Dolphin presence/absence probabilities on the Virginia and North Carolina coasts as correlated with sea surface temperature and chlorophyll-α levels

The North Atlantic Bight area of the northwest Atlantic is an area that serves as habitat for bottlenose dolphins (Tursiops truncates). The population there can be divided into three subgroups, or stocks: Offshore, Year-round Resident, and Seasonally Migratory. The movements of the migratory stock are of concern, due to the necessity of compliance with the Marine Mammal Protection Act of 1972. Studies have shown that the likelihood of presence of many marine mammals can be correlated with both the sea-surface temperature (SST) and the levels of chlorophyll-a (Sykes, et. Al., 2003). Previous work in this area has shown that the migratory stock may be responding to a critical temperature ranging from 13 degrees to 16 degrees Celsius, (Barco, et al., 1999; Shoemaker, et. Al. 2004) yet there have been sightings below this temperature. The Objective of this study is to attempt to refine the critical temperature conclusions of prior researchers, and to attempt to correlate the probability of presence with chlorophyll-a levels. By producing a better model of prediction of the presence or absence of this stock of species, it is hoped that remote sensing data may be used as a surrogate for the need for direct observation. This would allow any agency or group to be aware of the potential presence of this protected species based on this remote data, and to act accordingly to reduce any impacts upon them. Correlations between Probability of Sightings, Total Number Sighted and Temperature vs. Group Size were examined for both sea surface temperature and chlorophyll-a. Most dolphin groups tend to appear between 18 and 29 degrees Celsius. At 12 and 16 degrees Celsius, there is a minimal, but noticeable number. The amount of Chlorophyll-a was hypothesized to be an indicator of phytoplankton, which would draw various marine life from small fish all the way up the trophic level to dolphins. It was proven not to be the case that higher concentrations of chlorophyll-a meant a greater chance of sighting dolphins. Using field observations and archived data to compare against the satellite SST and chlorophyll-a data, there was a significant difference found in the temperature where dolphins were likely to be found. Chlorophyll-a data, however, showed no significant variation in the likelihood of encountering dolphins with the level of chlorophyll-a. The data will be presented in this paper.