Date of Award

Fall 2010

Document Type

Legacy Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Robert F. Young

Second Advisor

Christopher E. Hill

Third Advisor

Keshav Jagannathan

Abstract

Atlantic bottlenose dolphins (Tursiops truncatus) in the salt marshes of Bull Creek, Beaufort County, South Carolina employ a unique foraging tactic called strand- feeding, where a coordinated group of dolphins rushes a muddy creek bank, using their bow wave to push fish onto the shore. The dolphins then temporarily strand themselves to capture fish from the mud before returning to the water. This behavior attracts a variety of wading birds in the marsh, as it allows them to access a food resource that is not usually available to them. Field observations conducted throughout the summers of 2009 and 2010 indicated that there was a significant difference between species distributions of birds in the marsh and at strand-feeding events (p < 0.000001). Great egrets (Ardea alba) were the most common wading bird to associate with dolphins, and were more prevalent at strand-feeding events than they were in the overall marsh. Other birds that associated with strand-feeding dolphins were great blue herons (Ardea herodias), snowy egrets (Egretta thula) and wood storks (Mycteria americana); these species were all less prevalent at strand-feeding events than they were in the marsh. On average, great egrets and great blue herons foraging with strand-feeding dolphins experienced the same caloric intake per one-hour block over birds foraging away from dolphins (p > 0.05 for both species), and were unable to fulfill their calculated daily energetic needs by foraging with dolphins in a one-hour block. However, individuals of both species were observed capturing enough fish at a single strand-feed to more than meet their active metabolic needs. Observations of marked birds indicated that some birds repeatedly associate with strand-feeding dolphins over multiple days. While bird depredation of stranded fish occurs at 15% of all strand-feeds, the actual losses to strand- feeding dolphins are negligible and this unique relationship between two top predators appears to be commensal. Observations of the dolphin population in Bull Creek were compared to the results of previous studies of strand-feeding dolphins in this area. Overall behavioral and movement patterns of these dolphins appeared consistent with those reported by Petricig (1995). A higher number of individual dolphins were identified to use Bull Creek in this study, compared to Petricig (172 vs. 46), and a similar number of strand-feeding dolphins were identified. Petricig reported 100% calf mortality within the first year, but I observed five calves in both 2009 and 2010. Four dolphins from previous studies (Gubbins 2002; Petricig 1995) were re-sighted. Two of these individuals were first observed strand-feeding in 1988, and continued to employ this foraging technique in this study. One dolphin identified by Petricig as a transient non-strand-feeder appeared to be a seasonal resident in 2009-2010 and was frequently observed engaging in strand- feeding. Three adult dolphins that begged at boats in Bull Creek all exhibited severe tooth wear on the right side of their jaws, which may be an indication of previous strand- feeding behavior and a shift in foraging techniques when teeth become too eroded. The increase in observed calf survival and immigration and integration of new individuals seems to indicate that the resident population of dolphins in Bull Creek and their unique strand-feeding tactics will continue to persist. As part of a study on wading bird energetics and bird-dolphin energetics in a South Carolina salt marsh, Spraymore paintball land mines were used to apply dye to three foraging great egrets and one great blue heron in an environment where standard capture and marking techniques would have been difficult to employ. These mines, powered by compressed gas and filled with an inkjet printer ink-based dye, were electronically triggered by a remote control. The effective spray radius of these mines was 2-5 m, depending on gas pressure and environmental conditions. Two marked birds were re-sighted: one great blue heron was seen for two days following marking, and one great egret was re-sighted intermittently for 44 days after being marked. Both birds were re-sighted only within approximately 800 m of where they were originally marked, and showed no aversion to the location where they were marked, or to additional paint mines. Neither marked bird showed any apparent long-term changes in behavior, nor were any unusual interactions observed between marked birds and unmarked birds.

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