Effects of red tide on prey availability
By Damon Gannon, PhD, and Elizabeth Berens, MSc

Red tide can affect fish in several ways: by exposure to brevetoxin (the neurotoxin produced by the red tide organism, Karenia brevis) in the water; by consuming food that is tainted by brevetoxin; or by exposure to hypoxic water (water with unusually low concentrations of dissolved oxygen), which often accompanies severe red tides. Red tide causes an increase in the biomass of dead organisms in the water, and the process of decomposition uses up oxygen. Both brevetoxin and hypoxia can kill fish.

The 2005 severe red tide event in Sarasota Bay appears to have had a significant effect on the bay’s fish community. SDRP’s quantitative survey of fish resources in Sarasota Bay documented dramatic decreases in fish abundance coinciding with the red tide. Compared to the same period in 2004 (during which there was no red tide), overall catch rates of fish in the summer of 2005 dropped by 49.9%. Declines in abundance of the species typically eaten by dolphins were even greater than the average rate for all species. For example, combined catches of pinfish (-75.4%), pigfish (-99.7%), silver perch (-99.5%), spotted seatrout (-93.2%), mojarra (-68.9%), and hardhead catfish (-96.2%) decreased by 76.1%. We plan to continue our survey to monitor the recovery of the fish community from this ecological perturbation.