A sleeper shark photographed on deck during a NOAA Fisheries research cruise. Photo credit: NOAA.
Seeing any species of shark on the back deck of a fishing boat is exciting, but when I saw a flabby, dark brown, sausage with rounded fins and a mouth full of razor-sharp teeth in a catch of Pacific ocean perch I was sampling, I knew I was looking at the elusive Pacific sleeper shark (Somniosus pacificus). At 6 feet in length, the shark was lifted out of the fish hold by crane to prevent it from clogging access to the ship’s factory where the fish were processed. As a fisheries observer aboard a commercial fishing vessel, my primary goal was to gather data on the target species and as many incidentally-caught species (known as bycatch) as part of the National Oceanic and Atmospheric Administration’s (NOAA) mission to monitor and manage U.S. fisheries. Because many sharks are long-lived, have slow growth rates, and reach their age of sexual maturity late in life, the life-history traits make sharks vulnerable to commercial fishing operations that use bottom trawl or longline gear and therefore, are a concern to fishery managers.
What do scientists know about Pacific sleeper sharks?
The tubby shape of the Pacific sleeper shark has lead scientists to think they are slow-moving scavengers that feed on whatever falls to the bottom of the sea. But recent tagging and diet studies by NOAA scientists at the Alaska Fisheries Science Center (AFSC), have found that sleeper sharks actually are active, opportunistic feeders that swim throughout the water column eating a variety of fast-moving species like salmon and squid. Diet data from juvenile sleeper sharks (averaging 5.5 feet in length) in the Gulf of Alaska suggest the sharks eat low on the food chain, but there is speculation that larger sleeper sharks may consume marine mammals such as the Steller sea lion.
The age of Pacific sleeper sharks caught by fisheries could provide a clue on how old they get. Unfortunately, current ageing techniques don’t work for sleeper sharks. The age of sharks is typically assessed by counting band pairs laid down in vertebrae or dorsal spines.
“We tried a number of methods to age the vertebrae, including several different staining and microscopy techniques, and when that didn't work we tried other hard structures including neural arches and jaws, all to no avail,” said researcher Beth Matta with the AFSC's Age and Growth program of the work she and her coworker Chris Gburski have done trying to age sharks.
Sleeper sharks, like several other deep-sea species, do not lay down any discernable bands in their vertebrae and have no dorsal spines. The vertebral column is unlike any that Dr. Ken Goldman, a colleague at the Alaska Department of Fish and Game, has examined. “It resembles a weird corrogated hose-like structure and all of my attempts over the past 14 years to assess age through vertebrae have failed,” he said. AFSC scientists have tried different microscopy techniques, slicing thin sections of the different structures to try to clearly identify any banding patterns; all attempts have yielded no clues on how old these animals get.
To date, no reproductively mature Pacific sleeper shark has been caught in Alaska. Sleeper sharks approaching 25 feet in length have been captured in areas outside of Alaska, and information suggests that both sexes mature at around 14 feet in length, but scientists have no idea about their growth rates. However, incidentally captured Pacific sleeper sharks in Alaska’s commercial groundfish fisheries are seldom longer than 7-8 feet. This has left the scientists wondering if Alaskan waters are the preferred habitat for juvenile sleeper sharks. Whatever the reason, it raises another question: Are the commerical fisheries’ bycatch of juvenile sleeper sharks impacting future population levels?
How do Scientists Estimate Population Size of Sleeper Sharks?
NOAA Fisheries scientists during tagging operations of a sleeper shark. Photo credit: NOAA.
Annual catch limits are required for all federally managed fisheries, including non-target species such as sharks. Scientists at the AFSC have found that fishery catch data are the best data to monitor the Pacific sleeper shark populations in Alaska. When scientists present the trend of Pacific sleeper shark bycatch in all fisheries in the Gulf of Alaska to fishery managers, they can determine if the trend is a concern. A downward trend raises a red flag for both scientists and managers to then investigate if the trend is due to a change in fishing behavior, changes in the environment, or if it may be a real reflection of the Pacific sleeper shark’s population size. Currently, the average yearly incidental catch of the Pacific sleeper shark in the Gulf of Alaska has been in a sharp decline since 2005, but scientists are not certain what the cause may be; earlier tagging studies of sleeper sharks did not produce a clear understanding of any migratory or movement patterns that may explain any changes in the fishery catch data.
New data may provide insight in areas where Pacific sleeper sharks have been historically caught. In 2013, fishery observer coverage increased in some small boat fisheries, such as in the Pacific halibut longline fishery in the Gulf of Alaska. These data plus future research will bring much needed information to help scientists better understand the elusive Pacific sleeper shark and help fishery managers make informed decisions.