ADF&G bottom trawl finds few adult male Red King crab

By James Mason
Much scientific research in the Bering Sea region has been on hold because of the Covid pandemic. But the Alaska Department of Fish and Game was able to complete their annual bottom trawl survey of the waters of Norton Sound this past summer. What they found confirms the views of local commercial and subsistence crab fishermen about the state of the crab stocks near Nome. ADF&G biologist Jenefer Bell reported the findings in a Strait Science talk sponsored by the UAF’s Northwest Campus and Alaska Sea Grant.
The number of legal males is by far the lowest they’ve seen in the history of the trawl survey, according to Bell. On the upside, the large number of pre-recruit males looks good for the future. “There is hope that those crab are going to get bigger,” she said. “Right now we are at a historic low for legal size. This information is used in conjunction with other crab data, including the commercial fishery catch, to inform a model that’s used to set the Guideline Harvest Level.” A lot of other work goes into expanding this to cover the entire Norton Sound area.
In March the Norton Sound Advisory Council petitioned the Alaska Board of Fisheries to close the Red King Crab commercial fishery because of dangerously low crab stocks. The board rejected the petition. The petition was written by Charlie Lean, who stated, “We believe we have a conservation crisis and meet the criteria for an emergency closure of the fishery. Our petition outlines the biological conditions and the regulatory actions that are requirements given the stock status.”  
“One of the things we were not expecting in 2020 was the lack of growth in sublegal males,” said Jenefer Bell. “We did not see the expected growth of the cohort from 2019 to 2020 in our catch.” She explained the difficulty in understanding why the sublegal males did not grow. “It could be there was a mortality event between 2019 and 2020,” she said. It’s possible mature crab just weren’t in the path of the trawl or perhaps molting is an issue. “We don’t understand why we didn’t get the growth we were expecting.”
NOAA performed the bottom trawl survey from 1976 to 1991, then discontinued it. It was brought back to life by ADF&G in 1996 and was done every three years. Around 2000, Norton Sound Economic Development Corporation has been instrumental in making sure the survey continues. They also have helped with the purchase of sophisticated equipment used in the survey. In 2018 the decision was made to do the bottom trawl survey annually. It takes place the first two weeks of August.
The area trawled is organized in squares 10 by 10 nautical miles in size. The trawl is performed once in each of the squares. The net is 40 feet wide and the length of the trawl is one nautical mile. “We take the number of crab that we caught in the trawl net and multiply it by the ratio of the total area by the area swept,” said Bell. This is performed for each of the squares in the grid.
“We caught 322 crab that we detected,” said Bell. “If we break it down into just male crab, we caught 121 sublegal crab.” Sublegal is less than 105 mm carapace length, measured from the eye socket to the tail. Ninety-one of the crab were new shell, which indicates that they molted. The 15 legal crab were predominantly new shell as well. They captured 186 females, of which 29 were juveniles. Of the females 146 had eggs present and seven were barren. This is a very low percentage of barren females.
In order for a crab to grow, it has to molt. If it doesn’t molt the shell shows it. “It’s got growth on it, it’s like it’s been worn for more than a year,” said Bell. The question of how molting might be involved in the large percentage of sub-legal crab has not been answered. The role of mating in molting is also being examined. The biologists are doing live tank studies to determine at what size males can begin to mate.
The trawl also compiles information about other species such as Pacific cod and pollock, both of significant economic impact. The pollock numbers found in the survey were high but consistent with recent years. The second most abundant fish, after orange sea stars, is saffron cod, known to most as tomcod.
“It’s important that we continue to conduct our annual trawl survey,” said Bell. “We get a more refined picture of that if we can do it every year rather than have these data gaps, particularly as climate change becomes more obvious or apparent.”
“We struggle to have large males in this fishery. Our model predicts we should have a lot more large males,” Bell said.