Seasonal and Spatial Abundance and Size Distribution of Fishes Associated With a Petroleum Platform in the Northern Gulf of Mexico.

Abstract

The purpose of this study was to demonstrate the utility of dual-beam hydroacoustics in measuring abundance and target strength of fishes near a petroleum platform in the northern Gulf of Mexico. From September to November 1990 fish densities ranged from 0 to 1.88 fish/m$\sp3$ and varied with platform side, depth, time of day and month. Densities were highest adjacent to the platform and decreased significantly beyond 9 m. Approximately 2,400 to 8,300 fish were associated with the platform depending on month. Target strengths ranged from $-$56 to $-$24 dB and varied with side of the platform and month. Mean monthly target strengths ranged from $-$39.4 to $-$34.3 dB. The effect of the presence of SCUBA divers on fish density and target strengths was examined during November 1991 and February, March, and June of 1992. Mean densities and fish target strengths declined significantly when divers were present. Densities dropped from 41.5% to 76.5% and target strength decreased by 0.5% to 9.1% when divers were present. The decrease in target strengths were not uniform; fish with target strengths from $-$43 to $-$37 dB decreased at the highest rate. Stationary dual-beam hydroacoustics and visual point count surveys were used in tandem to measure the density, target strength and species composition of fishes associated with a petroleum platform. Fish densities ranged from 0 to 10.5 fish/m$\sp3$ for monthly sampling trips from January 1991 to May 1992. Density and target strength were spatially and temporally variable. Target strengths varied significantly with side of the platform, month, and east and north current vectors. Density varied significantly with platform side, month, east current vector and depth. No variation in density or target strength was detected over 24 hour periods or with water temperature. Fish densities were highest adjacent to the platform and decreased significantly beyond 16 m. Approximately 1,990 to 28,100 fish were associated with the platform depending on month. A total of 19 species were observed at the platform with Atlantic spadefish, bluerunner, bluefish, greater amberjack, gray triggerfish, red snapper and sheepshead constituting 97% of observed species.