Ocean glider opens new 'tool kit' in crab tracking efforts

Ocean glider opens new 'tool kit' in crab tracking efforts

A remotely piloted underwater glider is showing promise as a tool to track crabs in the Bering Sea, where their numbers have plummeted.

The Alaska Department of Fish and Game and the University of Alaska Fairbanks have tested the glider Shackleton for the past three years to locate tagged crabs. The 6-foot-long glider, which is piloted out of a Fairbanks laboratory at the UAF College of Fisheries and Ocean Sciences, can travel for three months and up to 1,500 kilometers on a battery pack before it's recovered.

The glider can survey vast areas in search of crabs outfitted with acoustic transmitter tags, and some transmitter signals can even indicate whether a tagged crab has been eaten by a predator.

"Crab are notoriously hard to follow, but some new tagging technologies have allowed us to expand our animal tracking tool kits," said Jared Weems, a Kodiak-based crab biologist for the Alaska Department of Fish and Game. "Our most recent 'crabby' glider survey, bang for the buck, produced an extraordinary dataset."

Finding better methods for tracking red king crabs and snow crabs is a priority for fishery managers as numbers have dropped in recent years. The decline may be driven by warming oceans that allow predatory fish to expand their ranges.

Shackleton's first tracking job came in 2022, after ADFG biologists seeded a Kodiak fjord system with 35 tagged Tanner crabs. It worked, leading to another test in the southeastern Bering Sea in 2023.

An expanded effort in 2024 added a new layer of technology. In May, ADFG biologists outfitted about 30 juvenile king crabs with a specially coated tag about the size of a multivitamin. The tag coating dissolves in the acidic conditions found in a fish's stomach. That changes its coded acoustic signal to let biologists know where and when the crab was presumably eaten.

During a week of sampling in the fall, three months after the tagged crabs were released, Shackleton located four tagged juvenile red king crabs, including one that gave a "predation signal." It was the first time the tags had been proven effective in Alaska marine waters.

Weems said this year's project was completed for a fraction of the cost of recovering tagged crabs with dedicated survey boats or fisheries gear. Funding was provided by ADFG, and the project shared vessel time with a separate juvenile red king crab study funded by the North Pacific Research Board.

Gliders are typically used by researchers to gather oceanographic data on temperature, water salinity and chlorophyll content. Capturing those details can also help biologists better understand environmental conditions that may correspond to crab movement and survival.

"The gliders provide a platform for doing persistent monitoring over a wide spatial area," said Hank Statscewich, the UAF glider program operations director. "In addition to tracking the animal movements, they give the study environmental context."

Weems is optimistic new crab tracking projects will follow in the coming years, especially with the apparent value of monitoring real-time predator-prey dynamics.

"The future potential and interest is very high," he said. "I would really love to see a devoted large project that looks at crab movement over multiple seasons and time scales."

ADDITIONAL CONTACTS: Hank Statscewich, [email protected]; Jared Weems, [email protected]

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