Dophin Strand Feeding
Dec 1, 2010
Department: Marine Science
Professor: Dr. Robert Young and Graduate Student Adam Fox
Graduate student Adam Fox just completed his thesis on strand feeding Bottlenose Dolphins and their relationship with local wading birds in Bull Creek, South Carolina. Strand feeding is a "unique foraging tactic" where a coordinated group of dolphins rush a muddy creek bank, using their bow wave to push fish onto the shore. The dolphins then temporarily strand themselves on the bank to capture the fish, followed by a return to the water. It is believed that dolphins do this because it is an efficient method of meeting their caloric needs. Fox spent 2 summers (2009 and 2010) studying the group in Bull Creek and made several interesting findings.
There were 172 individual dolphins in the area observed, 27 of which were involved in stranding behavior. It is unknown why some dolphins engage in stranding and some don't, but the ones who do are only found in Georgia and South Carolina. The muddy banks provide a soft landing as there are large patches of ground with no oyster shells. It is also possible that the angle of the creeks/banks make returning to the water easy.
Fox focused his research on the commensal relationship between the strand feeding dolphins and their interaction with birds who fed off the dolphin's catch. Great Egrets and Great Blue Herons were the most commonly found birds to follow the dolphins and feed with them. The relationship is interesting because top predators normally do not interact, and these predators had developed a working relationship. It was found that dolphins were not affected or bothered by the birds because they are not losing that much food. The birds wait until there are extra fish before grabbing them. It was determined that the birds could meet their daily caloric needs in 1 hour of strand feeding as opposed to 1 hour hunting away from dolphins.
Several birds were marked by Fox to determine their tracking and feeding behaviors. One bird was sighted daily for 44 days and others for just a few days. He observed that some of the marked birds frequently associate with strand-feeding dolphins over several days.
Dolphins can learn strand feeding by watching others forage. Some are year round residents who eat this way routinely and some come and go, averaging about 7 at a time. There were 2 dolphins sighted that have been in Bull Creek since at least 1988. The females keep their calves with them for 1-2 years, allowing them to teach their offspring this feeding technique. In the
1990's the calves had a 100% mortality rate in Bull Creek, but Fox observed several calves with their moms in consecutive summers during this study, indicating that the survivability has improved.
Fox states that future research is needed to determine if some birds are specialists- if they only feed off fish stranded by dolphins. He would also like to track the long-term behavior of the dolphins to determine the length of time that several individuals have been in Bull Creek and for what period of time.