Foraging Strategies

Bytfluke with fish
Ewa Krzyszczyk

Bottlenose dolphins feed on a wide variety of fish, cephalopods (squid, octopodes), crustaceans (e.g., shrimp, prawns) and occasionally stingrays, sharks, eels, and mollusks. Approximately 20 species of prey have been identified for Shark Bay, but we know little about the bulk of dolphin diets. Our observations of fish caught are biased towards large fish because the long time they spend carrying and breaking up these fish (up to an hour in some cases) allows for more accurate identifaction. Examination of stomach contents from Indian Ocean bottlenose dolphins caught in shark nets off Natal, South Africa found about 48 species of fish and cephalopods. Our latest research involves both using fatty acid signatures to match up dolphins with their suspected prey, along with increased underwater filming to capture images of the more benthic species they may be feeding on.

A number of foraging strategies have been described for bottlenose dolphins, and Shark Bay dolphins have been observed at least at least 13 different techniques. They use echolocation or sonar to track fish, but vision may be quite important too. Dolphins might also listen to sounds that fish produce, and use these to locate their prey.The variety and complexity of these techniques, as well as the manner in which they are passed down (primarly from mother to daughter) presents evidence for social learning and may even qualify as a type of animal “culture.”


Wedges and the Golden Trevally

In May of 1999, Dr. Janet Mann was surveying a group of dolphins and noted a very tiny newborn calf. The mother, Wedges, stayed close by her side, but left twice to chase a large dusky shark that was pestering the group of females and calves. The calf was clearly born out of season, and hence named “Whoops”. Newborn calves are hard to come by, so Janet enrolled Wedges in her long-term study of mothers and calves. During that first follow, Wedges began these tremendous high leaps in rapid pursuit of fish. She caught her prize, a large golden trevally, nearly one meter in length.

In 12 years of watching adult females, Janet had never seen a catch so impressive. Whoops seemed less impressed. She darted back and forth, whistling constantly as her mother b-lined for shallow water and broke the fish up in shallow sand flats. It took Wedges nearly an hour to consume the fish. Poor Whoops didn’t get to nurse the entire time. But there were other things to worry about. Twice, large tiger sharks cruised close by, seemingly attracted by the broken bits of fish. Each time Wedges and Whoops sped to even shallower water to continue feeding. At the time, Janet thought this event was merely a “fluke” and wouldn’t happen again. About two hours after the first trevally catch, Wedges caught another, although this one was slightly smaller. Her team has now followed Wedges for ten years and she catches a trevally every few hours. During one follow in 2002, Wedges finished off a large trevally and then lay still in 4 ft. of water for over four hours. Her slight arching suggested that she may have had an awful stomach ache. No surprise, given the size of the trevally bones. The team anchored and waited with her until sunset. By the next day, she was out hunting again. No other dolphin has been observed catching a fish this large, although Puck was seen carrying trevally twice, but both times Wedges was close behind her. Researchers suspect that Puck may have stolen her fish, a rare occurrence among dolphins. Whoops, who is nearly five and going through the weaning process, is too small to catch such spectacular fish, but they expect that one day, she will.

Sponging

One of the most exciting foraging strategies, known as “sponging,” is the only recorded instance of cetaceans using tools in the wild. Over 70 dolphins have been documented wearing sponges on their rostra (beaks) in Shark Bay. They use these sponges to ferret prey from the seafloor. This is generally a solitary activity, but sometimes more than one sponger will be tens of meters apart. Spongers generally stick to sponging and don’t use other foraging techniques, but there are some exceptions. This appears to be a “tradition” of sorts. The daughters of sponge-carriers clearly grow up to be sponge-carriers, but we aren’t sure about the sons. New research from the genetics team at the University of New South Wales will shed light on the relatedness of spongers.

Ewa Krzyszczyk

Sponging was discovered when Rachel Smolker, one of the founders of the Shark Bay Dolphin Research Project, was told in the mid-80s about a dolphin north of Monkey Mia with a horrendous growth on its nose and half a tail fluke. Not believing this fisher’s tale, she thought little of it. But, a year later, in the location the fisher described, she too, saw the dolphin with the growth and half a tail fluke. As she watched, she noticed that the dolphin managed to change its growth, and at closer inspection, she discovered that this was in fact, a marine sponge. She named the dolphin Half-fluke and soon found that others were carrying sponges too. About one year later, Half-fluke had a calf, Demi. Although Half-fluke and Demi were seen regularly, only Half-fluke carried a sponge. In 1989, when Demi was nearly 3 years old, researcher Janet Mann observed her wearing a tiny sponge on her beak, conical shaped like her mother’s, only much much smaller. She dived like her mother (flukes out) and seemed to be foraging like her. When she went back to nursing position (infant position), she dropped her sponge. Half-fluke had two more calves after Demi, but both died. Demi continued to sponge-carry after weaning and often associated with her aging mother. A year after Half-fluke’s death, Demi had her first calf, Dodger, before her 13th birthday. Janet and her graduate student Brooke Sargeant were very interested in Dodger and would observe her several times each year to see when she would pick up her first sponge. Soon after Dodger’s third birthday, she did. It was such an exciting event that a reporter-writer on our boat fell down and nearly knocked the rest of us off the boat. Dodger was soon joined by a sister, Ashton, who has also begun sponging. The tradition continues…

Other Strategies

Shark Bay dolphins also chase fish “belly-up” near the surface, a behavior we call “snacking.” Calves engage in this type of foraging most often. At Point Peron, northwest from our main study area, a small group of dolphins appear to strand-feed, trapping fish in very shallow water. This behavior can be viewed on the National Geographic film, Dolphins: The Wild Side.

BBC / BigWave

In the shallow seagrass beds out east, dolphins will arch their tails high before driving them forcefully into the water, creating a several meter splash and a resounding ‘kerplunk’ sound. We think dolphins learn the location of fish hiding in the seagrass when they are startled by ‘Kerplunks.’

One might also argue that the provisioned females in Monkey Mia, Shark Bay, have developed a unique foraging strategy of begging for fish from boats and tourists. Such “traditions” have continued across at least three generations. This may be true for other foraging tactics as well.

At other sites, dolphins can be seen corkscrewing into the sand after fish (Bahamas), strand-feeding on mud-banks in Portugal, Georgia and South Carolina, or stunning or killing fish with a tail-hit. Found worldwide in warm coastal waters, bottlenose dolphins have also learned to take advantage of human activity. For example, bottlenose dolphins have learned to feed on fish drawn to garbage barges, follow shrimp trawlers as they stir up the bottom, or steal bait from lines or crab pots. In Laguna, Brazil, fishermen and dolphins appear to cooperatively net mullet, with the dolphins herding the fish into the nets and feeding easily off the remains. Historical accounts of Australian aboriginal cooperative fishing with dolphins have also been reported.