Dolphin underwater bait-balling behaviors in relation to group and prey ball sizes

• We characterized dolphin feeding behaviors at two study sites, and related behaviors to dolphin group and prey ball sizes.
• Dolphins fed on larger prey balls in Argentina (maximum size >74 m2) than they did in NZ (maximum size 33 m2).
• Dolphins in larger groups more often swam under prey balls during herding passes.
• When dolphins fed on larger prey balls, they more often swam through prey balls to capture fish.
• Coordination of prey-capture behaviors appeared to make it easier for dolphins to capture fish due to entrapment.

We characterized dusky dolphin (Lagenorhynchus obscurus) feeding behaviors recorded on underwater video, and related behaviors to variation in prey ball sizes, dolphin group sizes, and study site (Argentina versus New Zealand, NZ). Herding behaviors most often involved dolphins swimming around the side or under prey balls, but dolphins in Argentina more often swam under prey balls (48% of passes) than did dolphins in NZ (34% of passes). This result may have been due to differences in group sizes between sites, since groups are larger in Argentina. Additionally, in NZ, group size was positively correlated with proportion of passes that occurred under prey balls (p < 0.001). Prey-capture attempts most often involved capturing fish from the side of prey balls, but dolphins in Argentina more often swam through prey balls (8% of attempts) than did dolphins in NZ (4% of attempts). This result may have been due to differences in prey ball sizes between sites, since dolphins fed on larger prey balls in Argentina (>74 m2) than in NZ (maximum 33 m2). Additionally, in NZ, dolphins were more likely to swim through prey balls to capture fish when they fed on larger prey balls (p = 0.025).