Effect of Hypoxia on the Distribution of Fishes Associated with a Petroleum Platform off Coastal Louisiana

Abstract

A combination of hydroacoustics and visual survey techniques were used to estimate the abundance, size distribution, and species composition of fishes at a petroleum platform in the northern Gulf of Mexico. The effect of dissolved oxygen (DO) concentration on the density and distribution of fishes at this de facto artificial reef was examined. The techniques used permitted surveys of the entire water column, providing a more accurate reflection of the effect of a highly variable phenomenon (i.e., hypoxia) that may not be measured with traditional fisheries sampling equipment. Hypoxic conditions were observed during two of the five trips to the petroleum platform, South Timbalier 54. When hypoxia was present, fish density was dramatically higher in the strata immediately above the hypoxia. It was difficult to determine the exact impact of hypoxia on target strength (a measure of fish size), as the larger mean target strengths observed in the spring and summer may be due to seasonal factors and not to hypoxia alone. With the exception of the fall 1995 survey (which had significantly higher density and abundance estimates than other surveys), the estimated number of fishes did not vary among any of the sample periods. Even when 32% (15–22 m) of the water column was hypoxic (summer 1996), abundance estimates were slightly higher than in other survey periods without hypoxia (except fall 1995). While hypoxia reduced the availability of the water column to fishes, they concentrated where favorable DO levels were found. This included demersal species such as red snapper Lutjanus campechanus, which were found higher in the water column than expected.