Bristol Bay, at the eastern edge of the Bering Sea, is home to America’s largest red king crab fishery. Alaska red king crab is one of the most valuable of the commercially harvested crabs, in demand for its succulent and flavorful snow-white meat. The Bristol Bay red king crab stock suffered a severe decline in the early 1980s; although it recovered at a lower level, recent years have seen further declines. In order to sustainably manage and responsibly harvest red king crab stocks, managers need to understand how crabs move throughout the seasons. To make better decisions regarding the protection of the important Bristol Bay fishery, researchers from the National Oceanic and Atmospheric Administration’s (NOAA) Alaska Fisheries Science Center and Bering Sea Fisheries Research Foundation (BSFRF) teamed up to track the movements of red king crab using Saildrone unmanned surface vehicles (USVs).
In June, researchers aboard a chartered fishing vessel tagged 148 mature male Alaska red king crabs with a shell length greater than 120 mm (4.7 inches). Only large male crabs are harvested in Alaska; females and smaller individuals are released to perpetuate the fishery. In late September, two Saildrone USVs were launched from Dutch Harbor, AK, to find out where the crab had gone.
Saildrone has performed several successful animal tracking missions, including the 2018 White Shark Café mission and multiple missions tracking northern fur seals in the Bering Sea, but those animals swim near the surface whereas crabs reside on the seafloor requiring different equipment and techniques to find them.
The saildrones, known as SD 1042 and SD 1048, are equipped with keel-mounted VEMCO acoustic receivers in addition to the standard sensor suite, which collects oceanographic and meteorological data above and below the sea surface. Saildrones are designed to perform in the most difficult ocean environments, from hot, light winds in the tropics to Southern Ocean storms. In the fall, conditions in the Bering Sea can range from very light wind to severe storms.
“Since this was the first time any of us had worked on this kind of project, it took a bit of time to figure out how best to search for crab. We were really happy with how quickly we could put in a request to alter the vehicle’s track and then see the saildrone changing direction in the Mission portal,” said NOAA Fisheries scientist Leah Zacher, who is leading the project. “Based on our research of previous Saildrone missions, particularly those that took place in the Bering Sea, our expectations for how the vehicles would perform were pretty high, but they were more than met.” Read more about how a Saildrone mission is managed.
Traditional tag and recapture methods involve tagging crabs and then relying on fisheries or research vessels to locate them. In the case of the fisheries, the effort is biased, depending on where the fishery decides to fish. Typically, a crab tracking project is deemed successful if around 10% of the tagged animals are located.
“Working with the fisheries has provided an invaluable source of information, but the problem is that we only get data from where fishing is taking place. That was one of the real advantages of using a saildrone; we could conduct a comprehensive search effort of any area. The fishermen are really good at finding hot spots, but even in areas where crab are not found, that’s still important data to have,” explained Zacher.
The two saildrones sailed a series of straight-line transects in the eastern Bering Sea, the location pre-determined by data from annual summer surveys and the location of the fishery, which opened in October. Crabs tend to aggregate, so when one of the saildrones located a crab, it would be re-tasked to sail a spiral in that location.
“Just by doing straight-line transects, we found 11 crab, which is a little over what we predicted. By sailing spirals around the crab, we were able to locate a lot more. The scale of how many we found was pretty impressive, above expectations,” said Zacher.
During the October mission, the two saildrones covered a total of 3,500 nautical miles and located a total of 50 crabs—34% of the tagged group.
Participants in the Bristol Bay red king crab fishery supported this project by releasing tagged crabs when they were caught in crab pots. Vessels also reported the locations and tag numbers for 20 tagged crabs, adding to the data set generated by Saildrone.
Though the fishery is technically open until January, most of the quota is usually harvested in October and November. Over the winter, Zacher and her team will work to interpret the data, including the temperature on the seafloor, which is recorded by the tags, as well as some surface environmental data collected by the saildrones.
The mission will resume in early spring 2020. There is very little crab distribution data at that time of year because there aren’t any fisheries or surveys. It’s also the time of year that the ground trawl fisheries are fishing for yellowfin sole and other flatfish; Alaska red king crabs are at risk for being caught as bycatch.
“It’s important to know the distribution of the crab in the winter and early spring so that managers can set better closure areas for trawling. Since we have very little information on crab distribution at that time of year, there are currently a lot of questions as to whether the closure areas are in the right location,” said Zacher.
Changes in the Bering Sea ecosystem affect the health of the Alaska red king crab, especially changes in temperature. The crab movements observed so far will help scientists and managers understand how red king crabs move from one season to the next.
Leah Zacher, “Tracking the Alaskan Red King Crab – Post 8,” NOAA Alaska Fisheries Science Center, November 5, 2019