Health Impacts and Recovery From Prolonged Freshwater Exposure in a Common Bottlenose Dolphin (Tursiops truncatus)

Abstract

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.

Fresh water skin disease in dolphins: a case definition based on pathology and environmental factors in Australia

“…Similar skin changes and lesions were reported for cetaceans either in natural habitats subjected to a salinity change for various reasons, or for individual animals who ended up in fresh water either by misadventure or following unusual weather or some other catastrophic event 9,13,14,51 . In addition to dermatitis and secondary infection, there are serum electrolyte imbalances including decreased osmolality and sodium and chloride levels associated with over-hydration due to increased transcutaneous water absorption and solute loss [56][57][58][59] . Corneal opacity (edema) has also been observed in dolphins residing in hypo-saline waters for prolonged periods or out-of-habitat dolphins rescued and repatriated from freshwater systems 51,59,60 .…”

“…In addition to dermatitis and secondary infection, there are serum electrolyte imbalances including decreased osmolality and sodium and chloride levels associated with over-hydration due to increased transcutaneous water absorption and solute loss [56][57][58][59] . Corneal opacity (edema) has also been observed in dolphins residing in hypo-saline waters for prolonged periods or out-of-habitat dolphins rescued and repatriated from freshwater systems 51,59,60 . The associated physiological stress of prolonged freshwater exposure eventually leads to mortality, as for the four dolphins in this report, but the pathophysiology is not completely understood.…”

“…For other suspected outbreaks of freshwater skin disease, some of these elements, particularl retrieval of fresh carcasses for necropsy, were missing e.g. 13 , 14 , 51 , 59 , 60 .…”

Common Bottlenose Dolphin, Tursiops truncatus, Behavioral Response to a Record-Breaking Flood Event in Pensacola Bay, Florida

“…Common bottlenose dolphins (hereafter referred to as 'dolphins'), Tursiops truncatus, that inhabit brackish inshore estuaries have been observed in salinities ranging from 15 to 25 ppt. These inshore dolphins can suffer adverse health effects from prolonged freshwater exposure and pollution introduced from surface runoff [1][2][3][4][5], which can occur as a result of natural climatic events. For example, a high-precipitation event that coincided with local agricultural pesticide applications and reduced bay salinities to <10 ppt for several months was likely associated with a dolphin Unusual Mortality Event (UME), a period in which there is a significant die-off of a marine mammal population, along the mid-Texas coast [6].…”

“…For example, a high-precipitation event that coincided with local agricultural pesticide applications and reduced bay salinities to <10 ppt for several months was likely associated with a dolphin Unusual Mortality Event (UME), a period in which there is a significant die-off of a marine mammal population, along the mid-Texas coast [6]. Poor water quality, prolonged freshwater exposure, and changes in water temperature were associated with increases in dolphin skin lesion prevalence and extent in multiple populations [1,2,4,7,8]. Additionally, abrupt changes in water quality and salinity caused by floods were associated with the development of poxvirus-like skin lesions on Indo-Pacific bottlenose dolphins, Tursiops aduncus [9].…”