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Foraging theory suggests that hungry animals balance a complex set of costs and benefits when determining what and how to eat . Prey distribution , patch size , and the presence of conspecifics are important factors influencing a predator's feeding tactics , including the decision to feed individually or socially . Dusky dolphins (Lagenorhynchus obscurus ) in New Zealand employ different feeding tactics in varying habitats and seasons . I used programmed survey routes and opportunistic sightings to examine the habitat use and feeding mechanics of dusky dolphins in Admiralty Bay , New Zealand , a protected shallow -water environment frequented by wintering dolphins . I encountered 253 dolphin groups , of which 58 .5 % were engaged in food -acquisition activities . Photographic efforts revealed a total of 177 individually -recognizable dolphins , 100 of which were returnees from previous seasons . Thirty -seven feeding groups and 70 bouts of feeding behavior were followed . Two -minute interval sampling as well as active acoustic sonar were used to test the hypothesis that diurnally -feeding dolphins would work in a coordinated manner to bring schooling fish to the surface . Feeding tactics observed in Admiralty Bay were then compared to foraging by some of the same animals in the unprotected , deep -water environment off Kaikoura , where large numbers of dusky dolphins feed during the night on organisms associated with a vertically -migrating scattering layer . Evidence supporting coordinated surface feeding was not statistically significant , but indicative of behavioral flexibility in feeding styles as part of a larger feeding repertoire . A potential shift in prey distribution from previous years may also explain some observed patterns . Feeding groups were positively correlated with seabirds and New Zealand fur seals (Arctocephalus forsteri ) . Mean group size of
6 .1 (± 8 .23 S .D . , n=253 ) in Admiralty Bay is dramatically less than groups observed off Kaikoura , a variation likely reflecting differences in prey number and distribution , as well as differences in predation risk by deep -water sharks and killer whales . Behavioral flexibility likely confers an adaptive advantage for species subject to environmental fluctuation , whether due to natural or anthropogenic sources . Further research is necessary to evaluate prey distribution in Admiralty Bay and its possible effects on feeding dusky dolphins . |
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