Karenia brevis, the single-celled alga responsible for Florida red tides can affect fishes in several ways.  Karenia brevis produces lethal neurotoxins, called brevetoxins, which affect the respiratory system.  Fishes can be exposed to the toxins by inhaling them directly from the water through their gills or by consuming food that contains the toxin.  Also, the many dead and decomposing organisms associated with a red tide can lead to hypoxia, or dangerously low levels of oxygen in the water (the process of decomposition uses oxygen).  Hypoxia can kill more fishes and invertebrates.  Red tides are a natural phenomenon, but they may be increasing in frequency and/or intensity.  In Sarasota, red tides occur almost every year.
     Our program is interested in finding out whether the episodes of high fish mortality that occur during red tides cause significant decreases in fish abundance (particularly for species that are important food sources for dolphins) or change the species composition of the fish community.  We also would like to know how long it takes for the fish community to recover once a red tide has ended.  Our sampling efforts of the fish community in Sarasota Bay continued this year.  Since the summer of 2004, we have completed 790 purse seine sets (a type of fish sampling net) and sampled 268,291 fish.  The vast majority of our work involves catch and release sampling.  This summer we set a record for both the number of purse seine sets completed (146) and the number of fish caught (85,032). 
     Two red tide events have occurred in Sarasota Bay since we started this research: a prolonged and intense one from February to December of 2005 and another from Mid-August to December of 2006.  In seagrass beds (one of the most productive habitats in Sarasota Bay), the average abundance of fish during the red tides was 10.3% lower during red tide conditions.  However, the changes in the abundance of fish species that are typically eaten by dolphins were much greater.  For example, pinfish (-71%), pigfish (-97%), silver perch (-94%), spotted seatrout (-85%), and mojarra (-55%) all decreased sharply.  One species, Atlantic thread herring, actually increased by 140% in the seagrass habitat during red tide.  Thread herring appear to be more tolerant of brevetoxin than do other species.  They are not normally an important part of the dolphin diet, but it is possible that thread herring are an alternative food source during red tides.  One of the more striking effects of red tide is its effect on biodiversity.  The number of fish species caught per purse seine set was cut in half during the red tide.
     So red tide reduces fish abundance, causes a decrease in species diversity, and causes a shift in the fish community from dominance by bottom-dwelling species like pinfish, mojarra, silver perch, and pigfish toward dominance by pelagic filter feeders, such as the thread herring.   The good news is that the estuarine fish community appears to be quite resilient and most species returned to pre-red tide abundance levels within two years.  There are still many unanswered questions regarding the effects of red tide on fishes, such as how it might affect growth rates, body conditions, reproductive rates, habitat selection, and behavior.   We hope to be able to answer more of these questions with continued sampling of Sarasota Bay’s fish community in 2008.

Striped (black) Mullet (Mugil cephalus)
Sheepshead  (Archosargus probatocephalus)
Pinfish  (Lagodon rhomboides)
From top to bottom (perch, spot, seatrout) (Bairdiella chrysoura) (Leiostomus xanthurus) (Cynoscion nebulosus)