Abundance of fishes associated with a petroleum platform as measured with dual-beam hydroacoustics

David R. Stanley and Charles A. Wilson

Stanley, D. R. and Wilson, C. A. 1996. Abundance of fishes associated with apetroleum platform as measured with dual-beam hydroacoustics. – ICES Journal ofMarine Science, 53: 473–475.

Stationary dual-beam hydroacoustics was used to measure the density of fishesassociated with a petroleum platform in the northern Gulf of Mexico from January1991 through May 1992. Mean monthly fish densities ranged from 0.19–0.41 fish m"3.Density varied significantly with platform side, month, and depth. Fish densities weresignificantly higher adjacent to the platform and decreased to background levels atdistances greater than 16 m. Approximately 1990 to 28 100 fish were associated withthe platform depending on month.

Copyright1996 International Council for the Exploration of the Sea

Key words: artificial reef, Gulf of Mexico, hydroacoustics, petroleum platform.

D. R. Stanley and C. A. Wilson: Coastal Fisheries Institute, Center for Coastal,Energy, and Environmental Resources, Louisiana State University, Baton Rouge,Louisiana 70803-7503, USA. Correspondence to Stanley [tel: +1 504 388 6284,fax: +1 504 388 6513].

Introduction

There are more than 4100 petroleum platforms presentin the northern Gulf of Mexico, providing an additional 5000 km2of hard substatum to the approximately 2800 km2 of existing natural hard bottom. These struc-tures act as artificial reefs and their addition is one of themost extensive habitat modifications in the world.

Research on the abundance and composition of fish assemblages surrounding petroleum platforms has been limited owing to logistical sampling diYculties and a lack of applicable fisheries-independent sampling methods. Previous researchers have used a variety of methodologies including catch per unit eVort studies and visual surveys by SCUBA divers, remotely operated underwater vehicles, and fixed underwater cameras; however, the majority of these studies were only short-term snapshots of the abundance and composition offishes (Continental Shelf Associates, 1982; Gallaway andLewbel, 1982; Putt, 1982; Stanley and Wilson, 1990,1991).

In response to the diYculty of estimating fish abun-dance at petroleum platforms we used a stationary array of dual-beam hydroacoustics. Hydroacoustics provides precise and accurate estimates of absolute abundance and size distribution, is not aVected by poor visibility, can sample at night, has target detection capabilities to macrozooplankton sizes, and has a greater range than do visual methods.

The objectives of this research were to use stationary dual-beam hydroacoustics over a 15-month period to: (1) measure and compare the abundance of fishes associ-ated with a petroleum structure over time, (2) define the spatial near-field influence of the platform on fish abundance.

Methods

The study was conducted at a production petroleum platform operated by Mobil Exploration and Produc-tion USA, Inc., located 80 km south of the Louisiana coast in 23 m of water. It was installed in 1978 and the nearest platform was 14.5 km.

Stationary dual-beam hydroacoustic surveys were conducted monthly at the platform from January 1991 through May 1992, with the exceptions of February and May 1991.

Two arrays of stationary dual-beam hydroacoustic equipment were used. Array 1 was designed to measure the density of fishes immediately adjacent to the plat-form. It consisted of four vertically oriented transducers; one transducer on each side of the platform providing acoustic coverage from 2 to 20 m. Array 2 estimated the local area of influence of the platform. It consisted of four horizontally aligned transducers, one deployed on each side of the platform at a depth of 11 m. This arrangement allowed for estimates of relative density from 2–72 m away from the platform.

Horizontal and vertical acoustic sampling was carried out over consecutive 24 h intervals for each sampling trip; 2 h of hydroacoustic data were collected encompassing dawn, noon, dusk, and midnight.

Acoustic data were collected and processed with Bio-Sonics hydroacoustic equipment; 120 kHz dual-beam transducers were deployed on the north and west sides of the platform, and 420 kHz dual-beam transducers were deployed on the south and east sides of the platform. Background noise levels were approximately 20 mV at 21 m for Array 1 and 40 mV at 72 m for Array 2. The voltage threshold used in later analyses was 100 mV, corresponding to a minimum detectable target strength of "56 dB, or a fish of 2.5 cm total length according to Love (1971).

