ACCESS NUMBER: 30660-19961 

STUDY TITLE: Offshore Petroleum Platforms: Functional Significance for Larval Fish 

Across Longitudinal and Latitudinal Gradients 

REPORT TITLE: Offshore Petroleum Platforms: Functional Significance for Larval Fish 

Across Longitudinal and Latitudinal Gradients 

CONTRACT NUMBER: 14-35-0001-30660-19961 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): North-central Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1999; 2000; 2001 

COMPLETION DATE OF REPORT: June 2002 

COST(S): FY 1999: $113,458; FY 2000: $120,067; FY 2001: $86,673; CUMULATIVE 

PROJECT COST: $320,198 

PROJECT MANAGER: Richard F. Shaw 

AFFILIATION: Coastal Fisheries Institute, Louisiana State University 

ADDRESS: Wetland Resource Building, Baton Rouge, Louisiana 70803 

PRINCIPAL INVESTIGATOR(S)*: M. Benfield, T. Farooqi, D. Lindquist, R. Shaw, J. 

Plunket 

KEY WORDS: North-central Gulf of Mexico; oil and gas platforms; fish larvae and 

juveniles; ichthyoplankton; artificial habitat; light-traps; diversity; spatial distribution; 

temporal periodicity; reef fishes. 

BACKGROUND: The introduction and proliferation of offshore oil and gas platforms in 

the northern Gulf has undoubtedly affected the marine ecosystem. These structures 

provide additional hard-substrate to areas of the continental shelf that are dominated by 

sand/silt/mud. Because fish populations are usually limited by available energy, 

recruitment or habitat, it is important to determine if platforms: 1) serve as new or 

additional spawning habitat; 2) provide critical habitat for early life history stages; 3) 

influence energy flow through the ecosystem by aggregating prey. 

OBJECTIVES: (1) To characterize the larval and juvenile fish assemblages at 

platforms by depth gradients (latitudinal) and by east/west of the Mississippi River Delta 

(longitudinal). (2) To assess how platforms affect the local distribution and abundance 

of larval and juvenile fishes. (3) To provide ecological information on the early life 

history stages of reef fishes, and, in particular, address whether platforms have a 

nursery/refugia function for these habitat-limited fishes. 
ACCESS NUMBER: 30660-19961 

DESCRIPTION: Larval and juvenile fishes were sampled at two platforms east of the 

Delta. Santa Fe-Snyder’s Main Pass (MP) 259, which stands in 120 m of water on the 

outer shelf (29o19’32”N, 88o01’12”W), was sampled twice-monthly during new and full 

moon phases from May to September 1999. Murphy Oil’s Viosca Knoll (VK) 203, which 

stands in 35 m of water at mid-shelf (29o46’53”N, 88o19’59”W), was also sampled twicemonthly during new and full moon phases from May to October 2000. Larval and 

juvenile fishes were collected in surface waters within the platform using passivelyfished plankton nets and light traps, and off-platform (20 m down-current of the platform) 

using light traps. Environmental data for water temperature, salinity, turbidity, 

microzooplankton biomass, and current speed and direction were also collected at each 

platform. Data collected at MP 259 and VK 203 were analyzed with data from previous 

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

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

platform Green Canyon (GC) 18]. 

SIGNIFICANT CONCLUSIONS: The higher abundance and diversity of larval and 

juvenile fishes within the platform, as compared with adjacent open-waters, indicated 

that platforms have a significant impact on the distribution of larval and juvenile fishes in 

the northern Gulf. These fishes may have been attracted by the platform’s nighttime 

light-field and/or concentrations of prey that may be found in the waters around 

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

i.e., blenniids and pomacentrids. Blenniids and pomacentrids were the most commonly 

collected settlement-size reef larvae at platforms. Their collection in discrete pulses 

during darker moon phases further suggests that blenniids and pomacentrids settled at 

platforms. Other settlement-size reef taxa (e.g., lutjanids and serranids) were relatively 

rare as settlement-size larvae and, therefore, probably do not utilize platforms as 

nursery/recruitment habitat. 

STUDY RESULTS: Patterns of larval and juvenile fish abundance and diversity were 

primarily influenced by across-shelf gradients of increasing water depth. Larval fish 

total densities from plankton nets were highest for the inner shelf platform ST 54, and 

decreased with increasing depth. Light trap total CPUEs were highest for the mid-shelf 

platforms VK 203 and GI 94, and were generally low at the outer shelf platforms. 

