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How to Track the Most Elusive Shark in Alaska

Sep 13, 2021

A Pacific sleeper shark returning to its deep-water habitat after being tagged in Resurrection Bay in August 2021. Photo by Thomas Farrugia.

When most people think of sharks in Alaska, they think of either salmon sharks (Lamna ditropis) or spiny dogfish sharks (Squalus suckleyi). But there’s another shark species that resides in Alaskan waters, one that is rarely encountered and poorly understood – the Pacific sleeper shark (Somniosus pacificus).

A close relative to the Greenland shark (Somniosus microcephalus), Pacific sleeper sharks are deep-water sharks with loose skin, flabby muscles, and the demeanor of a sleepy giant. Full grown adults can attain over 4 m in length and may live to be well over 200 years old. When they are encountered, it is usually by commercial gear fishing in deep waters, and often it feels like a huge log being hauled up. Once at the surface, they seem very lethargic, lazily rolling rather than the wild thrashing expected from most shark species.

This lethargic nature is one reason why it was surprising when a recent study showed evidence of sleeper sharks feeding on Steller sea lions. And it also led to questions about where these animals moved, how far they could travel, and whether there was a seasonal pattern to their movements. To address these questions, Markus Horning (Wildlife Technology Frontiers) and Amy Bishop (UAF) have been tagging Pacific sleeper sharks in Resurrection Bay. In 2021, Thomas Farrugia (AOOS) worked remotely from Seward to assist in capturing and tagging sleeper sharks.

Pacific sleeper sharks were captured using baited hooks set at 600 to 950 ft in Resurrection Bay. Sharks were brought to the surface and directed into a floating sling tied to the tagging vessel, where they were measured, tagged and monitored before being released. This way, the sharks were never taken out of the water and stress to the shark was minimized. A total of eight sharks ranging from 129cm to 350cm total length were tagged – seven with electronic tags and the smallest one with just a numbered spaghetti tag. Four sharks received a single ‘miniPAT’ pop-up archival satellite transmitting tag. These tags record depth and temperature data while on the shark, and after a pre-determined length of time (in this case one or two years) the tag releases from the shark, floats to the surface and transmits its final location and depth and temperature data through the Argos satellite system back to the researchers.

In addition, three sharks received a total of four tags, including a ‘miniPAT’, two ‘mrPAT’ tags, and one acoustic transmitters. The ‘mrPAT’ tags are similar to ‘miniPAT’ tags except that they do not record data and only transmit the final location when they pop off. One tag was set to release after 4 months, and one tag after 8 months. Therefore, for the sharks with all four tags, we should get a location of where they are after 4, 8 and 12 months, revealing any seasonal pattern in their movements. Finally, the acoustic transmitters on these sharks transmit a unique acoustic signal every few minutes. This signal can be detected by either glider-mounted or moored receivers if the sharks come within about 500m of the receiver. As more gliders are used throughout Alaskan waters, and moored receivers are deployed in Resurrection Bay to complement those already in Prince William Sound, the chances of detecting tagged sharks will increase. Now it’s a waiting game – either for acoustic transmitters to be detected, or for the satellite tags to pop off and transmit their data. These data will fill in a crucial knowledge gap about the behavior, life history and swimming capabilities of Pacific sleeper sharks. Additionally, movement models developed by Julie Nielsen and Garrett Dunne (UAF) will recreate the movement patterns of the tagged animals. Further tagging of sleeper sharks in Resurrection Bay and Prince William Sound will be undertaken in 2022.