 10
67

3.6 Comparison of Sonnier Bank High-Relief Pinnacle Area and 

VR 279 

Sonnier Bank is located approximately 10 km to the southeast of the platform situated at VR 

279. Maximum depth at both localities is 70 m, although we were only able to resolve the data 

for the upper 60 m of the water column. A comparison of biomass at the two high-relief features 

(Sonnier Bank pinnacle and VR279) showed inverse trends: at VR 279, mean biomass 

decreased with depth, while mean biomass at the Sonnier Bank pinnacle increased with depth 

(Figure 31, 34). LS means biomass was substantially lower at all depths at Sonnier Bank 

compared to VR 279 (Figure 34). Mean target strength increased marginally with increasing 

depth at both sites, but the higher values at VR 279 (Figure 32, 35) indicated that fishes 

associated with the platform are also generally larger than those at the pinnacle. Fish densities in 

the upper 20 m of the water column at VR 279 (Figure 33) were vastly higher than those found at 

the pinnacle (Figure 37). Densities of fishes at both sites were similarly low at depths of or 

below 30 m. The composition of fish functional groups found at VR 279 and the pinnacle 

feature were quite similar (Figure 38), being heavily dominated by planktivores. The dominant 

planktivorous species at the features were, however, quite different. Blue runners dominated at 

VR 279 while Atlantic creolefish dominated at the pinnacle feature. At the low-relief feature, 

carnivores were the dominant functional group followed by planktivores and a number of 

grazers. 
68

Figure 31. LS mean volume backscatter (SV in dB ± 1 SE) at depth (m) at VR 279, northern 

Gulf of Mexico. Means with same letters are not significantly different at α= 

0.005. 

Figure 32. LS mean target strength (dB ± 1 SE) at depth (m) at VR 279, northern Gulf of 

Mexico. Means with same letters are not significantly different at α= 0.005. 
69

Figure 33. LS mean density (fish per cubic meter) at depth (m) at VR 279, northern Gulf of 

Mexico. Means with same letters are not significantly different at α= 0.005. 
70

Figure 34. LS mean Volume backscatter (Sv) at the standing platform (VR279) compared 

to Sv at the small feature and pinnacle feature at Sonnier Bank. 

Figure 35. LS mean Target Strength (TS) at the standing platform (VR279) compared to TS 

at the small feature and pinnacle feature at Sonnier Bank. 
71

Figure 36. Fish Energy at the standing platform (VR279) compared to Sv at the small 

feature and pinnacle feature at Sonnier Bank. 

Figure 37. Fish Density at the standing platform (VR279) compared to Sv at the small 

feature and pinnacle feature at Sonnier Bank. 
72

Figure 38. Comparison of the proportion of each fish functional group found at Sonnier 

Pinnacle, Sonnier low-relief feature, and VR279. 
73

4.0 DISCUSSION 

4.1 Technical and Analysis Logistics 

This project, intended to last three years, has taken seven years to complete. During this 

extended period of time we experienced significant changes in personnel, technology for data 

collection, data processing, and data analysis. Through the course of the project, several of the 

cooperating oil and gas companies merged, leading to changes in our internal industry contacts 

and offshore field personnel. A lightning strike during the first year of the project crippled data 

collection as it destroyed several transducers and cables and they had to be replaced. In the 

second year poor planning on our part led to the failure of a lift strap for moving our gear from a 

platform to the boat; our offshore box struck the water, split open, and immediately sank to the 

bottom. Thanks to the generosity of Chevron we recovered the equipment. However we 

suffered another setback: it took roughly six months to replace the equipment that was damaged 

by saltwater. One platform was lost to hurricane damage, another was sold and turned into an 

un-manned facility and another was shutdown permanently before a second visit could be 

executed. Problems encountered during this study reinforce how complicated it is to work in the 

marine environment, even when facilitated by access to oil and gas platforms and associated 

infrastructure. 

The current hydroacoustic technology has become very portable, durable, and easy to use. 

Target discrimination and analysis have also been greatly improved. The greatest advancement 

we recently encountered was the addition of Echoview processing software. There are a number 

of features to this new software that greatly enhance our ability to derive useful data from the 

acoustic returns. For example, Figure 39 illustrates an echogram image from the Sonnier Bank 

mobile acoustic survey; the software automatically defines the bottom, which is color-coded by 

bottom hardness, and the blue scale in intensity of the targets above the bottom illustrates target 

size. The software automatically analyzes those targets between the surface and the identified 

bottom. The operator can also select a specific area in the echogram for analysis or manually edit 

out areas of poor data or poorly defined bottom. Figure 40 is an echogram of an area of Sonnier 

Bank where we encountered numerous gas seeps. We edited these bad data regions to exclude 

them from the final analysis. 