Vertical density (log(density+1)) data were modeled using a randomized block ANOVA, blocking on side of the platform, against depth, time of day, month, and their interactions (SAS 1986). Horizontal density (log(density+1)) data were modeled using a randomized block ANOVA, blocking on side of the platform, against distance, time of day, month, and their interac-tions. The total abundance estimates at the platform were calculated by determining the local area of influ-ence of the platform, then multiplying mean density values for each month and platform side, in number of fish m "3, by the volume of water on each side of the platform.

Results

Fish density was highly variable over the study period, with monthly means ranging from 0.04–0.50 fish m -³. Vertical fish density varied significantly with month, platform side, and water depth. Significant variations in densities were observed with time of day within month although a consistent diel pattern was not observed. Little pattern in fish densities from month to month was observed; densities varied by as much as a factor of five between months. Fish density also varied with depth and was significantly greater from 4–12 m than from 16–20 m.

Horizontal density varied with month, side of the platform, and distance from the platform. Fish density decreased significantly with distance from the platform. Densities within 16 m were significantly higher than densities from 16–72 m (Fig. 1). Since the density of fishes dropped to background levels (i.e. less than 0.05 fish m"3) (Stanley and Wilson, unpubl. data) at dis-tances greater than 16 m from the platform we defined the local area of influence as 16 m.

The population of fish associated with the platform fluctuated with month from a low of 1988 &413 in January of 1991 to a high of 28 138 &5532 in February 1992. Total abundance estimates were highly variable from month to month with a maximum variation of approximately 6.6 times. The average number of fish at the platform over the study period was 12 473&3251.

Figure 1. Mean horizontal fish density (number of fish m"3) from 2–72 m from a petroleum platform for each side of the platform for the time period January 1991 to May 1992.

Discussion

Dual-beam hydroacoustics was a useful method for assessing the abundance of fishes and measuring the local area of influence of a petroleum platform. The most dramatic results were the large monthly variations in density of fishes. This is consistent with previous research at natural and artificial reefs which exhibit large variations in density with time (Bohnsak et al., 1991; Ebeling and Hixon, 1991; Sale, 1991). The current hypothesis regarding fish abundance at reefs is the open non-equilibrial system, which proposes that substantial spatial and temporal changes occurring at local reef populations are due to variable recruitment, emigration, and immigration (Doherty and Williams, 1988; Ebeling and Hixon, 1991; Sale, 1991).

Since fish density varied with both platform side and depth, and target strength varied with side of the plat-form, comprehensive spatial sampling is required to describe the assemblages associated with these artificial reefs. The ability to measure changes in density through-out the water column is crucial and we were able to accurately survey nekton in the low visibility zones which could not be assessed using visual surveys.

Dual-beam hydroacoustics also gave the first measurement of the local area of influence of an artificial reef. Past researchers have inferred it to range from 5–50 m (Continental Shelf Associates, 1982; Gerlotto et al. , 1989). Based on the horizontal density data results, we estimate the local area of influence at 16 m; at greater distances the fish densities are similar to those detected in the open water of the Gulf of Mexico (Stanley and Wilson, unpubl. data).

Total fish abundance estimates at this platform were higher than those reported in other platform studies, with an average of 12 473 fish documented over the study period. Other researchers found 283–3955 fish associated with individual platforms (Continental Shelf Associates, 1982; Putt, 1982; Gerlotto et al., 1989). DiVerences in the estimated fish abundances between this study and previous investigations may be due to the larger area of influence measured in this study, the ability to sample in low visibility conditions, and the unobtrusive nature of hydroacoustics.

Acknowledgements

The authors thank Mobil Exploration and Production USA, Inc. (MEPUS) for logistical support and access to the West Cameron 352 petroleum platform. We also thank the sta V of WC 352 and Messrs ‘‘Doc’’ Brinkley, Curtis Cain, Jerry Hagen, Clyde Jones, JeV Passmore, and Alan Mathieson for their help and patience with our activities. Comments by two anonymous reviewers greatly improved the quality of the manuscript. This study is part of a dissertation written by the senior author and was supported by the Louisiana Department of Wildlife and Fisheries, Coastal Fisheries Institute, Louisiana State University, MEPUS, and the Mobil Foundation. This is LSU-CFI Publication 95-2.

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