Potential climate change effects on dolphins
By Randall Wells, PhD

     Because of the unique long-term datasets developed and maintained by the SDRP, Randall Wells was an invited participant to the International Whaling Commission’s Second Workshop on Climate Change and Cetaceans, held 21-25 February 2009 in Siena, Italy. While it is widely recognized that Arctic and Antarctic marine mammals are expected to exhibit the strongest climate-related signals, at least initially, large scale changes are also likely to affect small cetaceans inhabiting shallow, inshore habitats, including estuaries and rivers.

     Bottlenose dolphins, which occur along many coastlines in temperate and warm waters, may serve as one sentinel of climate change effects as they exhibit a wide range of behavioral and physiological plasticity that may provide the basis for detectable responses to environmental changes. This species has been the subject of numerous research projects, providing potential baseline data for evaluation of changes at a number of sites. At the northern extent of the species’ range on both the east and west coasts of the United States, bottlenose dolphins have demonstrated the capacity to dramatically alter spatial or temporal aspects of their ranging patterns in apparent response to changing environmental conditions. Many coastal bottlenose dolphin populations live well away from their range limits, within a matrix of established, long-term resident communities. Under these circumstances, large-scale range shifts into waters already inhabited by other bottlenose dolphins may not be an option – these dolphins may live in an “ecological cul-de-sac”. The datasets developed beginning in 1970 through our long-term study of resident bottlenose dolphins on the central west coast of Florida, including sightings, reproductive histories, health and body condition, strandings, behavioral observations, and prey distribution and abundance, provide a time series of data for examination of possible climate change signals and effects for resident populations of small cetaceans in inshore waters.

     Sea surface temperature increase is likely to be one of the first climate change experiences for small cetaceans in shallow, coastal, non-polar waters. The resident dolphins have remained in the Sarasota Bay area for decades, at least, and through large scale environmental perturbations such as severe red tides and hurricanes. Existing data suggest that as these animals remain in warming waters, they may face increasing health problems, through increases in harmful algal bloom exposure or thermoregulatory issues. High summer metabolic rates and mortality rates suggest current thermal challenges, as water temperature approaches body temperature; this situation may be exacerbated through climate change. Thermal stresses may combine with toxicological stresses to increase mortality under warm water conditions. Lipids released from thinning blubber as waters warm can transport associated toxic environmental contaminants (e.g., PCBs, DDT and metabolites) to target organs or to organs where biotransformation can modify toxicity, leading to compromised immune function. Warmer waters are likely to support a variety of old and new pathogens, reduce dolphin host resistance, and/or increase the duration of exposure. Transfer of contaminants via lactation has been suggested as one cause of the increased mortality documented for first-born calves in the area. Taken together, these factors suggest that seasonal warming appears to lead to health challenges for Sarasota Bay bottlenose dolphins, perhaps approaching a tipping point, potentially leading to cascading declines in individual health. Information is needed to identify and detect signals of climate change, predict where and how impacts on dolphins and their habitat are likely to occur, and prioritize pre-emptive management actions for providing these animals with as much capacity as possible to respond to climate change.