MOST angling gear is designed to hold onto fish. But some new equipment is fine-tuned to be more selective — holding tight to some fish and letting go of others, especially if they are Atlantic bluefin tuna — a fish whose numbers have fallen sharply.

Starting this month, commercial fishing vessels that drop long lines in the Gulf of Mexico in search of tuna are mandated to use lightweight circular hooks that retain approved fish like yellowfin tuna, but flatten under the weight of the far heavier bluefin and allow them to swim free.

Bluefin populations have declined precipitously, and bluefin fishing is prohibited in the gulf. But the bluefin, which spawn there, are sometimes snared accidentally by long-line fishermen. Tuna fishing vessels are allowed to retain some bluefin caught accidentally, depending on the pounds of the intended catch on board. The new “weak” hook, as it is known, is aimed at reducing this unintended catch, said John Mitchell, who helped develop it. He is a unit leader in Pascagoula, Miss., at the Southeast Fisheries Science Center of the National Oceanic and Atmospheric Administration.

Hook design alone won’t save the world’s endangered sea creatures, said John E. Graves, a professor of marine science at the Virginia Institute of Marine Science in Gloucester Point, Va., but he added that the new product is a good idea. Dr. Graves’s work at the institute, which is part of the College of William & Mary in Williamsburg, Va., includes research on ways to reduce incidental catching of fish by long lines.