Environmental conditions and catch rates of predatory fishes associated with a mass mortality on the West Florida Shelf

• Large scale fish kill observed in offshore waters of the eastern Gulf of Mexico.
• Compared water quality and catch data collected within area over a 20 year period.
• Anomalies in some environmental variables were evident.
• Catch was significantly lower during fish kill than in previous years.
• Red grouper identified as the most severely impacted species of among large fishes.

While conducting a standardized fisheries-independent longline survey in the northern Gulf of Mexico on August 20–21, 2014, dead and/or moribund fishes, estimated to number in the thousands, were observed within a well-defined area of the West Florida Shelf. Fishes from 15 families were identified; however, numerous individuals of relatively large-bodied serranid species were decomposed beyond a state that would allow for identification below the family level. Based on survey catch data from previous years and morphological characteristics associated with the decomposing fishes, it was determined that most of the large unidentified fishes were red grouper (Epinephelus morio). Water profiler cast data collected within the area demonstrated that when compared to previous years (1995–2013) bottom temperature and salinity were consistent with what would be expected; however, dissolved oxygen concentration was lower than normal, and in some cases, hypoxic and chlorophyll a and transmissivity values were anomalously high and low, respectively. Hypoxia, high chlorophyll a concentrations and low transmissivity are thought to have resulted from a bloom of Karenia brevis, which was documented to have occurred in close proximity to the sampling area. As necropsies were not performed, it was not possible to state a definitive cause of death as the effects of brevetoxins are species-specific. However, numerous individuals of most impacted species were observed floating incapacitated, yet alive, in normoxic surface waters suggesting that the impacts we observed were due to the neurotoxicological and/or hemolytic effects of a harmful algal bloom.