Diversity and taxonomic richness were both generally greatest at the mid-shelf 

platforms. Cluster analysis found three general assemblages of non-clupeiform larval 

and juvenile fishes at platforms: an inner shelf assemblage (ST 54), a mid-shelf 

assemblage (GI 94 and VK 203) and an outer shelf assemblage (GC 18 and MP 259). 

High densities of sciaenids, synodontids and other coastal taxa characterized the inner 

shelf assemblage. The mid-shelf assemblages were characterized by blenniids, 

synodontids, and Bregmaceros cantori. The mid-shelf platforms also had the highest 

abundance and diversity of reef fishes, particularly blenniids, lutjanids and 

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

taxa, such as carangids and scombrids, and mesopelagic taxa. The only differences 

observed in the larval and juvenile fish assemblages across longitudinal gradients (i.e., 

east or west of the Delta) were differences in the abundance of certain taxa. 
ACCESS NUMBER: 30660-19961 

The waters within platforms had higher abundance and diversity of larval and juvenile 

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

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

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

platforms included scombrids and possibly carangids. Reef fish were occasionally 

found in higher abundance within-platform, yet pomacentrids and the blenniid 

Hypsoblennius invemar were often found in higher abundance off-platform. These 

specimens, though, were collected at settlement-size and may represent individuals 

settling to the platforms. 

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

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

surface water temperatures were at their highest, i.e., June-August. There were strong 

differences in the numbers and developmental stages collected between new and full 

moons. Many preflexion reef larvae were collected in higher numbers on either new or 

full moons. Postflexion larvae, however, were almost exclusively collected on new 

moons, which was consistent with the peak periods of settlement for many reef fishes. 

Coefficients of variation calculated from the mean light trap catches per sampling night 

and sampling trip, indicated pulses in the catches of several dominant taxa. Blenniids 

and pomacentrids, in particular, were primarily found in very discrete pulses that 

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

these pulses during darker moon phases (i.e., first quarter and new moons) further 

suggests that they were settlement-related. 

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

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

habitat provided by platforms. Recently-hatched and preflexion blenniids, holocentrids, 

labrid, lutjanid, scarid and serranid larvae were collected at every platform. The 

preponderance of platforms, as compared with natural hard bottom habitat, within the 

transport envelope of the larvae makes platforms the most probable source of these 

larvae. Blenniids and pomacentrids were the most common settlement-size reef taxa 

collected at platforms while settlement-size larvae of other reef taxa were relatively rare, 

except for possibly lutjanids at the mid-shelf platforms. Because blenniids and 

pomacentrids are benthically-spawned, as compared with other reef taxa that spawn 

pelagic eggs, their larvae may have shorter larval durations and limited dispersal and, 

therefore, may be able to remain in areas where suitable settlement habitat (i.e., 

platforms) is more available. 

STUDY PRODUCT(S): Shaw, R.F., D.C. Lindquist, M.C. Benfield, T. Farooqi and J.T. 

Plunket. 2002. Offshore Petroleum Platforms: Functional Significance for Larval Fish 

Across Longitudinal and Latitudinal Gradients. A final report for the U.S. Department of 

Interior, Minerals Management Service Gulf of Mexico OCS Region, New Orleans, LA. 

Contract No. 14-35-0001-30660-19961. 122 pp. 

*P.I.’s affiliation may be different than that listed for Project Manager(s).
ACCESS NUM

Location of oil and gas platforms sampled in the northern Gulf of Mexico. Circles represent platforms sampled during the preserepresent platforms sampled during previous studies, used for comparisons. 

Latitude ( °

N)

Longitude (°W)

LA

MS AL 

New Orleans

200 m

0 25 50 75 100 km

Location of oil and gas platforms sampled in the northern Gulf of Mexico. Circles represent platforms sampled during the preserepresent platforms sampled during previous studies, used for comparisons. 

Latitude ( °

N)

Longitude (°W)

LA

MS AL 

New Orleans

200 m

0 25 50 75 100 km

Latitude ( °

N)

Longitude (°W)

LA

MS AL 

New Orleans

200 m

Latitude ( °

N)

Longitude (°W)

Latitude ( °

N)

Longitude (°W)

LA

MS AL 

New Orleans

200 m

0 25 50 75 100 km