Effects of red tide on dolphins
By Spencer Fire, PhD-ABD, University of California, Santa Cruz

For decades "red tides" have been considered to be a nuisance along Florida’s Gulf coast and have had a significant impact on the economy, wildlife, and human health of many coastal regions of the U.S. This type of harmful algal bloom (HAB) has been responsible for shellfish poisoning, fishery closures, loss of tourism and marine animal die-offs, including marine mammals. In recent years, several Unusual Mortality Events involving bottlenose dolphins in the Gulf of Mexico and on the Atlantic coast have been suggested to be caused by red tide. Brevetoxins, the neurotoxins produced by the red tide alga Karenia brevis, have been shown to have harmful effects on a wide variety of organisms, but their effects on bottlenose dolphins are unclear. The aim of my research has been to gain an understanding of the impact of red tide on the diet, biology, and behavior of bottlenose dolphins in Florida’s Sarasota Bay area.

My study involves quantifying brevetoxin levels in dolphin carcasses, in dolphin prey items and in live dolphins sampled during red tide events. This gives insight into what level of brevetoxin exposure is associated with dolphin mortalities in areas frequently affected by red tides. It is unknown whether dolphin behavior is affected by red tide, and this study has sought to observe their behavioral states and record their movements relative to Karenia brevis concentrations. It is hoped that an increased understanding of how dolphins are affected by red tide will help conservation efforts in the future.

Laboratory analysis of samples collected from dolphin carcasses show that the majority of animal carcasses recovered during active red tide events have detectable levels of brevetoxins. 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 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. Brevetoxins have 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. Additionally, live dolphins sampled during red tide events had detectable levels of brevetoxins. Behavioral data from dolphins observed during red tide indicate that these animals have a higher incidence of chuffing (a type of explosive exhalation) compared with non-exposed animals. Also data suggest that dolphins utilize waters with lower K. brevis levels compared with levels found in nearby locations. Support for lab analyses and field observations has been generously provided by Long Marine Laboratory, Disney Wildlife Conservation Fund, National Marine Fisheries Service, Harbor Branch Oceanographic Institution’s Protect Wild Dolphins program, and the Chicago Zoological Society.