No standard method has been either prescribed or endorsed for analysis of hydroacoustic data. 

An obvious advantage of this is the ability both to tailor analyses to answer a specific question 

and to allow for analytical improvements as technology and research advance. The disadvantage, 

however, is that comparability among projects is reduced. The development of novel analysis 

techniques has provided significant improvements in processing acoustic data, resulting in a 

dataset of much higher resolution, but these improvements may also necessitate significant 

change in our approach to analysis. With Echoview, for example, we can isolate both bad data 

regions and questionable data from the final analysis (Figure 38). By removing the bad data 

regions we effectively change the overall sample size in the analysis. The appropriate method 

for treating observations with different sample sizes is to weight each observation by its 

respective sample size; therefore the means of those observations are inherently weighted. In 
74

previous studies, we have not used the weighted means technique because this option was not 

available in older analysis software. 

When we compared weighted versus non-weighted means, we observed differences in the 

magnitudes of biomass estimates. The trends observed in the data were, however, consistent 

regardless of the method used for deriving the mean (weighted or non-weighted). To maintain 

comparability with previously reported analyses (Wilson and Stanley 1991, Wilson et al. 2003), 

we chose to analyze the data without weighting for differences in sample size. It is likely that a 

complete re-analysis of the current dataset using weighted means would provide biomass 

estimates that are more accurate than those reported here, but the overall trends among sites 

would not differ from this analysis. 

Hydroacoustics is an extremely useful technology for comparing and estimating fish biomass, 

particularly that biomass associated with specific habitats. However, the users must be cognizant 

of the limitations of data derived from hydroacoustics. The resultant data should be construed as 

an estimate of the acoustic biomass; without extensive groundtruthing, those estimates are not 

precise, but relative. Similarly, estimates of target strength are an average of all the acoustic 

returns during the sample period in a particular depth stratum. Consequently, when the acoustic 

biomass is divided by the average target strength to estimate fish density, one must recognize 

that the error associated with that estimate is fairly large . 

4.2 Patterns in Acoustic Variables 

The results of this multiyear research project continue to reinforce our earlier conclusions that 

there is a relatively high fish biomass associated with operating oil and gas platforms when 

compared to other habitats in the northern Gulf of Mexico. We continue to observe the general 

trend that fish biomass around platforms is consistently higher than that of the adjacent soft 

bottom habit from the coastal waters of Louisiana out to the edge of the outer continental shelf 

(OCS). This general pattern was true along the eastern, central, and western regions. 

Unlike previous techniques used to assess fish biomass in specific habitats, hydroacoustics 

affords us the ability to be very specific in spatial and temporal perspectives. We observed a 

significant difference in fish biomass across the three regions. Fish biomass per unit area was 

highest in the central region and lowest along both the eastern and western regions (Figure 6a, 7). 

The lower sample size along the western region lead, however, to higher variability of this 

estimate. This higher biomass in the central region is largely due to a very high planktivore 

abundance. It was interesting to note that a number of variables were not significant in the 

various models. Several sites had been on location for over 20 years, yet platform age was not 

significant in statistical models. We also did not observe a statistical effect of number of legs, or 

number of conductor guides, although these variables were confounded by platform identity and 

depth. 

We continue to find fish around oil and gas platforms vary predictably with platform side, time 

of day, and especially, depth. Fish biomass is highest near the surface waters and declines with 
75

increased water depth. At water depths greater than 100 m, fish biomass is not statistically 

different from zero (Figure 6b). This continues to reinforce the fishery value of platforms, in 

particular platforms at water depths less than 50 m. Although several factors were statistically 

significant, depth consistently explained a large degree of variation in fish biomass, target 

strength and abundance. Fish biomass was highest at midnight and dawn and lowest at noon 

(Figure 6d). The fact that fish biomass is highest during the hours of darkness is evidence either 

that the lights at a platform create a halo effect attracting fishes to the platform or crepuscular 

fish emerge from within the confines of the platform at night and become acoustically visible. 

Platform side also significantly affected fish biomass; in the current study fish biomass was 

consistently and statistically higher on the west side (Figure 6c). Stanley and Wilson (1997, 

2000) reported the same observation but offered no explanation. 

