Migratory Bottlenose Dolphin Movements and Numbers Along the Mid-Atlantic Coast and Their Correlation with Remotely Sensed Chlorophyll-a and Sea Surface Temperatures

Along the Mid-Atlantic coast of the United States, there are different sub-populations, or stocks of bottlenose dolphins. The bottlenose dolphin, Tursiops truncatus, has both resident and migratory stocks. The focus of this study is the northern migratory population. This group of animals moves north and south along the coast in response to seasonal changes. The need for study arises from this mobile nature. Determination of the environmental cues that may be used to predict the presence or absence of these animals will aid in efforts to avoid disturbance to this protected species. This stock was also greatly affected during the 1987-1988 epizootic event that killed an estimated 50% of the migratory stock. This disease event was likely worsened by exposure to environmental toxins. The main areas of the field work, the lower James and Elizabeth Rivers of Virginia, are of interest due to their high toxin loads and frequent usage by bottlenose dolphins. The Elizabeth River is largely developed along its length. It also has a very high level of traffic: commercial, military and recreational.

Since this species represents the highest level on its food chain, our hypothesis is that the movement north represents can be correlated with the movements of their prey species. These prey species are known to be themselves migratory with temperature. As a surrogate for the in situ detection of the prey species, we feel that sea surface temperature (SST) and chlorophyll-a levels can be used. Both of these factors can be sensed remotely, removing the need for local observations. Sea surface temperature can serve to represent the movement of the prey species, and chlorophyll-a levels can be used to show the primary productivity, and thus the total food energy available in the ecosystem. The presence and absence data on these animals is then to be compared with the remotely sensed SST and chlorophyll-a data. These data were derived from a number of sources. MODIS-Aqua and AVHRR data was obtained from Goddard Space Flight Centers Ocean Color web archive. Additional AVHRR data was obtained from the Jet Propulsion Laboratory’s PO.DAAC Ocean ESIP Tool (POET) website. Field observations were based on archives from the Christopher Newport University Dolphin Project, and from the Ocean Biogeographic Information System (OBIS) archive of Duke University.

The results of the correlations show that the critical temperature in determining the presence or absence of bottlenose dolphins is between 16° and 18° C. While there were two sightings below this temperature, there were 694 above. A t-test show that there was a significant (p=0.003) difference between the mean temperatures of sighting and non-sighting efforts. When compared to the numbers of animals sighted at the different temperatures, again the 16° and 18° critical temperatures showed up. There were only 2 animals sighted below 16°, while there were 5400 sighted above. An ANOVA analysis showed a significant (p<0.01) difference between the two temperature ranges when it came to group size. A t-test for the mean group size showed no significant difference in the sizes of groups between 18° and 28°. While there was some variation in the chlorophyll levels (measured in mg/m3), a t-test showed no significant (p>0.1) difference between the means of sighting and non-sighting levels. In comparing chlorophyll-a levels with group size, there was a significant (p<0.001) difference, but this was likely due to the fact that coastal waters never drop below moderate chlorophyll-a levels. Based on these findings, it becomes clear that in determining the migratory movements of bottlenose dolphins sea surface temperature is the preferred environmental variable.