For the past couple of months, I’ve been taking an online class at the University of Florida (“Intro to Marine Wildlife”). This week, Dr. Randall S. Wells, of Mote Marine Laboratory in Sarasota, FL presented a lecture on “Dolphin Mortalities,” which really sparked my interest. Dr. Wells is the co-founder and current director of the Sarasota Dolphin Research Program.
Climate change and human activity have had dramatic effects on oceanic environments around the world, impacting the lives of not only cetaceans, but all marine animals. This threat to dolphins has been heavily debated; studying the impacts of these threats is difficult, and requires that data sets be collected over long periods of time. In order to cope with such environmental changes, marine mammals have 3 options:
2. Adapt physiologically or behaviorally
3. Go extinct
Bottlenose dolphins have demonstrated significant physiological and behavioral adaptability, limited only by the range of these functions. In 1982-1983, a severe El Nino event warmed the waters of the California coast and resulted in 12 human deaths. A community of dolphins that had spent their whole lives in the waters of San Diego moved up the coast to Monterey Bay (670km). At the time, there were no other bottlenose dolphins in the area, so these dolphins were free to establish residence in Monterey Bay, and have remained there ever since. This demonstrates that bottlenose dolphins are capable of leaving a harmful environment, when they have access to an open habitat. But what does this mean for bottlenose dolphins in situations where open habitats aren’t in close proximity? Such is the case along the west coast of Florida, where dolphins live in separate social communities. These dolphins persist throughout generations in the same waters, even when faced with challenges such as severe red tides, hurricanes, and increased human activities.
Long-term resident dolphins remain tied to their home ranges, even during strong hurricanes, for example: Hurricane Charley in 2004. Although Hurricane Charley caused massive devastation to the ecosystem, 94% of the resident dolphins remained in Charlotte Harbor (Kim Bassos Hull, SDRP). If an environmental disaster such as this does not motivate these animals to leave the area, how will these animals adapt as the climate continues to change?
Already, the bottlenose dolphins in the Florida waters live in conditions where summer temperatures approach their body temperatures. Studies have found that free-ranging bottlenose dolphins have highest metabolic rates during the summer, which indicates thermal stress. It would have been expected that the dolphins have higher metabolic rates during the winter, in order to stay warm in a colder climate. However, blubber provides enough insulation so that these animals don’t have to expend extra energy to stay warm. During the summer, the dolphins have to work harder to get rid of the extra heat. This results in higher metabolic rates, and in turn causes thermal stress. Evidence of this is a higher number of bottlenose dolphin deaths during the summer months than during the rest of the year. The average temperature during a summer day in Sarasota is 25.5degrees Celsius. Dolphin mortalities typically occur at 27.5degrees Celsius. Thus, if water temperatures continue to increase and approach or exceed their body temperature, the dolphins will have an even greater difficulty in cooling themselves. And in order to compensate for increased metabolic rates, they’ll need more food.
With rising water temperatures, dolphins are going to have a tough time staying cool. In addition, warmer waters will enable new pathogens to survive in the waters, and they’ll be able to live in the waters for longer periods of time. Pathogens are already an issue and generate health problems in bottlenose dolphins. In addition, dolphins give birth during summer months, which places calves at higher risk for pathogen entry. “Understanding dolphin deaths is essential to learning how best to protect dolphin populations and species” (Dr. Wells).
Credit: Wells, R.S., L.J. Hansen, A. Baldridge, T.P. Dohl, D.L. Kelly and R.H. Defran. 1990. Northward extension of the range of bottlenose dolphins along the California coast. Pp. 421-431 In: S. Leatherwood and R.R. Reeves (eds.) The Bottlenose Dolphin. Academic Press, San Diego. 653 pp.