Our experimental design allowed us to analyze a change in acoustic fish biomass with distance 

away from the platforms sampled. Our objective was to gain some insight into the "reef effect": 

how far all way from a platform (artificial reef) is there a measurable effect of the artificial 

habitat. We found that fish biomass was highest within 20 m of the platforms sampled and that 

biomass decreased with increased distance. However, we report that the range of influence of 

platforms over those sampled extends out beyond the 20 m reported by Stanley and Wilson 

(1997) and is more on the order of 50 m. This may in fact be a visual cue that varies with water 

clarity. 

We also compared target strength, which is an acoustic estimate of fish size. Although the target 

strengths reported are five minute averages of fish size over 10 m depth range, we did observe 

some statistical differences worth reporting. Average acoustic fish size varied across regions and 

was highest in the central region and lowest in the western region (Figure 12a, 13). In fact, 

targets along the western region averaged 30% smaller than those of the central or eastern 

regions. Targets along the eastern region were also slightly smaller than target size along the 

central region. Mean target size varied with the depth and was largest near the bottom and 

smallest near the surface (Figure 12b). The larger reef associated species tend to be near the 

bottom as reported by Stanley and Wilson (1997). Mean target size was statistically lower at 

midnight and greatest at dusk (Figure 12d). The higher biomass and smaller target size at 

midnight suggests that the smaller, crepuscular organisms were, in fact, emerging from the 

confines of the platform at night. Mean target size also varied with platform side and targets 

were largest on the east side and smallest on the north side (Figure 12c); this is an observation 

we cannot explain. 
76

Figure 39. Hydroacoustic echogram of Sonnier Bank pinnacle feature illustrating the high 

density of fish associated with the two pinnacle features. 

Figure 40. Hydroacoustic echogram of Sonnier Bank small feature illustrating a few 

acoustic targets associated with the small feature and a natural gas plume. The 

echogram illustrates how the natural gas and can be eliminated from analysis. 
77

4.3 Patterns in Species Composition 

Species richness was higher in the central and western GOM compared to the eastern GOM. 

This observation may be tied to the proportion of functional groups observed in each of these 

regions. The fish fauna in the eastern GOM showed a much higher proportion of carnivores and 

top predators. Platforms in this region are also located very close to edge of the continental shelf 

and a very steep shelf slope. Open water carnivores and top predators that are not strictly reefassociated species may be opportunistically visiting platforms in this region to take advantage of 

abundant prey resources. We speculate that this heavy predation pressure may, in fact, restrict 

the diversity of reef associated prey species that are able to effectively reside at platforms. 

The proportions of species (Figures 19-22) in the functional groups delineated in Table 7 show 

that these groups are differentially distributed across the study area. The westward flow of the 

nutrient-rich Mississippi River plume likely contributes to the proportional dominance of 

planktivores in the Central region off southwestern Louisiana (Figure 19). These nutrients 

support greater primary and secondary production that in turn support larger populations of 

planktivorous fishes. The East and West regions are comparatively nutrient poor and would be 

expected to support fewer planktivores. Grazer species (Figure 20) were only abundant at one 

locality (MP142) and were otherwise confined to those localities both closest to shore and at the 

shallowest depths. We suspect that the distribution of grazers is largely affected by the inshore 

distributions of their preferred crustacean and molluscan prey species that may be replaced by 

other species at greater depths. Carnivorous species (Figure 21) and top predators (Figure 22) 

show similar patterns in the proportional abundances. Both are proportionally abundant in the 

East region and show lesser proportions at the deepwater platforms in the Central and West 

regions. The carnivores include many species targeted by both commercial and recreational 

fishers, so their numbers may be affected by local fishing pressure. Amberjacks and groupers 

are obligate deepwater species; thus they would be expected at those platforms at the greatest 

depths. Barracudas and sharks are less specific in their depth requirements and are to be found in 

varying numbers at most platforms throughout the study area. Every platform in the northern 

Gulf may have at least one barracuda that has taken up residence; however, our experience both 

as scientists and as fishers has shown that barracudas are especially abundant at the platforms at 

the edge of the OCS. 

Red snapper are an important species for both commercial and recreational interests in the 

northern GOM and as such warrant some special discussion in light of our findings in this study. 

Of all the habitats included in this study, red snappers were proportionally most dominant at the 

low-relief feature at Sonnier Bank. This finding is similar to that reported by Wilson et al. 

(2003) for toppled and partially removed artificial reef structures. 

