A research team with members at the Alaska SeaLife Center and California State University Long Beach captured a juvenile Pacific sleeper shark last month, keeping it at the Seward aquarium for two weeks in the team’s first successful attempt to measure the metabolic rate of this little-studied species.
The shark was released Monday, but will still provide the team with valuable data: the shark carries a data logger which will eventually detach itself, float to the surface and transmit information about the animal’s activity after its release.
This is the first time that a team led by Dr. Markus Horning has brought a Pacific sleeper shark to the Alaska SeaLife Center for extended study, but not for lack of trying. In the two seasons since the project began, the team has captured around a dozen specimens, all but one of which were too large to transport.
This one came in around six feet in length, suggesting that it is still young – for a sleeper shark. The sharks’ nearest relative currently holds the title for longest-lived vertebrate: the Greenland shark has a lifespan estimated around 400 years.
“If this age determination on the Greenland sharks is accurate and if it applies for Pacific sleeper sharks … this animal could have been a 40- or 50-year-old youngster,” Horning said.
The shark’s two week stay at the Alaska SeaLife Center was not an unmitigated success. The team released the shark because it did not eat during its captivity. Nevertheless, the short stay in “hotel ASLC” is a milestone for the center’s research on the species.
“The one thing which we were able to accomplish very well was done by Taylor Smith, a graduate student at California State University,” Horning said. “She is doing the respirometry … the equivalent of putting the shark on a treadmill and measuring its metabolic rate.”
The metabolic rate reflects the rate at which an organism consumes energy and is a key figure in modelling how species interact at the population level. It tells researchers how much energy an animal needs over time, which predicts other factors like how much and how often it eats.
Horning, a specialist in diving, air-breathing vertebrates, such as sea lions, did not originally set out to study the sleeper shark, he told the Seward Journal. Instead, a long-standing study of sea lions inadvertently brought the shark to his attention.
It began with an effort to track the life histories of juvenile sea lions, in particular to establish survival rates and causes of mortality.
“For that purpose we developed these life history transmitters,” Horning said. “They were developed in my lab in collaboration with Wildlife Computers,” a research company specializing in wildlife telemetry.
“I always describe them as an electronic black box for animals like a black box for airplanes,” Horning said. “They are life-long implants that are surgically implanted into the gut cavity of the animals.
“Those tags are a satellite transmitter that will only connect to a satellite after the animal has died. … It comes out of the decomposing, digested or dismembered carcass, and we get a very sad email that says Animal 57 has just died – but we also know where, when and how.”
Horning has been tracking 45 sea lions with such implants. Twenty of those sea lions have died so far, which Horning says is right around the expected mortality rate. Eighteen of those sea lions became prey for another animal. A subset of those triggered Horning’s interest.
“Data from some of those tags looked unusual,” Horning said. “In five of those 18 instances, the data specifically suggest pacific sleeper sharks as the predator.”
The tracking devices record a variety of parameters that can help establish how an animal died, such as ambient light and temperature. It was the second that led to Horning’s hypothesis that pacific sleeper sharks – typically viewed as sluggish scavengers – may in fact occasionally prey on sea lions.
“Basically, the data suggested that those tags were ingested by a cold-bodied predator,” Horning said. “It couldn’t have been killer whales because they’re warm-blooded. And it couldn’t have been white sharks or salmon sharks, because they’re partially warm-bodied. Sleeper sharks are the only potential predator of sea lions that are really at ambient deep water temperature.”
With his co-author Jo-Ann E. Mellish, Horning published these findings in the National Marine Fisheries Service Fishery Bulletin in 2014. They called the article “In cold blood: evidence of Pacific sleeper shark predation of Steller sea lions in the Gulf of Alaska.”
“This led me to look into sleeper sharks, and I realized that we didn’t know much about them,” Horning said. “They are basically ignored because they are considered nuisance by catch. They are not really palatable for human consumption.”
The Pacific sleeper shark shares this feature with its closest relative, the Greenland shark. Both sharks have high concentrations of urea and trimethylamine oxide in their tissues, which renders it toxic to humans – although the cured flesh of the Greenland shark is one of the national dishes of Iceland.