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

Abstract

Hypoxic bottom waters (dissolved oxygen ≤2.0 mg l -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 lowoxygenated bottom water, hypoxia can induce vertical or horizontal displacement of fish mediating potential indirect bioenergetic or trophic interactions.

Modeling the Population Effects of Hypoxia on Atlantic Croaker (Micropogonias undulatus) in the Northwestern Gulf of Mexico: Part 2—Realistic Hypoxia and Eutrophication

“…In addition, while we allowed croaker to very effectively avoid low DO horizontally in our 2-D model, we did not allow for vertical movements. While there is evidence that croaker do not extensively move vertically as part of avoidance (Craig 2012), some other species are likely more inclined to use vertical movement to avoid low DO (Hazen et al 2009). Simulating realistic 3-D behavioral movement of fish remains a challenge (LaBone 2016).…”

Aggregation on the edge: effects of hypoxia avoidance on the spatial distribution of brown shrimp and demersal fishes in the Northern Gulf of Mexico

“…Two recent hydroacoustic studies reported pelagic fish biomass overlying the bottom hypoxic layer, suggesting that low bottom DO may have indirect effects on processes in the upper water column (Hazen et al 2009); however neither study identified the species composition of the pelagic fish community above the hypoxic layer. Analysis of paired mongoose and shrimp trawls, which sampled different portions of the bottom water column, suggests that these pelagic fish may have been mostly Atlantic bumper.…”

“…I calculated the proportion of each of 10 finfish species that occurred at stations with positive catches of brown shrimp and compared this index of spatial overlap across the 3 yr differing in the severity of hypoxia. I predicted that spatial overlap would be lowest in 2003, the year of least severe hypoxia, greatest in 2002, the year of most severe hypoxia, Vertical avoidance of bottom water hypoxia Hydroacoustic sampling at a subset of stations in 2004 indicated fish-sized biomass in the water column above the bottom hypoxic layer, raising the possibility that organisms may also avoid bottom water hypoxia by moving vertically in the water column (Hazen et al 2009). Another hydroacoustic study on the Louisiana shelf also reported pelagic, fish-sized biomass in the water column above the hypoxic zone ).…”

Modeling the Population Effects of Hypoxia on Atlantic Croaker (Micropogonias undulatus) in the Northwestern Gulf of Mexico: Part 1—Model Description and Idealized Hypoxia

“…In our simulations, individuals had to swim horizontally to minimize exposure to low DO, while higher DO water may be available a few meters higher in the water column. Hydroacoustic and trawling studies indicate that fish biomass, not resolvable to the species level, is present in the waters immediately overlaying the top of hypoxic layer suggesting vertical avoidance (Hazen et al 2009;Zhang et al 2009). Even so, based on an analysis of paired trawls that sampled different portions of the lower water column, Craig (2012) found no evidence that croaker moved vertically to avoid low DO.…”