Snow crab model raises questions over quota
Crabbers are anxious about survey estimates for snow crab, and even more anxious about how those estimates don’t synch with allocation models.
Bristol Bay red king crab and tanner crab are less than last year’s biomass levels, but still roughly on par with long-term averages. Allocations for snow crab, the largest of the three main commercial crab harvests, could take a worse dive this season, resulting from a questionable modeling method that could make the Alaska Department of Fish and Game manage the fishery more cautiously than usual, up to and including lowering crabbers’ quota.
The department, or ADFG, sets the total allowable catch, or TAC, for crab fisheries each year. Scientists and stakeholders organized by the North Pacific Fishery Management Council complete the surveys and scientific models that inform ADFG’s quota decisions.
The department releases the TAC for crab species in the first week of October, with the season beginning Oct. 15. Bristol Bay red king crab is harvested first to reach holiday markets, and snow and tanner crab fishing doesn’t begin until January.
A yearly trawl survey assessment estimates the amount of crab in the sea, or biomass. To account for uncertainties in the area-limited trawl survey, scientists use different models to extrapolate overfishing limits and acceptable biological catch, from which ADFG’s derives the total allowable catch.
The Crab Plan Team, a group of stakeholders and scientists within the North Pacific Fishery Management Council, spent a large chunk of its Seattle meeting fretting over inconsistencies in the modeling and survey numbers, an ongoing issue with snow crab management.
“Last year, biomass went up, but the (overfishing limit) the model spit out went down,” said Ruth Christiansen, scientific policy coordinator at Alaska Bering Sea Crabbers, an industry cooperative. “So there’s always been this trend, where (overfishing limits) and biomass estimates are contradicting themselves, which to a certain degree is expected. This year it was even more so.”
In the 2013-14 season, the biomass dropped to 279.9 million pounds. The overfishing limit, however, rose 22.6 million pounds.
The biomass for 2015-16 snow crab, as estimated by a trawl survey, is 271 million pounds compared to 284 million pounds last year.
The Crab Plan Team, however, accepted an estimate model that places the overfishing limit at 183.2 million pounds, along with an acceptable biological catch of 137.4 million pounds.
The Alaska Department of Fish and Game cannot set the TAC greater than the allowable biological catch, and the Crab Plan Team changed the rubric for calculating that number.
Because of the disparity between the survey’s biomass estimate and the Crab Plan Team model’s overfishing limit, the team changed the formula for calculating the acceptable biological catch from 90 percent of the overfishing limit to 75 percent.
ADFG could simply reduce the TAC out of caution, recognizing a level of uncertainty in the model. Fisheries managers rate various stocks on a tier system from one to five. The higher the number, the less information managers have regarding that specific stock.
Snow crab is a tier three stock; managers have a fairly full information set. The model could bump that tier status down, which would cause managers to act more conservatively.
Christiansen fears ADFG will not use any of the Crab Plan Team models and set the TAC solely on the low trawl survey assessment, or use the problematic model and come up with the wrong TAC.
“We’re stuck between a rock and hard place,” said Christiansen. “The rock is an assessment model you know isn’t functioning as it should. The hard place is the inability to pick another option because you don’t have adequate information.”
The Scientific and Statistical Committee of the North Pacific council will review the models and make recommendations to ADFG. Potentially, the scientists could scrap the model altogether and make their own recommendations, though such actions are infrequent.
For the other two main commercial crab stocks, survey estimates show a similar decline. Lower biomass estimates typically correspond to lower overfishing limits and acceptable biological catches, which in turn lower the final TAC.
The legal-sized male Bristol Bay red king crab biomass is estimated at 54.4 million pounds, 5 million pounds less than the 2014-15 biomass. The Crab Plan Team accepted an overfishing limit of 14.8 million pounds and an acceptable biological catch of 13.4 million pounds.
In 2014, the Bristol Bay red king crab harvest retained 10 million pounds of a 10-million pound TAC, based on an acceptable biological catch of 13.5 million pounds and a biomass of 60.1 million pounds.
The biomass of legally harvestable tanner crab is estimated at 117.6 million pounds, down 40 million pounds from last year. The overfishing limit has been set at 60.4 million pounds, and the allowable biological catch at 48.3 million pounds.
The numbers hint at a lower catch limit this year.
In the 2014-15 season, the tanner crab fishery had a TAC of 15.1 million pounds, and retained 13.6 million pounds, based on an acceptable biological catch of 55.5 million pounds.
DJ Summers can be reached at email@example.com.