While many Americans mark spring by dyeing chicken eggs, Bill Glick marks spring by dyeing sockeye salmon smolt.
Each spring, Glick, an Alaska Department of Fish and Game fish biologist, captures a few thousand Kenai River sockeye smolt, dyes them gold and releases them back into the river. Kenai River smolt dyeing began in 2005 as part of a Fish and Game mark and recapture project to estimate out-migrating smolt abundance.
In the future, however, the spring parade of gold smolt swimming down the Kenai River could come to an end if, as Fish and Game biologists hope, a genetic-based method of estimating smolt abundance can be used to replace the dye-based method. A genetic-based method would reduce smolt handling and could produce more accurate estimates, said Mark Willette, a Fish and Game research biologist for upper Cook Inlet.
The out-migrating smolt abundance estimate is one of several indicators biologists use to forecast the strength of future runs of returning adult sockeye salmon. Biologists using the dye-based mark and recapture method to estimate smolt abundance begin by dyeing the smolt. The dyed smolt are released and recaptured downstream. Biologists then use the total number of smolt caught downstream and the proportion of those fish that are dyed to help them estimate the total out-migrating smolt abundance.
In the Kenai River, biologists using the dye-based mark and recapture method, capture sockeye smolt at the mouth of Torpedo Creek with a fyke net, a net that guides the smolt through a series of hoops into a trap called a live box.
Biologists then recapture the smolt downstream at the Kenai Keys in two inclined-plane traps and sometimes a fyke net. For the best results, the dye-based method needs to capture, dye and release 2,000 Kenai River smolt per week from May 11 until the end of June.
If biologists could replace the dye-based method for estimating Kenai River smolt with a genetic-based method, however, they would require fewer captured smolt and would no longer need to dye them. But the genetic-based method for estimating smolt abundance cannot be used to estimate salmon smolt abundance in just any river.
In the Kenai River, the genetic-based method is possible because the river’s out-migrating sockeye salmon smolt include smolt originating in the Russian River system. Russian River smolt differ considerably from other smolt migrating through the Kenai River and can be easily distinguished through genetic testing.
Biologists using the genetic-based method to estimate Kenai River smolt abundance can take advantage of this fact by using genetic tests to determine what proportion of the smolt captured at the Kenai Keys are Russian River smolt.
By comparing the proportion of Russian River smolt captured at the Kenai Keys with the Russian River’s total estimated smolt abundance, biologists can then estimate the Kenai River smolt abundance. Russian River smolt abundances, however, must still be estimated using the dye-based mark and recapture method.
Fish and Game began testing the genetic-based method in the Kenai River alongside the dye-based method last year, and says it could help resolve some problems with the dye-based method. Biologists sometimes struggle to capture enough smolt from the Kenai River to achieve the best possible abundance estimate. This is particularly true in years when the out-migrating smolt include a lot of bigger smolt, which are not easily caught in nets and traps.
“Larger fish detect the gear and avoid it,” Willette said. “They can actually just zip out of the way.”
In 2005, for example, the smolt averaged 40 to 50 millimeters in length and Fish and Game had no problem catching enough smolt. In the last two years, however, smolt have averaged about 70 millimeters in length and catches have dramatically decreased.
A couple of factors are believed to have influenced sockeye salmon smolt size fluctuations in recent years. Smolt migrating down the river in 2005, for example, are believed to have been small because they grew up as part of a particularly large fry population and, consequently, had more competition for food. And some of the recent increase in smolt sizes is believed to be due in part to a shift in the age composition of out-migrating smolt.
An out-migrating smolt population comprised of 95 percent to 98 percent 1-year-old smolt used to be typical, but recently more smolt are spending two years in freshwater, a behavior referred to as holding over. For example, 20 percent of smolt from the 2004 brood year held over, and 30 percent from the 2005 brood year held over. Data tables for subsequent brood years have not yet been fully calculated, but preliminary observations indicate the tendency to hold over has continued to climb, Willette said.
“It has gone up,” he said. “We’ve seen the proportion of age 2 smolt greater than 50 percent. It’s really changed, which is why the smolt are much larger.”
Smolt likely hold over when they fail to reach a certain threshold size to out-migrate at age 1, Willette said.
Continued improvements are needed in smolt-capturing techniques to reduce size selectivity and, consequently, improve the accuracy of smolt abundance estimates. However, by reducing the necessary smolt catch rate, the genetic-based method of estimating smolt abundance would make it easier for Fish and Game to capture enough smolt to achieve the best possible estimates even in years when smolt have been big.
Any switch that may occur from the dye-based method to the genetic-based method would occur slowly. Although the first couple of years of testing the genetic-based method look promising so far, Fish and Game cannot be entirely certain if any particular abundance estimating method has worked well until years of data have been accumulated.
“We think it’s working better,” Willette said. “But until we get the adult returns back we don’t know for sure how well our smolt programs are working.”