Effects of Red Tide on Prey Availability
By Damon Gannon,PhD, Elizabeth Berens, MSc, and Sandra Camilleri, BSc

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 and may, therefore, decrease the amount of food available to dolphins.

Sarasota Bay experienced a severe red tide event (›100,000 cells/liter) from February to October 2005, which 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 (in which there was no red tide), overall catch rates of fish in the summer of 2005 dropped by 50%. 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%. Despite these massive declines in fish abundance in 2005, some species returned to their pre-red tide levels by the early summer of 2006. Some of the longer-lived, slower-growing species, such as spotted seatrout, have not yet fully recovered. The abundance of one species, thread herring, actually appears to be positively correlated with the presence of red tide. It is possible that thread herring, which is normally not an important prey species for dolphins, provide a food source for dolphins during severe red tides.

Our investigation of the effects of red tide on dolphin prey is in its early stages, and so far the work has generated more questions than answers. We plan to continue our survey to monitor how the fish community responds to ecological perturbations caused by red tide, including the red tide that started in August 2006.