Vertical distribution of fish biomass in hypoxic waters on the Gulf of Mexico shelf

Hypoxic bottom waters (dissolved oxygen <= 2.0 mg 1(-1)) in the northwestern Gulf of Mexico are largely due to nutrient loading from the Mississippi-Atchafalaya watershed. Loss of benthic habitat has been documented in bottom trawl surveys, but little is known about the effect of hypoxia on the vertical distribution of fish biomass. To investigate these effects, we used a 120 kHz split-beam echosounder to compare the vertical distribution of fish biomass at stations with hypoxic bottom waters to those with normoxic bottom waters. We also used paired mongoose and flat trawls to assess species composition, and a CTD to measure physical characteristics of the water column. Atlantic croaker Micropogonias undulatus, Atlantic bumper Chloroscombrus chrysurus, and anchovies (Anchoa spp.) comprised 92% (by number) of fish sampled. Dissolved oxygen, time of day, and depth within the water column were the major factors explaining variation in acoustic biomass. Stations inside and outside of the hypoxic zone had similar overall density but differed in vertical distribution. Hypoxic stations had greater biomass in the upper 7 m of the water column and much less biomass below 13 m compared to normoxic stations, consistent with aggregation of organisms above the bottom hypoxic layer. We did not find evidence of strong aggregation at the hypoxic edge throughout the entire water column. While the pelagic habitat is not directly impacted by low-oxygenated bottom water, hypoxia can induce vertical or horizontal displacement of fish mediating potential indirect bioenergetic or trophic interactions.