Since 2004 we have been measuring fish abundance, distribution, and species composition in Sarasota Bay, with particular interest in species that are important food sources of bottlenose dolphins. This has provided us with an opportunity to examine red tide effects on the estuarine fish community. The phenomenon known as red tide is a familiar sight to those living along the coasts of Florida. It is caused by the single-celled alga Karenia brevis which, under the right conditions, can bloom to over 1,000,000 algae cells per liter of sea water. K. brevis produces lethal neurotoxins, called brevetoxins, which affect the respiratory system of vertebrates including fish, manatees, sea turtles, seabirds, and dolphins. Fish are exposed to brevetoxins by inhalation through their gills or by consuming food containing the toxins. Severe red tide blooms will result in extensive fish kills. The toxic conditions resulting from red tide blooms can be exacerbated by hypoxia (low levels of oxygen in the water) caused by the large numbers of decomposing fish. For dolphins, the main route of bevetoxin exposure is most likely food consumption. However, strong indirect effects could result from dramatic drops in prey abundances and/or shifts in species composition.
        To date, we have completed 869 purse seine (sampling gear) sets in 5 distinct habitats, sampled 350,215 fish (primarily catch and release sampling), and identified over 132 different species. Our 2008 summer (Jun.-Sept.) sampling targeted the seagrass habitat because it is one of the most productive habitats in Sarasota Bay. We completed 40 purse seine sets, sampled 50,610 fish, and identified 56 species during this sampling period. Since 2004, two red tide periods have occurred within Sarasota Bay. A severe and prolonged red tide occurred from February to December of 2005 and covered much of the west coast of Florida. A moderate red tide occurred from mid-August to December 2006. Our previous sampling has shown that in general the relative abundance of fish decreased 10% when non-red tide periods and red tide periods from 2004-2007 were compared. Previous work suggests that soniferous (noise-making) fishes, such as pigfish and spotted seatrout, are selected and consumed by dolphins using passive listening. These soniferous species decreased 85-97% in relative abundance during red tide periods as compared to non-red tide periods. Additionally, species diversity dropped and the fish community shifted from dominance by bottom-dwelling species like pinfish, mojarra, silver perch, and pigfish toward dominance by pelagic filter feeders, such as the Atlantic thread herring during red tide periods.
        Within the last 20 years, Sarasota Bay has been affected by red tide about every 2-3 years. We must not only understand the effect that red tide has on fish and dolphin populations but we must also understand the speed at which these populations recover. Since the 2005 and 2006 red tide events, we have seen a complete recovery of the Sarasota Bay estuarine community. In the seagrass habitat, the overall relative abundance of fishes rebounded by summer 2006 (132% over 2004 levels) despite the moderate red tide during the late summer months. However abundances were mainly due to the red-tide-induced shift in community structure, resulting in an increase in the abundance of pelagic filter feeders. Species richness remained 86% of 2004 levels.

        By 2007, Sarasota Bay had experienced two red tides and fish abundance in the seagrass habitat was 74% of 2004 levels. Currently, most fish species have reached or exceeded their 2004 pre-red tide abundance levels. Fish caught per purse seine set in the seagrass habitat increased by 40% from summer 2004 to summer 2008 and soniferous fishes increased by 137% within that same period (Fig. 1). Overall species richness reached and exceeded pre-red tide levels by summer 2007, and the fish community structure has shifted back to being dominated by bottom-dwelling species once again. Our data indicate that some species take longer to recover from red tide than others, fish abundance on its own may not be an accurate reflection of the estuarine community’s recovery, and both species richness and soniferous dolphin prey abundance may take up to two years to recover to pre-red-tide levels.
        While we have determined that severe red tides greatly decrease fish abundance, decrease species composition, and shift fish community structure, within two years most species recovered to or exceeded pre-red-tide abundance levels and measures of species richness. Questions we would like to focus on in the future include the effect of red tides on the growth rates, body condition, reproductive rates, habitat selection, and behavior of both fishes and dolphins. We hope to answer more of these questions by extending our long-term fish sampling program in Sarasota Bay.