New Technology is Key to Study of Bristol Bay Red King Crab
September 1, 2019
Collaborators in an innovative research effort to track the trail of Bristol Bay red king crab are hoping that the combination of acoustic tags and an unmanned drone will give them answers to help better manage this multi-million-dollar fishery.
"We still have a lot to learn about why king crabs aggregate and how they more across the ocean floor," says Leah Zacher, a research fisheries biologist with the Alaska Fisheries Science Center's Shellfish Assessment Program at Kodiak. "We hope that this study will answer many of these questions.
"Males, females and juveniles do tend to be found in different areas at certain times of the year. In Bristol Bay males generally move further out in the Bay and into deeper waters after mating with females in near-shore areas in the spring or early summer. We are interested in studying why king crabs aggregate in certain areas and we'll be looking at several different potential explanations, such as water temperature and bottom substrate type," she said.
Zacher is the project leader for a team of federal fisheries researchers, working with the bering sea Fisheries Research Foundation, that used pots to capture Bristol Bay red king crabs in June, during the summer survey. Females and smaller individual male king crab were thrown back to allow the population to reproduce and replenish.
A total of 148 of the mature male crabs were tagged with VEMCO acoustic tags that can communicate with unmanned drones that sail on the surface of the water. The tags were attached with a harness looped through a muscle in the back of the carapace, so the tags would be retained through crab molting. Each tagged crab had its tag switched on so that it began pinging, and the pings were tested for reception as each crab was released.
"Tagging mortality is always an important question and can be challenging to determine in a natural environment," Zacher said. "The most tricky part for tagging crabs is that they molt and it's difficult to find a place to attach the tag so that it won't get lost during the molt.
"The isthmus muscle has been used to tag king crab with simple spaghetti tags since at least the 1950s. We are using this time-tested method, with the addition of an acoustic tag attached to the spaghetti tag. We tested our tagging method in the laboratory and found that all crabs retained the tag when they molted, with a high survival rate through the molt, always a stressful time for any crab," she said.
The acoustic tags send a ping into the water column that can be detected by the Saildrones equipped with VEMCO receivers.
The Saildrones themselves are wind and solar-powered unmanned surface vehicles designed for up to 12 months of ocean data collection missions for science, research and commercial purposes. Each Saildrone is equipped with GPS and an onboard computer, enabling them to navigate following prescribed waypoints. Around the clock supervision of the Saildrones is provided by trained operators at the Saildrone Mission Control in Alameda, Calif.
Saildrones have been used previously to track northern fur seals with a small GPS unit glued to their back, but this technique won't work for crabs because of molting and because they don't swim to the sea surface where the data can be transmitted via satellite.
Saildrone technology was used in 2018 to successfully track great white sharks swimming near the sea surface, but this is the first time it is being applied for acoustic tracking for crustaceans.
In October two Saildrones are to be deployed from Dutch Harbor to sail out to the area of the study grounds to relocate the crabs tagged in June. The study is designed to determine crab movement from June to immediately prior to the start of the commercial crab fishery season in mid-October. Then in the spring of 2020, in March and April, the Saildrones will again be deployed to relocate tagged crabs and continue monitoring crab movement.
"At this point we are only tagging mature male red king crab, which means their carapace length is at least 120 mm," Zacher said. "Crabs cannot be aged, but crabs this size are around seven years old. This is an important life stage, as these are the crabs that are part of the king crab fishery or will be entering the fishery in the next year or two."
Researchers note that bottom temperature are one of the most interesting environmental variables impacting red king crab, but they have not previously been collected over a large geographic area at other times of the year, so in addition to the ID number, the acoustic tags will transmit water temperature via the Saildrones.
"This technology will collect many data points for each crab released," said Scott Goodman, executive director of BSFRF, and president of Natural Resources Consultants Inc., in Seattle.
"So little is known about where crabs are and how they move. We have only snapshots from summer surveys. This research will fill in the life history gaps to better inform the management of red king crab as both target and bycatch."
While some areas have been set aside as Red King crab Savings Areas, to protect the crab, there is still little data available to determine those protected areas. Scientists and seafood harvesters alike want to know if they are in the right spot at the correct time of year and if the areas might vary from year to year.
"Where animals are is really important," Zacher said.
That sounds basic, but it's really hard to know for an animal that's living on the seafloor, that doesn't come to the surface, and that's way offshore. This project is gathering data that will be of both scientific interest and help managers make the best possible decisions."
Researchers are hoping to compile data that will be useful in advance of the 2019/2020 Bristol Bay red king crab fishery, with the total allowable catch to be announced in mid-October.
Last year's harvest of the 4.3 million-pound TAC garnered harvesters $10.33 a pound, up from $9.20 a pound the previous year, when the TAC was 6.6 million pounds. For the 2016/2017 season the TAC was 8,469,000 pounds and for 2015/2016 it was 9,974,000 pounds.
BSFRF also notes in its summer newsletter the continuation of its research into the growth rate of Tanner and snow crab, again to better inform crab stock assessment models used to make harvest management decisions.
That research team is led by Madison Shipley, a fisheries analyst with Natural Resources Consultants, Erin Fedewa, a research biologist with NOAA Fisheries in Kodiak, and Charles Heller, a scientific observer with Natural Resource Consultants. Their research is looking at how much crab grow between molts and how to improve on the limited samples of growth per molt that are currently available from bering sea crabs. BDFRF is chartered the F/V Half Moon Bay this past spring to complete the growth sample collections. Researchers used a specialized Naphrops trawl to catch very small crabs while maintaining optimum contact with the sea bottom. In 34 trawl tows they caught several thousand crabs of which they retained 464 crab samples within targeted size classes of from 15 mm to 105 mm.
The crabs were sorted so researchers could retain only pre-molt crabs, which were held in mesh bags in tanks on deck during the charter. Once back at port, those crabs were taken to the Kodiak NOAA shellfish laboratory and placed individually into stacked plastic containers, where they were monitored daily for molting. Once the molt happened, the old carapace and the newly formed carapace widths were noted and the growth increment recorded.
Researchers said the growth per molt of both bering sea snow crab and southern Tanner crab has proven to be a critical population parameter that informs the annual status estimates of these crabs. Their goal is for understanding of growth for snow and Tanner crab to improve stock assessments, management and sustainability of these crustaceans.
NOAA scientists have projected an accelerating decline in king crab biomass, with a total cessation of commercial fishing by the year 2100, the Alaska Sea Grant report noted. While less is known about snow crab dynamics, there is little reason to think their scenarios will be better, the report said. On the bright side, the crabs may be able to migrate into the Arctic and new habitat, the Sea Grant report said.