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2004 Bottlenose Dolphin Unusual Mortality Event Along the Florida Panhandle

bottlenose dolphin in watery grass bed
Bottlenose Dolphin mortality
(Tursiops truncatus)
Photo: NOAA

Executive Summary
This interim report outlines the initial findings and the ongoing analyses in the investigation of the bottlenose dolphin (Tursiops truncatus) mortality event that began in March 2004. One hundred and seven (107) bottlenose dolphins stranded dead along the Florida Panhandle between March 10 and April 13, 2004. Hundreds of dead fish and marine invertebrates were also discovered in the area. The National Marine Fisheries Service formally declared the dolphin deaths an "Unusual Mortality Event" (UME) after consulting with the Working Group on Marine Mammal Unusual Mortality Events. A multi-agency investigation was initiated and is being conducted by Federal and state marine wildlife officials working in partnership with private research organizations and universities.

Analyses conducted to date found brevetoxins, naturally occurring neurotoxins produced by Karenia brevis, the Florida red tide, at high levels in the stomach contents of all dolphins examined to date and at variable levels in the tissues of these animals. Metabolite concentrations in tissues of fish or dolphins are not yet completed. The concentrations of brevetoxins observed in the analyzed subsample of the stomach contents are greater than or equal to those observed in previous marine mammal mortality events associated with Florida red tides (Karenia brevis blooms) in the Gulf of Mexico. In most of the dolphins, the first chamber of the stomachs was gorged with large amounts of fish, some of which were partially whole and undigested indicating recent feeding. Fish (planktivorous, herbivorous, and omnivorous fish species) collected from St. Joseph Bay on March 18 tested positive for brevetoxins in stomach contents and in muscle, liver, and gill tissues whereas fish collected on March 28-31 also tested positive for brevetoxin but at much lower levels (except in sea trout). Satellite imagery of the northern Gulf of Mexico indicated elevated chlorophyll levels in the UME area March 9-11, 2004 but water samples collected March 11 and later in the area of the UME did not contain significant quantities of Karenia brevis, although low levels of brevetoxins were detected. Thus, the source of the brevetoxins involved in this event remains uncertain, but the presence of toxic fish and water suggests that there was an undetected bloom somewhere either in the bay itself or in waters in which the fish or dolphins were feeding. A similar dolphin UME occurred in 1999-2000 in the same area of Florida and was correlated with a Karenia brevis bloom.

In addition to brevetoxin, low levels of domoic acid were detected in the stomachs, urine and feces of some dolphins. Further, Pseudo-nitzschia delicatissima, a diatom that can produce domoic acid was present at low to moderate levels in water samples. However, the levels of domoic acid detected were 1-2 orders of magnitude lower than those found in domoic acid related marine mammal mortalities on the California coast. Therefore, its role in the current event would appear to be secondary to brevetoxin if, in fact, it is involved at all.

The evidence based on gross and histological findings does not indicate an infectious process and viral testing has ruled out morbillivirus, a virus known to cause high morbidity and mortality in dolphin populations. The lack of evidence of infectious disease, the wide age class spread of the mortalities, and the fact that most animals had recently fed (demonstrated by full stomachs) does implicate a toxin of some type as one of the causes of the event. The Investigative Team is continuing to look at all potential causes of the mortality event and will conduct further analyses of the brevetoxin metabolites, prey and stomach contents, and genetic identification of the Florida Panhandle population. Additional work includes quantification of the estimated dose of toxins received by the animals, examination of any predisposing factors, and searching for evidence of any other toxicants that might be in dolphin prey species. The fact that southwest Florida has significant blooms of Karenia brevis almost annually without unusual numbers of bottlenose dolphin mortalities makes this investigation even more perplexing. In order to fully understand what is happening in these dolphin mortalities, the investigation will continue to work with an interdisciplinary team of scientists using an ecosystem approach to understand the factors that contributed to this mortality event.

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