Red snappers were proportionally abundant at several platforms in all three regions of the study 

area (Figure 41); most of these platforms were at the shallowest depths included in the survey 

design. Few red snappers were observed at those platforms at the edge of the OCS. The 

movements of red snapper over their potential 50 year lifespan can help to explain some of the 

observed distribution. After spending the best part of their first two years in shallow, inshore 
78

waters, red snapper begin an offshore migration in search of more suitable, deepwater habitats 

that might include reefs, lumps, depressions, and wrecks (Nieland and Wilson 2003). In the 

northern Gulf, these suitable habitats include the many oil and gas platforms that have been sited 

at moderate depths in the nearshore waters. After a residence at the platforms of perhaps 5-8 

years, the red snapper migrate away from the platforms and spend the rest of their lives in a 

relatively solitary existence at natural or anthropogenic, non-platform structures in deeper waters 

toward the edge of the OCS (Nieland and Wilson 2003). 

4.4 Comparison of Sonnier Bank and a Standing Platform 

A mobile hydroacoustic survey has allowed us, for the first time, to compare hydroacoustic 

biomasses of a natural, high-profile hard bottom feature (Sonnier Bank pinnacle) with that of an 

adjacent oil and gas platform (VR 279). Although similar in form in many respects, the obvious 

difference between the Sonnier Bank pinnacle and VR 279 is their respective profiles in the 

water column: the summit of the Sonnier Bank pinnacle is at a depth of about 20 m while VR 

279 spans from the bottom to the surface. Unlike the most of the platforms surveyed to date, 

including VR 279, acoustic biomass at the Sonnier Bank pinnacle was highest near the bottom 

and lowest at the surface (Figures 32 and 35). Even though the Sonnier Bank pinnacle rises very 

high in the water column, biomass near the surface was, still very close to zero. Structureoriented, epipelagic species, such as blue runner, that were found in abundance at VR 279, were 

absent from the Sonnier Bank pinnacle. 

Although the dominant species differ substantially between the pinnacle and VR 279, the 

proportion of fish observed in each functional group were remarkably similar. At both locations, 

the ichthyofauna was overwhelmingly dominated by planktivores. We conclude therefore that, 

planktivores will dominate fish abundances at a feature given a certain vertical extent. This 

remains true whether the feature is natural or artificial in nature. This certain vertical extent does 

not however, necessarily translate into higher fish biomass. Fish biomass reported at the 

pinnacle was, in fact, much lower even than biomass reported at the low-relief feature (Figurer 

36, 37). 

The low-relief feature at Sonnier Bank held a higher fish biomass per unit area compared to the 

pinnacle area. We speculate that at least part of this observation can be explained by the spatial 

configuration of the two areas. The “pinnacle feature” is actually a field of four major highrelief features separated from the nearest high-relief feature by approximately 500 m. Fish in 

this region were not only tightly clustered around the feature, but were also found dispersed 

between two features, and may move readily between the pinnacle features (Figure 39). The 

low-relief feature, is a single, more isolated feature at least 1km distant from the nearest feature 

having significant relief (Figure 3). Fish may therefore remain more closely associated with the 

low-relief feature. 

The low-relief feature seems to provide a habitat that is most similar in species composition, to 

toppled or partially removed artificial reefs. The species found at both toppled and partially 

removed platforms also hold a significant fraction of carnivores and top predators. Wilson et al. 
79

(2003) reported that the most abundance species at a partially removed platform (HIA 355) were 

greater amberjack, red snapper, almaco jack and Spanish hogfish. 

This research continues to support the premise that standing oil and gas platforms do make 

useful artificial reefs because they support fish densities that can be from 10 to 1000 times 

greater than the densities found over adjacent sand and mud bottom habitats. Also, fish densities 

at standing platforms almost always exceed those found both at artificial reefs (both partially 

removed and toppled) and at natural habitats such as Sonnier Bank. However, the fish species 

associated with artificial reefs (including standing platforms) differ from those found on natural 

habitats. Future research efforts might be directed toward determining the reasons for this 

difference. 

Figure 41. Proportion of red snapper observed at each study platform based on ROV 

surveys. Note that data from Sonnier Bank (SONN_PINN and SONN_SF) are 

also included.
81

BIBLIOGRAPHY 

Abe, K., S. Honda, T. Oshima, K. Nakano, K. Miyashita, K. Iida, and T. Mukai. 2002. In situ 

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The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

for most of our nationally owned public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; 

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As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) 

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Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

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