Common bottlenose dolphins (Tursiops truncatus) exposed to freshwater or low salinity (<10 practical salinity units; PSU) for prolonged periods of time have been documented to develop skin lesions, corneal edema and electrolyte abnormalities, and in some instances they have died. Here we review a case of an out-of-habitat subadult, female common bottlenose dolphin that remained in a freshwater lake in Seminole, Alabama for at least 32 days. Due to concerns for the dolphin's health a rescue was initiated. At the time of rescue bloodwork results indicated minor electrolyte abnormalities (hyponatremia, hypochloremia, hypoosmolality). Renal function was not affected (normal creatinine and urea nitrogen) and all other bloodwork parameters (hemogram; serum biochemistry analytes) were within normal limits. The dolphin was deemed healthy enough for immediate relocation and release. A satellite-linked tag was attached to the dorsal fin to track the dolphin following its relocation to a nearby brackish water bay (Perdido Bay, AL), a known habitat for bottlenose dolphins. Twelve weeks following release, the dolphin was found dead as a result of a fisheries interaction (peracute underwater entrapment). A full necropsy was conducted and there was complete resolution of the skin pallor and skin lesions and no evidence of chronic renal or central nervous system lesions. Post-mortem analysis of vitreous humor (used as a proxy for serum analytes and to determine post-mortem interval) was challenging to interpret and has not been validated in dolphins. This supports the need for future research in cetaceans to establish a species-specific approach. Elevated barium (Ba) concentrations in tooth dentin corresponded to increased seasonal freshwater discharge patterns, confirming repeated annual exposure to low salinity conditions prior to death and indicating freshwater exposure may pose an ongoing threat to dolphins in the region. This case provides a unique opportunity to follow the progression of prolonged freshwater exposure and recovery in a bottlenose dolphin and highlights that dolphins in nearshore habitats face a combination of persistent natural and human associated threats.

The potential role of morbillivirus was evaluated in the deaths of >1100 bottlenose dolphins Tursiops truncatus and other small cetaceans that stranded from February 2010 through July 2014, during the northern Gulf of Mexico (GoM) unusual mortality event (UME). Morbillivirus analysis was carried out on 142 live or freshly dead cetaceans and results were combined with samples from 102 live, free-ranging bottlenose dolphins sampled during capture-release health assessments conducted from 2011 to 2014. Polymerase chain reaction (PCR) testing for morbillivirus showed that 9.9% (14/142) of the stranded cetaceans and 1% (1/83) of the free-ranging live dolphins were positive for dolphin morbilliviral (DMV) RNA. In contrast, previous DMV dolphin die-offs had DMV detectable by PCR in 61 to 97% of animals tested. Histologic findings consistent with morbillivirus infection, including lymphoid depletion, bronchointerstitial pneumonia, syncytial cell formation, or meningoencephalitis, were found in 6.6% (9/136) of the cetaceans that underwent histologic examinations. Serological analysis using a virus neutralization assay found that 29% (5/17) of live stranded and 23% (23/102) of live free-ranging bottlenose dolphins had titers of 64 or greater for cetacean morbillivirus, indicating prior but not necessarily recent exposure to morbillivirus. Current findings suggest that DMV infection, although present in the northern GoM, was sporadic and occurred at low levels and therefore was not the primary cause of the northern GoM UME. Confirmation of DMV infections and existing DMV titers demonstrate continued exposure to morbillivirus among northern GoM cetaceans since the first detection of this virus in the early 1990s.

Human activity affects marine mammal stranding rates in two major ways; through human interaction (HI) that may lead to mortality and through search and response efforts that enable carcass detection. To better quantify the combined effects of these interacting human influences, we analyzed strandings for bottlenose dolphins (Tursiops truncatus) in the northern Gulf of Mexico (nGOM), an area of high cetacean strandings. To identify hotspots of human influence, we first determined the number of total and HI-related bottlenose dolphin strandings normalized to shoreline length in each nGOM U.S. state, which represent major response areas. To estimate the effects of response effort on stranding numbers (for HI and non-HI strandings), we used the Deepwater Horizon oil spill (DWHOS) as an established benchmark to compare periods of lower (pre-spill) and higher (post-spill) response effort. Strandings in Alabama waters were used as a case study to detail spatial and temporal variation due to human influences during the 39-year period of retrospective study. We found an increase in strandings from Louisiana through Alabama following the DWHOS. Non-oil related HI strandings increased in total number in AL, and they increased as a proportion of total strandings in Alabama (16%) and the Florida panhandle (12%). The increase in HI-related strandings in Alabama was driven by mortality of many types, but particularly fishery interactions and cases of apparent intentional harm. The Alabama case study clearly detected lower stranding numbers during periods of low or intermittent response coverage. Our findings are consistent with an overall increase in stranding numbers due to a combination of increased stranding occurrence and response effort following the DWHOS. Importantly, we provide evidence that HI-related standings increased independently from the DWHOS, with ongoing increases in at least one hotspot (Alabama). These findings provide a first step to parsing out different effects of human influences on stranding data for a common coastal cetacean. Our approach establishes baselines for future damage and recovery assessments, identifies areas where resources can be focused for management and education, and highlights the power of response and monitoring agencies to positively influence stranding datasets.