ACCESS NUMBER: 30660-19915

STUDY TITLE: Seasonal and Spatial Variation in the Biomass and Size Frequency 

Distribution of Fish Associated with Oil and Gas Platforms in the Northern Gulf Of 

Mexico 

REPORT TITLE: Seasonal and Spatial Variation in the Biomass and Size Frequency 

Distribution of Fish Associated with Oil and Gas Platforms in the Northern Gulf Of 

Mexico 

CONTRACT NUMBER: 14-35-0001-30660-19915 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEARS OF FUNDING: 1998, 1999, 2000 

COMPLETION DATE OF REPORT: February 2000 

COSTS: FY 1998: $182,550; FY 1999: $137,032; FY 2000: $71,003 

CUMULATIVE PROJECT COST: $390,585 

PROJECT MANAGERS: D. Stanley and C. Wilson 

AFFILIATION: Coastal Fisheries Institute, Center for Coastal Energy and 

Environmental Resources 

ADDRESS: Louisiana State University, Baton Rouge LA, 70803 

PRINCIPAL INVESTIGATORS: D. Stanley and C. Wilson 

KEYWORDS: artificial reef, oil and gas platform, hydroacoustics, fish abundance, fish 

density, target strength, visual survey, Gulf of Mexico, species composition, hypoxia, 

continental shelf, continental slope 

BACKGROUND: The largest artificial reef complex in the world, although unplanned, is 

composed of the 4,000 petroleum platforms scattered across the outer continental shelf 

(OCS) of the northern Gulf of Mexico (GOM). Scientists have hypothesized that 

artificial reefs and platforms improve and/or diversify habitat, increase resources, modify 

the assemblages of organisms in the region or concentrate existing resources. The 

placement of these defacto reefs has undoubtedly impacted the regional marine 

community although little information is available. Only recently have assessment 

methods been developed to test these and other hypotheses concerning artificial reefs. 

Scientific investigations of fish assemblages at petroleum platforms did not start until the 

late 1970's. The reports consisted of visual surveys conducted by SCUBA divers, 
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remotely operated underwater vehicles (ROV) and stationary cameras; the majority 

were short term, often only “snapshots” of the fishes at each site. The results of this 

early research provided insights into structure associated assemblages; abundance and 

species composition varied with platform, water depth and time of the year. However, 

results were difficult to compare due to problems with limited visibility, gear bias, diver 

avoidance and a lack of standardized survey methodology. 

OBJECTIVES:

1) Measure and compare the species composition, abundance and size frequency 

distribution of fish associated with three petroleum structures of varying depths. 

2) Determine the effect of temporal, physical and chemical variables on species 

composition, biomass and size distribution of fish associated with platforms. 

3) Define the spatial near field influence of each platform on the abundance of fish and 

estimate the associated fish population. 

DESCRIPTION: We used complimentary sampling methods of visual surveys and 

quantitative dual beam hydroacoustic surveys to document the assemblage of fishes 

associated with three petroleum platforms in the northern Gulf of Mexico. With the 

sampling protocol established, the objectives of this research were to use dual beam 

hydroacoustics in conjunction with visual point count surveys to measure the density 

and size distribution of fishes associated with three petroleum platforms off the 

Louisiana coast. The goals of this research were to determine the effect of water depth 

on fish density, size distribution and species composition and ultimately to measure the 

fisheries value of platforms of different depths in the same geographical region. 

SIGNIFICANT CONCLUSIONS: The results of this project demonstrate the variability 

in abundance, size and species composition of fishes associated with petroleum 

platforms. Similar results from earlier studies have been found with natural and artificial 

reefs. The variability in density and the size distribution of fishes at petroleum platforms 

in this project was linked to temporal, spatial and environmental variables. 

This research confirms the variability of fish assemblages associated with petroleum 

platforms and reinforces the need to sample on each side and throughout the water 

column to obtain an accurate estimate of fish abundance. The high abundance of 

fishes found at the sites demonstrates the importance of petroleum platforms to the 

marine environment of the northern GOM. Although some variance was observed, 

10,000 to 30,000 fishes were found per site at any one time and since over 1,000 

platforms are located in similar water depths it is clear that these structures impact the 

fisheries of the region. 

In response to the use of platforms by fishers in the region Louisiana and Texas both 

created artificial reef programs where the materials of choice are retired platform 

jackets. The standard deployment of these structures as reefs involves placing the 

jacket on its side, however, this minimizes vertical relief. If a platform such as GC18 

were deployed in this manner it would extend approximately 80 m off the bottom. 

Based on our results at the site very few fishes would utilize the structure in this 
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orientation and its value, as an artificial reef would be questionable. This project is the 

first demonstrating the importance of vertical relief in maximizing the effectiveness of 

platforms as artificial reefs, especially with respect to deepwater environments. 

This study continues to demonstrate the utility of merging hydroacoustics and visual 

survey techniques to study the assemblage of fishes associated with petroleum 

platforms. The combination of these techniques allows for the measurement of the area 

of influence of these defacto artificial reefs, as well as estimates of abundance, size 

distribution and species composition throughout the water column and over long time 

periods. 

STUDY RESULTS: At the shallowest site, South Timbalier 54 (ST54, water depth 22 

m), the decline in fish density with distance from the platform was precipitous; beyond 

18 m fish densities were similar to that of the open waters of the northern Gulf of 

Mexico. At the deeper sites a drop in fish density with distance from the site existed, 

although fish density was much higher to greater distances. At both Grand Isle 94 

(GI94, water depth 60 m) and Green Canyon 18 (GC18, water depth 219 m) fish density 

approached that found in the open waters of the Gulf of Mexico after a distance of 

approximately 50 m. Despite some complications hydroacoustics again illustrated its 

effectiveness in defining “area of influence” of the platform or the effective size of the 

artificial reef to the fish assemblage. As we defined it, the area of influence extended 10 

m at GC18 and 18 m at GI94; fish densities within these distances were significantly 

higher than densities at greater distances and while this may result in conservative 

estimates of total abundance we feel it more accurately reflects the true abundance of 

fishes at the sites. 

