Bottlenose Dolphins of the St. Johns River, FL: Stranding Data, Freshwater Overexposure Cases, Unusual Mortality Event 2010
Bottlenose dolphins, Tursiops truncatus, inhabiting the St. Johns River (SJR), Florida are a sparsely documented population of cetaceans.1,2 Between July 10 and September 4, 2010, 11 dolphin carcasses were recovered from brackish inshore areas of the SJR, where this species had not previously been formally documented, prompting the declaration of an Unusual Marine Mammal Mortality Event (UME) by National Oceanographic and Atmospheric Association (NOAA). The UME followed a harmful algal bloom and a chronic, unusual fish kill that occurred in late May through early July 2010, and was co-associated with dredging activities that spanned the SJR where dolphins transition from mesohaline to oligohaline regions.
For years 2000–2009, the average annual stranding rate was about 2 dolphins per year in the SJR, (range = 0–4). During those years, some 10 dolphin carcasses were recovered further upstream than Reddie Point, more than 15 river miles inshore. Live dolphin interventions for 2000–2009 involved 4 SJR dolphins, including 3 located further upstream than Reddie Point, all of which had mild to severe skin lesions compatible with freshwater overexposure. Two of the freshwater overexposure cases had substantial injuries in addition to skin lesions, such as line entrapment and scoliosis. A third rescued animal having mild skin lesions and no overt injury or illness had ventured upstream to the most southern region of the SJR known to be recorded for a bottlenose dolphin. During the UME, 2 animals recovered in the southern SJR had mild or severe skin lesions.
Preceding the UME, the chronic, unusual fish kill occurred in a low salinity 30 mile stretch of river from downtown Jacksonville south, lasted about 6 weeks, and was not associated with low dissolved oxygen levels or other poor water quality. Adult red drum and several other euryhaline species were primarily targeted, but not all fish species in the area were affected. Internal hemorrhage and pathological indicators of a toxicosis, presumptively from hemolysis were consistently noted in moribund and freshly dead fish. The fish kill followed the crash of an unusual bloom of the freshwater Aphanizomenon flos-aquae that was triggered following a salinity influx. Low levels of microcystin and other cyanotoxins were noted in the SJR near the time of the bloom and fish kill. However, levels of microcystin or other cyanotoxins were not likely high enough to be directly responsible for fish deaths, but could have played a role in the chronic nature of the fish kill. A multi-factorial etiology for the fish kill involving the algal bloom, cyanobacteriolytic bacteria, hemolysins, and cyanotoxins has been proposed.
Dolphin tissues are thus far negative for brevetoxin, domoic acid and saxitoxin. Analysis for cyanotoxins, metals and organic contaminants are being completed. Histopathology has not been informative due to carcass condition. Scrutiny of UME age and sex data reveal several nursing animals, including 5 recovered near the site of a dredging project that spanned the Talleyrand section of river just before dolphin carcass recoveries. Consideration of these nursing animals has prompted discussion regarding potential combinations of anthropogenic events, the algal bloom, and the fish kill, that may have impacted SJR dolphins.
Gratitude is expressed to the research biologists who provided repetitive field response and extended support for the UME investigative effort: Ryan Berger, Rachel Cimino, Janice Price, and Nadia Gordon of the Florida Fish and Wildlife Commission Jacksonville Office, George Biedenbach and Matt Denny of Georgia Aquarium's Conservation Field Station.
1. Caldwell M. Social and genetic structure of bottlenose dolphin (Tursiops truncatus) in Jacksonville, FL. Ph.D. Thesis, University of Miami; 2001.
2. Caldwell M, Gibson QA, Richmond JP, Burks R, Mazzoil M, Borkowski R. Post Unusual Mortality Event survey data suggest long term habitat use and possible range extension of bottlenose dolphins (Tursiops truncatus) in Jacksonville, FL. Society for Marine Mammalogy 19th Biennial Conference, Tampa, FL; 2011.