Effects of red tide on bottlenose dolphins
By Spencer Fire, PhD candidate, University of California, Santa Cruz

For decades “red tide” has been a nuisance along Florida’s Gulf Coast and has had a significant impact on the economy, wildlife, and human health of many coastal regions of the U.S. It has been responsible for shellfish poisoning, fishery closures, loss of tourism and die-offs of marine animals, including marine mammals. In recent years, several large mortality events of bottlenose dolphins in the Gulf of Mexico and Atlantic Ocean have been suggested to be caused by red tide. Brevetoxin, the neurotoxin produced by the red tide alga Karenia brevis, has been shown to have harmful effects on a wide variety of organisms, but its effects on bottlenose dolphins are unclear. The aim of my research is to gain an understanding of the impact of red tide on the diet, health, and behavior of bottlenose dolphins in Florida’s Sarasota Bay area.

My study involves quantifying brevetoxin levels in the tissues of fish eaten by dolphins, as well as in dolphin tissues recovered from carcasses stranded during red tide events. This will give insight to what levels of brevetoxin are present in the dolphins’ diet and how the toxin is distributed throughout the animal once ingested. The behavioral response of dolphins to the presence of red tide is also being investigated. It is unknown whether dolphins are aware of (or react to) the presence of high concentrations of Karenia brevis during red tide events. By observing their behavioral states and recording their movements relative to concentrations of Karenia brevis, we may be able to determine if there is a response to the toxic event. The purpose of all these efforts is to estimate the levels of harmful toxin to which dolphins are exposed, and through which pathways the exposure presents itself. It is hoped that an increased understanding of how dolphins are affected by red tide will help conservation efforts in the future.

Preliminary findings show that the majority of dolphin carcasses recovered during active red tide events have detectable levels of brevetoxin. Samples of liver, kidney, lung, muscle and blubber tissue, as well as stomach contents, urine and fecal samples have concentrations of brevetoxin ranging from 7 to 2,900 nanograms per gram (ng/g) of tissue (as point of reference, the current regulatory limit for brevetoxin in shellfish for humans is 800 ng/g). Samples of dolphin prey fish (pinfish, spot, pigfish, mullet) taken during a red tide event also had detectable levels of brevetoxin in their tissues, with concentrations ranging from 3 to 261 ng/g. Brevetoxin has also been detected in samples from dolphin carcasses and fish recovered more than 6 months after the last red tide occurrence in the same area, which raises further questions about the residence time of the toxin in these animals. Support for lab analyses and field observations has been generously provided by Long Marine Laboratory, Disney Wildlife Conservation Fund, and Harbor Branch Oceanographic Institution’s Protect Wild Dolphins Program.