Halibut scientists report whale losses and a shift in when females mature

Scientists for the international commission that manages Pacific halibut briefed its science advisers in Seattle this week on two problems with the numbers behind Alaska catch limits: how much halibut whales are taking off the gear, which nobody can measure, and when female halibut mature, which has moved.

On the first, staff said some longline grounds out of Dutch Harbor and near the Pribilof Islands are effectively unfishable when killer whales are working them, stripping halibut off the hooks as the gear comes up. In the central and eastern Gulf of Alaska it is sperm whales. Fifteen years of trying to quantify the losses through logbooks, observers and surveys has not produced a usable estimate, and the commission's assessment calls the effort a dead end.

That leaves halibut eaten off the gear uncounted, which makes the stock look less productive than it is. Staff called it serious for individual boats and particular hot spots rather than critical for the coastwide assessment. For the boats that run into it, board chair Olaf Jensen said, the losses can be the "difference between profitable and unprofitable."

Work has shifted from measuring the problem to preventing it. In a test last May, an underwater device researchers call a shuttle brought up 93.3 percent of the halibut hooked, though only 52.2 percent of the sablefish. The results are preliminary.

On maturity, research biologist Colin Jones said new coastwide tissue analysis moved the age at which half of female halibut are mature from 11.6 years down to 11.0, "a slight shift in the left of the curve to younger maturing females." That curve is already in the 2025 stock assessment. Over a 23-year series, he said, maturity age climbed from 2004 to 2016 before falling sharply through 2022.

Both findings land on the same figure, the spawning biomass estimate that determines how much halibut can be caught. Jones said work on whether the largest older females produce proportionally more eggs should yield preliminary data next June, ahead of the full stock assessment in 2028.

The science board's recommendations go to the full commission at its interim meeting Dec. 1.