The next swell catches me off guard, and as my feet stumble, the momentum jerks my view off the water to the shoreline and back in quick succession. I immediately feel my stomach protest. Don’t get sick. Don’t. Get. Sick. Dontgetsickdontgetsick. 

I lower the binoculars briefly, allowing myself a deep breath of cold damp air and tug my hood down a bit farther onto my hat brim. The Chugach mountains, usually towering on all sides of the narrow Alaskan fjord, are hidden in the low grey clouds that almost reach down to touch the similarly grey waters where our boat, the R/V Tiburon, is currently sloshing around. I breathe again, set my feet, lift the binoculars back into position, and resume scanning for our target: a small grey tag no bigger than the palm of your hand that after a year of traveling the deep dark waters on the back of a Pacific sleeper shark is finally within our grasp.  

Image Credit

Amy Bishop

Not many people have heard of the Pacific sleeper shark, but their closest relative, the Greenland shark, made the news a few years ago when scientists discovered they may live anywhere from 250-512 years! Though less popular and woefully understudied, Pacific sleeper sharks are similarly large, slow-moving sharks found throughout the Arctic and Pacific, anywhere that waters are deep and/or cold including Hawaii and the Solomon Islands. They’re commonly bycaught in fisheries (their meat can be toxic, so it isn’t commonly consumed), but there isn’t a good estimate of population abundance.  There has also been a prevailing assumption that they are likely too slow to catch prey and rely on scavenging. Combined, this lack of basic knowledge has resulted in Pacific sleeper sharks being ignored when it comes to ecosystem studies.  
 

Our interest in Sleeper sharks actually came from a different direction--a desire to understand if predators may be limiting the recovery of endangered populations of marine mammals. Killer whales are typically the primary predator for seals and sea lions in the North Pacific, but some interesting data from the Gulf of Alaska suggested there may be additional, large, cold, predators taking a bite out of young sea lions. As there aren’t many sharks that are residents in the colder Alaskan waters, the Pacific sleeper shark was the only one that fit the criteria of being large (11-20ft), having a similar internal temperature to the surrounding waters, and overlapping in space and time with their potential sea lion prey. 

Image Credit

Amy Bishop

Additionally, recent studies of Greenland shark diets suggest they may not be as ‘sleepy’ as we thought and in addition to scavenging are predators on a wide range of fish, squid, and marine mammals.  

So, our goal was really to start at the beginning and learn as much as we can about their basic biology to understand the likely important, but ignored, role these sharks play in Arctic and North Pacific marine ecosystems as both predators and prey.  

Bio-telemetry, or tagging, is a powerful way to answer basic biological questions for wildlife that are difficult to monitor from observations or on land. The first shark we ever tagged was a very large female (cue: I think we need a bigger boat), and since then we have tagged over 50 animals with satellite devices that collect information on how deep sharks are diving, the temperature of their environment, and light levels. The tags are programmed to ‘pop-off’ and float to the surface after a set amount of time, typically 6-12months later. Once at the surface, the tag can communicate with satellites to calculate a location, and download some of the data it has collected. There is always much more data stored than can be quickly sent to a satellite, which is why we were braving sea-sickness on a cold August day, scanning the waters for a veritable needle-in a haystack and the goldmine of data that it held. (I’m happy to say we have gotten quite good at recovering tags when they are close to shore, with over 10 found on beaches or bobbing in the waves!) 

Image Credit

Amy Bishop

From this data we have begun to fill in some of the gaps for these mysterious deep dwelling sharks. Firstly, we found that some sharks appear to be residential, staying within a few miles of where we caught them for a whole year, while others travel close to 600 miles in just 6 months. In terms of the habitat they prefer, though sleeper sharks have been seen in waters as deep as 3000-6000 meters, we generally catch sleeper sharks in 200-300m of water with our hooks near the ocean floor. The tags however revealed their movements throughout the year are much more varied with instances where they were found within 50m of the surface, and the sharks routinely swam into (relatively) warm waters at night in the autumn. Most interestingly, the data from the recovered tags showed signs that sharks were accelerating occasionally, in an upward trajectory, which could be a signal of a foraging attempt. Together this suggests some previous assumptions about sleeper sharks may be incorrect and our data will help us understand how sharks may respond in the future to a changing ecosystem: will they move to new habitats? Need to rely on different prey? Interact more frequently with fisheries and human activities? 

Not every day on the water is damp and rocky. There are stunningly sunny days with glass-calm seas, so full of life they almost appear emerald green, reflecting the towering mountains and glaciers around us. There are days we catch sharks, and days we don’t. There are the winter days in the office pouring over the data with a cup of tea. There are days spent in the garage repairing lines, and hooks and days spent collecting fish heads to use as bait. I never saw myself as chasing after sharks, but there is something exciting about science at a new frontier—exploring a species and a part of the oceans we know relatively little about. In the end, it's the pursuit of the unknown that drives us. 

Each new discovery is a piece of a much larger puzzle, and with every breakthrough, we inch closer to understanding not only sharks but the complex systems that govern these waters. And none of this would be possible without funding that supports our efforts, enabling us to dive deeper and uncover mysteries that remain hidden in the depths of the ocean.