“You are what you eat” holds true for the salmon shark, at least in name. These warm-blooded, energetic sharks prey on the anadromous (freshwater spawning) fishes that swim up the North American west coast each summer. So it isn’t surprising that the salmon shark is found high in the northern Pacific, following the salmon runs into the estuaries of the big rivers in Alaska and the Yukon.
What is surprising, given our usual enthusiasm for large, active predators, is that scientists know little about the salmon shark – at least on the North American side of the Pacific. Only a handful of studies have been done outside of Russia and Japan. At this point its not even known if the western and eastern Pacific salmon sharks are distinct groups, or if sharks seen in North American waters also make their way to the other side of the Pacific, and vice versa.
Other missing pieces of the puzzle include the life span, age of sexual maturity, length of gestation (how long it takes for fertilized eggs to grow into full-term pups in the female), and fecundity, or rate of births per female in the population. All of these bits of information are necessary to determine how the population of salmon sharks will grow over time. Without population growth models, it’s impossible to tell whether or not salmon sharks could support a sustainable fishery, or if current pressures are all the shark can take, while still maintaining a viable population. By the way, for species that have been extensively studied, this information seems elementary, but going about gathering it for the salmon shark is no easy feat. Ken Goldman, a researcher at the Virginia Institute of Marine Science who studies salmon sharks in Alaska’s Prince William Sound, is one of the few scientists studying the species.
Ken Goldman says fishing regulators have given the salmon shark, and its biological researchers, a fantastic break.
The good news is fishing regulators have given the salmon shark, and its biological researchers, a break: There is no legalized commercial fishery and only a very limited sport fishery – ten licensed boats in all. This gives Goldman and his colleagues time to assess the population with relatively little fishing pressure. With a small team and one boat, Goldman can only do so much first hand observation, but some important data can be gathered from another source. By-catch is the term used by fisherman and fisheries officers to describe species other than the target animal caught in commercial fishing operations. By-catch, which may ultimately prove to be a threat to the salmon shark, is also a ready source of encounters with the animals, whether they be alive or dead. Every fishery accidentally catches animals it wasn’t fishing for. In both Canada and the U.S. the fishery departments require vessels to monitor their by-catch. The numbers, logged separately by species, can not only give researchers an idea of the population density, but also how the accidental fishery would impact any fishery targeting salmon sharks. “If you don’t know how many salmon sharks are being caught as by-catch and you go with all the information we’ve collected and you say here’s a responsible management plan for a sport fishery and a commercial fishery, you’re losing animals that you’re unaware of in that population,” Goldman says.
A salmon shark caught by Michigan sport fishermen becomes a specimen for Goldman’s research
The exact impact of by-catch on salmon sharks is not known, but the numbers are being added up, and in the U.S., nine years worth of data is being processed. The Canadian numbers aren’t being crunched, but if salmon sharks are caught as part of commercial fishery by-catch in Canadian waters, they are being recorded. Every trawling vessel has to carry a certified observer and every boat involved in ground fishing has to check in with port observers, where each non-target fish aboard is recorded by species. Besides the raw numbers, salmon shark bodies that are collected as a result of by-catch can add to Goldman’s knowledge of the species. Records of the shark’s length and size, whether or not the individual has reached sexual maturity, and what the animal has been eating, help the researchers piece together the shark’s life history. Tag, you’re it. There’s also the potential that a tagged animal would wind up in a net or on a fisherman’s trawl line, but unfortunately not all tags make it back to the research lab. Goldman says it’s hit or miss whether tags are returned. Whether or not tags make it back to the researcher depends on fisherman finding them and seeing them as a help, rather than a hindrance to their business. The reality is, not all commercial fishermen will turn in found tags. “I’ve been on lots of commercial boats where the whole upward part right above the wheelhouse is just filled with tags stuck in to the wheelhouse ceiling, that they’ve taken off of fish, kind of like trophy tags,” says Goldman. However, every tag that does come in helps, and Goldman stresses that he works with everybody to gather as much information as possible on the little-known salmon shark. So far, of 300 tagged animals, only one tag has been found and returned. Goldman received that one from a commercial salmon fisherman right on the border of southeast Alaska and Canada – indicating the animal had travelled close to 1050 kilometres south in 49 days.