Tracking Alaska Red King Crabs Using Saildrones

Popularized by the TV documentary Deadliest Catch, red king crab is one of the most valuable of the commercially harvested crabs. Though they have been fished and studied for nearly a century in Bristol Bay, AK, very little is known about when, where, and why crabs move across the ocean floor, especially during the winter months when it’s not possible to conduct standard vessel surveys. To answer some of these questions, the National Oceanic and Atmospheric Administration’s (NOAA) Alaska Fisheries Science Center and the Bering Sea Fisheries Research Foundation have teamed up on a project to track Alaska red king crab using acoustic tags and a Saildrone unmanned surface vehicle (USV).

“We’re specifically tracking mature male crab greater than 120 mm carapace (shell) length. Only large male king crab are harvested in Alaska and there are a lot of unanswered questions on the movement of these crabs,” explained NOAA Fisheries scientist Leah Zacher, who is leading the project. Females and smaller individuals are thrown back to allow the population to reproduce and replenish.

In order to set effective rules for fishing, managers need to understand how crabs move through the seasons and which habitats are essential to their protection. Bristol Bay red king crab are sustainably managed and responsibly harvested following a series of regulations, but in recent years, crab stocks have declined. 

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Informed by NOAA’s annual summer survey in the Bering Sea, researchers tagged 148 mature male crabs in June while aboard a chartered fishing vessel. Two Saildrone USVs will be deployed in October 2019 and again in April 2020 to relocate the tagged crabs.

The North Pacific Fishery Management Council is one the primary bodies making decisions regarding the protection of red king crab. The mission will attempt to answer two different sets of questions. In the fall, researchers are interested in how crab move from the summer survey area onto known fishing grounds. Trawl fisheries are active in the winter and spring, so the question is whether or not red king crab are in areas where they’re at risk of being caught as bycatch.

“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,” said Scott Goodman of the Bering Sea Fisheries Research Foundation, the organization that has contracted the project.

Red king crab molt every few years, completely shedding their shell and growing a new one. Saildrones have previously been used to track northern fur seals with a small GPS unit glued to their back, but this is impossible with crabs because of the molting, and because they don’t swim to the sea surface where data can be transmitted via satellite. Instead, each crab gets an “earring” looped through a muscle in its back (where the carapace meets the bottom shell) to which a small acoustic tag can be attached. This way the crabs can molt and still retain the tag.

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The acoustic tags send a ping into the water column that can be detected by the saildrones equipped with VEMCO receivers. Saildrone USVs carry a payload of approximately 20 science sensors; the VEMCO receiver is part of the enhanced sensor suite. When a ping is received, the receiver on the saildrone can detect the ID number of the crab.  

This is the first time Saildrone will apply acoustic tracking to crustaceans. In 2018, Saildrone successfully tracked great white sharks swimming near the sea surface. In the case of the crab, the VEMCO receiver will be positioned downward on the USVs to detect the animals moving across the ocean floor.

Bottom temperature is one of the most interesting environmental variables affecting red king crab, but existing data sets are limited to observations made during the summer survey. Bottom temperatures have not previously been collected over a large geographic area at other times of the year. In addition to the ID number, the acoustic tags will also transmit temperature, via the saildrones, providing researchers with a data set of utmost importance, which was previously cost-prohibitive to collect.

“This technology will collect many data points for each crab released. It is a new paradigm for tag release and recovery studies,” said Goodman.

Areas have been set aside to protect the crab, specifically, the Red King Crab Savings Area, but there has been very little data available to determine those protected areas. Scientists and fishermen want to know if they are in the correct location at the correct time of year and if the areas might vary from year to year. This study is expected to show if the protected areas are actually protecting crab.

“Where animals are is really important. 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,” said Zacher. “This project is gathering data that will be of both scientific interest and help managers make the best possible decisions.”

Read "Record Number of Crabs Located During First Crustacean Tracking MIssion."