Shrimp trawlers may influence the social structure of dolphins

Shrimp trawlers may influence the social structure of dolphins

Robin Perrtree

Robin Perrtree

Bottlenose dolphins that live near people often beg for food, and begging rates are particularly high in the waters near Savannah, Georgia. Dolphins there also follow commercial shrimp trawlers, eating fish that are stuck in or stirred up by the long, submerged nets. But researchers at Savannah State University wondered if the impact of these interactions with people goes beyond foraging.

In new research published in PLOS ONE, they report that it does: Savannah dolphins are split into social groups that do and don’t follow shrimp trawlers. “Dolphins that engaged in trawler foraging spent more time with each other, even when there were no trawlers present,” says study co-author Robin Perrtree.

This discovery is based on photo-identification surveys of 137 dolphins. Specifically, the researchers watched to see if dolphins begged or followed trawlers, and then estimated how often they were seen together based on statistical analysis of the photos.

Perrtree and her colleagues found that 55 of the dolphins begged while 51 followed trawlers, and that the dolphins were divided into six social groups. Surprisingly, beggars and non-beggars were spread across the groups. This was unexpected because studies elsewhere have shown that begging dolphins form distinct social groups. Instead, the researchers found that half of the groups near Savannah followed trawlers, and half did not.

This suggests that trawler foraging may be learned socially, which makes sense because this is likely to be a group activity. “Dolphins in Australia have been observed manipulating the net to open the end, and feeding on the fish that were released,” Perrtree says, adding that “trawl nets are large and heavy, so it might take several animals to move them around.”

Manipulating nets could put trawler dolphins at risk. “If there are too many injuries or deaths, then efforts will be made to reduce the impact humans are having on the population,” she says.

In addition, the human-influenced social division of Savannah dolphins could become entrenched. Explains Perrtree, “If the two groups are split in terms of who they spend time with, it could effectively create two sub-populations within the area, as opportunities for reproduction across groups may also be limited.” Indeed, trawler dolphins frequented open waters transited by shrimp trawlers, while non-trawler dolphins preferred estuarine creeks and rivers.

Reference: Kovacs CJ, Perrtree RM, Cox TM (2017) Social Differentiation in Common Bottlenose Dolphins (Tursiops truncatus) that Engage in Human-Related Foraging Behaviors. PLoS ONE 12(2): e0170151. doi:10.1371/journal.pone.0170151

Image Credit: Carolyn Kovacs (21Jun12 Photo by C. Kovacs under Permit #14219); Robin Pertree

Author

Robin is a freelance science writer based in the San Francisco Bay Area, covering water, energy and the environment in the western US, and all things biology from biomechanics to behavior.

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