Clever Marking System Separates Wild from Ranched Salmon
A source of Alaska streamside pride is an angler's ability to instantly distinguish among different kinds of salmon; a pro can tell a king (chinook) salmon, from a pink (humpback) salmon, from a silver (coho) salmon, from a chum (dog) salmon, from a red (sockeye) salmon. If salmon names are this complicated, imagine catching two pink salmon in the ocean and trying to tell a hatchery-born pink from one born in a stream.
This nearly impossible task has become simpler thanks to an ingenious fish-marking technique adapted by University of Alaska scientists for use in Southeast Alaska at Douglas Island Pink and Chum salmon hatchery with the help of the Alaska Department of Fish and Game and the Alaska Science and Technology Foundation. Without touching the fish, hatchery workers are able to mark millions of salmon fry before they wriggle out to the ocean.
In fisheries like the North Pacific, where hatchery-born fish mix freely with their wild brethren, fisheries biologists have the responsibility to assure about 40 percent of wild salmon avoid fishermen's nets and make it back up to streams to spawn. The problem is that unmarked hatchery salmon are virtually indistinguishable from wild salmon, which makes it easy for a fisheries manager to misinterpret a plentiful return of mostly hatchery salmon as a sign wild salmon are abundant. Rather than closing a fishery to protect wild salmon, a manager might choose to allow commercial fishermen to keep scooping up fish, only to later find disturbingly small numbers of wild salmon returning to streams.
For almost 30 years, hatchery workers have tried to help out managers by tagging a small percentage of the millions of salmon fry born each year in Alaska hatcheries. In the tagging process, workers remove an unnecessary fin and insert a metal tag into a salmon fry's nose. When netted in the ocean, these punkish looking fish can be traced to the hatchery of their birth.
With this traditional method, however, only one-tenth to one-half of a percent of the salmon in a hatchery can be marked, according to Bill Smoker, a fisheries scientist with UAF's School of Fisheries and Ocean Sciences, Juneau Center. Managers sought a more statistically significant way to mark fish, and found it in a method developed at Cornell University in New York and Uppsala University in Sweden.
Researchers found that a salmon's otolith--a kidney-bean shaped bone in a salmon's inner ear--reacts to water temperature changes by adding darker layers of calcium to its expanding shell. By raising the temperature of water running over salmon eggs about 3 degrees C, holding it there for two days and then dropping the temperature back down to normal temperature, salmon ranchers induce a dark ring visible when the otolith is later removed, cut in half, and viewed in cross-section.
"If you artificially control the timing of the temperature drop, you can induce a series of these rings that look very much like a bar code," Smoker said.
Each hatchery can imprint its own custom code. At one hatchery, for example, workers might induce five dark rings, while another hatchery cultivating the same species of fish might induce seven rings. Smoker said the beauty of the system is that every salmon fry released from an otolith-marking hatchery can later be distinguished from wild fish. This gives managers more confidence in their statistical evaluations of a salmon run.
"Managers need only take a relatively small sample of the fishery, perhaps 200 fish," Smoker said. "With that, they can get a precise estimate. It's more cost effective (than the traditional method), and much more statistically powerful."
