MEPS 608:199-219 (2019)  -  DOI: https://doi.org/10.3354/meps12772

ABSTRACT: River discharge and seasonal, eutrophication-driven hypoxia within coastal waters of the northern Gulf of Mexico create a wide range of environmental conditions for fishes. Environmental conditions are most dynamic on the Louisiana shelf, a region in which oil and gas platforms (hereafter platforms) are abundant and serve as artificial reefs. Platforms provide a unique, vertically oriented substrate for fouling organisms and nekton throughout the water column and often span substantial gradients of salinity, temperature, dissolved oxygen (DO), and water clarity. During the summers of 2013 and 2014, we used paired video and hydrographic samples taken at 150 small platforms sited in nearshore waters (3.7-18.0 m water depth) to document the responses of platform-associated fishes to coastal water quality. Responses of the 26 analyzed fish species included substantial changes in habitat selection coupled with vertical habitat compression. The vertical extent of the water column suitable for fishes was compressed due to avoidance of hypoxic bottom water (DO <50% saturation) and apparent avoidance of eutrophic surface strata with supersaturated oxygen conditions (140.7 ± 7.0% saturation [95% CI]). Generalized linear mixed models, habitat suitability indices, and factor analysis suggested that fish habitat-selection patterns varied significantly in the presence of hypoxia by up to 10 salinity units, 10°C, 30% DO saturation, 6 m water depth, and 4.5 m Secchi depth. The differences in intraspecific distributions occurring in the presence and absence of hypoxia were often greater than interspecies separations within comparisons. These findings demonstrate an important indirect effect of eutrophication-driven hypoxia on fishes.

KEY WORDS: Hypoxia · Habitat compression · Eutrophication · River-influenced ocean margins · Physicochemical fish responses · Artificial reefs