Estuaries are highly productive and ecologically rich areas that are important habitats for fish and bird species.
Over the past few decades, the frequency and duration of harmful algal blooms (HABs) have been increasing globally in coastal areas.
HABs, especially those caused by the red tide organism, Karenia brevis, occur frequently along Florida’s west coast, causing episodes of high mortality in fish, sea turtles, birds, bottlenose dolphins
Although red tide is known to cause episodes of mass mortality among marine animals, it is not known whether this disturbance results in significant declines in animal populations or changes in community structure.
This project determined the extent that the red tide toxin, brevetoxin, contributed to illness and death in stranded fish-eating birds from the Sarasota Bay area. In addition, how red tide influenced the abundance, habitat use, and behavior of fish-eating birds in the Sarasota Bay estuary was investigated.
Red tide bloom events caused by the dinoflagellate K. brevis occurred along the central west Florida coast from February 2005 through December 2005, and August 2006 through December 2006.
During these events, from February 4, 2005 through November 28, 2006, sick sea birds admitted for rehabilitation showed clinical signs including disorientation, inability to stand, incoordination, and seizures.
Testing for brevetoxin by enzyme-linked immunosorbent assay found toxin present in 69% (n=95) of rehabilitating sea birds.
Twelve of the 19 species of birds tested positive for brevetoxin exposure.
Double-crested cormorants were the most commonly affected species and presented with more severe neurological signs as compared to other species. Serial blood and fecal samples taken from several live sea birds during rehabilitation showed that brevetoxin was cleared within 5-10 from the animals’ bodies.
More than 34,000 bird observations from boat-based surveys were obtained during summer and winter seasons from 2006 through 2009 in Sarasota Bay, involving more than 20 different species.
The most abundant bird species were double-crested cormorants, laughing gulls, and brown pelicans. Periods of high red tide cell densities (>105 cell l-1) occurred during the summer 2006 and the winter 2007 seasons.
Overall bird densities were lower during red tide blooms than they were during non-red tide bloom conditions. In particular the lower density of birds was attributed to decreased abundance of double-crested cormorants in all habitats during red tide bloom conditions.
In contrast, brown pelicans and laughing gulls had no change or increased in abundance during red tide conditions. I
t is probable that cormorants are consuming different prey than pelicans and gulls and may be exposed to a higher dose of toxin leading to increased illness and death, and lower abundances during red tide events.
This project was supported by a Morris Animal Foundation Research Grant, Florida’s State Wildlife Grants Program, and an EPA Star Fellowship