Dolphins that die after accidentally becoming entangled in fishing nets are often related, with mothers and their offspring being among the most likely to perish together, according to a new study in the latest issue of PLoS ONE.
It has long been suspected that related dolphins die together as bycatch, but the new study is among the first to prove it using genetic analysis.
The research focused on Franciscana dolphins -- one of the world's smallest dolphins -- but porpoises, other dolphins and small marine mammals anywhere are likely at risk too. Franciscana dolphins live in the Atlantic off the coasts of Brazil, Uruguay and Argentina
"Any fishery that impacts small cetaceans as bycatch could potentially impact family groups," project leader Martin Mendez, a postdoctoral researcher at the Sackler Institute for Comparative Genomics at the American Museum of Natural History, told Discovery News.
He said such mammals die together as bycatch because "they are highly social animals that frequently move around in family groups."
Mendez and his team looked at over 250 Franciscana dolphin bycatch deaths that occurred over a decade. DNA connections showed that most of the deceased were genetic relatives that perished in nets set out by artisanal fisheries.
These fisheries "target hake, white croaker, small houndshark and other coastal fish," Mendez said.
The researchers estimate that between 2 percent to 5 percent of the Franciscana dolphin population near Argentina dies each year as bycatch. That's equal to the average population growth rate of this species.
An additional problem affecting population growth is that since reproducing females and the next generation of dolphins are the most common bycatch victims, the species is particularly vulnerable to decline.
The females are attracted to the fish-full nets and their young calves simply tag along with them.
Trevor Spradlin, a marine mammal biologist for the NOAA Fisheries Office of Protected Resources, told Discovery News that pilot whales and killer whales, both members of the dolphin family, also frequently travel together and therefore relatives can get entangled in fishing nets at the same time.
Sue Rocca, a biologist at the Whale and Dolphin Conservation Society of North America, told Discovery News that whales, dolphins and other marine mammals get entangled in lobster pot gear too.
"Even if they can get to the surface, the line can wrap around the mouth or cut around bone, which has got to be incredibly painful," she said, adding that shellfish consumers should look for lobster caught in Massachusetts waters, since the state promotes whale-safe lobster fishing practices.
In Argentina, study co-author Pablo Bordino and his colleagues at Fundacion Aquamarina have been trying a number of different strategies to minimize Franciscana dolphin deaths.
"We have used pingers (subaquatic alarms to alert dolphins about fishing rates); have provided fishermen with 'reflective nets' that are acoustically easier to detect than regular gillnets; have proposed seasonal shifting of fishing operations in areas we suspect are frequently used by Franciscanas to calve or feed; and are currently implementing a combination of all these strategies," Mendez said.
Thus far, no single solution appears to be foolproof.
"The pinger is supposed to drive away animals," Rocca noted, "but for some it's like a dinner bell."