Entire Dolphin Families Dying in Fishing Nets

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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.

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."