LA ROCHELLE, France — At dawn on a recent Saturday, the crew of the fishing trawler L’Arlequin II pulled their cone-shaped net up from the Bay of Biscay, and found the usual catch crammed into the bottom: hundreds of bass.

And the bodies of two dead dolphins.

Such scenes have become far too common: A record 1,200 dolphins have washed up on France’s Atlantic coast since January, most of them with wounds suggesting that the mammals had died after being trapped in fishing nets.

For every carcass that ends up on a beach, several more decay at sea, wildlife biologists say, which suggests that as many as 6,000 of the 200,000 common dolphins living in the bay may have perished in less than four months because of fishing.

So a disturbing question hangs over the port cities that dot the coastline from Brittany to the Basque Country, and their hundreds of fishing vessels, with no clear answer.