Six species made up over 90% of the fishes observed at each site on any survey and 

platform assemblages could be characterized as not specious and dominated by a few 

individual species. By site the dominant six species (highest to lowest abundance) 

were; GC18; creolefish, blue runner, Bermuda chub, almaco jack, amberjack, and 

barracuda; GI94; blue runner, horseeye jack, red snapper, mangrove snapper, gray 

triggerfish and barracuda; ST54; Atlantic spadefish, bluefish, blue runner, mangrove 

snapper, red snapper and sheepshead. While the species found at each site were 

somewhat unique and the dominant species at each site was different, a large amount 

of overlap of observed species existed between sites, especially adjacent sites. 

Seasonal migrations were common at each of the study sites. Since this research took 

place throughout the year we were able to document the seasonality alluded to by past 

research. Common seasonal migrations observed in the winter were the appearance of 

significant numbers of Florida pompano and bluefish at ST54; and the appearance of 

blue runner and barracuda at GI94 and GC18. Summer seasonal observations 

included the appearance of tarpon and cobia at ST54, an increase in abundance of 

cobia, red snapper, greater amberjack and almaco jack at GI94, while no summer 

migrants were observed at GC18. 

Comparison of results from this research with other petroleum platform studies from the 

northern Gulf of Mexico revealed similarities as well as significant differences in density 
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and abundance. Comparison of acoustically derived estimates of density from our past 

research showed similar values with those from this project especially at ST54 and 

GI94. Mean densities from our earlier work at a site in 24 m of water were 0.244 (+/- 

0.062, 95% confidence interval) fishm-3, while mean densities found during this project 

were 0.333 (+/- 0.034) fishm-3 at ST54, 0.496 (+/- 0.017) fishm-3 at GI94 and 0.029 

(+/- 0.003) fishm-3 at GC18. 

Total fish abundance estimates from the platforms in this project summer within the 

range of estimates of fishes at platforms from our earlier research in the Gulf of Mexico 

but are higher than those derived by visual only surveys. Our earlier research found an 

average of 12,473 (+/- 6,522, 95% confidence interval) fishes at a site in 24m of water, 

while during this project estimates of the total number of fish per site were 13,472 (+/- 

1,346) at ST54, 28,952 (+/- 1,806) at GI94 and 13,856 (+/- 1,324) at GC18. One of the 

most interesting results of the project was the comparison of abundance between the 

sites. GC18 was 3 to 10 times larger than the others but the total abundance estimates 

were not significantly different than the smallest site (ST54), while the abundance 

estimates from GI94 were twice that of the other sites. The low abundances observed 

at GC18 are likely due to its location and water depth at the site. Fish abundance was 

essentially zero below 100m and densities above 100 m were lower than those 

observed at the other sites. Because of the location of GC18 it is not influenced by the 

eutrophic waters on the continental shelf and the outflow from the Mississippi River, it 

most influenced by oligotrophic oceanic waters from the Caribbean. The combination of 

water depth and cooler oligotrophic water conditions likely reduced the abundance of 

fishes around GC18. 

STUDY PRODUCTS:

Stanley, D.R. and C.A. Wilson. 2000. Seasonal and spatial variation in the biomass and 

size frequency distribution of fish associated with oil and gas platforms in the 

northern Gulf of Mexico. A final report for the U.S. Department of the Interior, 

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

OCS Study MMS 2000-005. 

Stanley, D.R., and C.A. Wilson. In press. Variation in the density and species 

composition of fishes associated with three petroleum platforms as measured 

with dual beam hydroacoustics. Proceedings 1997 AFS Acoustic Symposium, 

American Fisheries Society, Monterey CA. Fisheries Science. 

Stanley, D.R., and C.A. Wilson. 1999. Spatial variation in fish density at three 

petroleum platforms as measured with dual-beam hydroacoustics in the northern 

Gulf of Mexico. Proceedings 1997 AFS Artificial Reef Symposium, American 

Fisheries Society, Monterey CA. Gulf Science. 

Stanley, D.R., and C.A. Wilson. 1996. The use of hydroacoustics to determine 

abundance and size distribution of fishes associated with a petroleum platform. 

International Council on the Exploration of the Sea, Journal of Marine Science. 
ACCESS NUMBER: 30660-19915

53:473-475. 

Stanley, D.R., and C.A. Wilson. 1999. The effect of depth on the abundance and 

species composition of fishes associated with petroleum platforms in the northern 

Gulf of Mexico. Proceedings: Seventeenth Annual Gulf of Mexico Information 

Transfer Meeting. U.S. Department of the Interior, MMS, New Orleans, LA. OCS 

Study MMS 99-0042. 

Stanley, D.R., and C.A. Wilson. 1996. Variation in the abundance of fishes associated 

with a petroleum platform in the northern Gulf of Mexico. Proceedings: Fifteenth 

annual Gulf of Mexico Information Transfer Meeting. U.S. Department of the 

Interior, MMS, New Orleans LA. OCS Study MMS 96-0056. 

 

Stanley, D.R., and C.A. Wilson. 1996. Seasonal and spatial abundance and size 

distribution of fishes associated with a petroleum platform in the northern Gulf of 

Mexico. Proceedings: Fourteenth Annual Gulf of Mexico Information Transfer 

Meeting. U.S. Department of the Interior, MMS, New Orleans LA.