Counting cohos
Project monitors health of silvers
At some point this fall, just about every angler, boater or homeowner on a stretch of the Kenai River from Centennial Park in Soldotna to Skilak Lake has seen them. They ply the waters of the Kenai in boats with radio antennas and nets, tracking the movements of the coho salmon.
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Alaska Department of Fish and Game employees have been very visible on the river for the last two and a half months as part of an annual coho tagging project, part of a broader effort that seeks to get a handle on the overall health of the river's coho population.
And while all the data has yet to be analyzed, this year's strong run of silvers has given researchers crucial information toward developing a long-term population estimate of adult Kenai River cohos.
Researchers have tagged and recaptured salmon throughout the fall escapement monitoring project with the use of fish wheels and nets. Some fish receive radio transmitters, while others are fitted with "spaghetti" tags. The fish are then netted upstream and checked for the markings.
Most of the hardware was pulled from the river last week, but not before this year's strong run led to the tagging of more than 3,300 fish, significantly more than last year's total of 450.
"We have been busy this fall," said Jay Carlon, a fisheries biologist and the research project leader. "It appears that the 2000 run looks much better than the last four years."
Mallette uses a portable scanner to locate a marker in a fish that was tagged in the spring of 1999 as Strausbaugh holds it steady in a cradle. The fish was marked with a different type of tag and released again.
Photo by M. Scott Moon
Carlon said the fall spawning escapement project for cohos differs from surveys for chinook or sockeye salmon because sonar is not used.
"We looked at sonar to count cohos in 1993," Carlon said. "But we found by radio telemetry that sockeye and coho were intermingling at the same site. The sonar couldn't discriminate one from the other."
He said the fall counting project began in 1997.
Carlon said two fish wheels were set up on each side of the Kenai at river mile 27.5. The wheels were checked each day and every fish found was tagged with either a radio transmitter or spaghetti tag. Farther upstream, Fish and Game employees in boats equipped with radio tracking devices and nets targeted silvers in two locations: between river mile 36.3 and the Moose River confluence and between river mile 30.4 and the Funny River confluence. Carlon said Wednesday that netting was expected to continue for three more days.
"With the netting, we're looking for the marked proportion on the adults," Carlon said of tracking the spawning fish. "We used the nets to tell us what percentage we were catching at the fish wheel."
Carlon said 200 radio tags are used per season, adding that an improved design allows for quicker tagging, thus shortening the time each fish is out of the water. The tags are designed with two prongs that are placed into the base of the fish's dorsal fin.
A tag is inserted into the back of the fish. Researchers will try to catch the same fish later this season to track its movements.
Photo by M. Scott Moon
Along with boats equipped with radio tracking equipment, eight data receiving stations are scattered at various points on the river shoreline from Centennial Park to the outlet of Skilak Lake. Some Fish and Game trucks used in the project also are equipped with radio tracking devices.
"The radio tags tell us how many fish remain in the study, and we have tracked fish to many of the tributaries, so it seems to be effective," Carlon said. "We put out 200 radios per season -- everything else gets a spaghetti tag."
In 1999, 40 percent of the radio tagged fish didn't move upstream, Carlon said. They either died from handling stress or left the system. Last year's return wasn't a strong one, he added.
"Cohos are very susceptible to handling stress," Carlon said. "That's one reason we're not comfortable with the 1999 results. As for this year, until all the telemetry data is in, it will be hard to tell what proportion of radios remained in the study, but it looks a lot better than the 1999 data."
Initial plans were to tag fish through the end of September, but Carlon said a push of silvers during the last week of the month resulted in a surge in fish wheel catches.
"On the 29th (of September) we decided to fish another week to see what fish were coming in in October," Carlon said. "We will follow the radio-tagged fish through the end of October and into November."
Carlon said the number of fish entering the river decreases from October through December.
"We see some that arrive late in the season as well as some that come in September and stay for five months," Carlon said. "One fish tagged September 15th was caught at the outlet of Skilak Lake on February 24th."
But the fall survey is just one part of the bigger picture for biologists as they try to measure the overall health of the coho population in the Kenai.
In the spring, the Fish and Game focus is on the Moose River, where from mid-May to the end of June around 100,000 coho smolt are tagged and their adipose fins clipped. Carlon said the purpose of the spring project is to formulate an estimate of smolt abundance also while seeking to measure the commercial harvest of Kenai River coho.
The 2-year-old, 6-inch smolt are fitted with a small, coded wire tag placed in their noses. At that age, the smolt are preparing to head out to sea and will return in a year, something no other salmon does, Carlon said.
"The coho is an amazing fish," Carlon said. "They leave the river as 6-inch smolt and return a year later with an average length of 27 to 28 inches. We've seen them as big as 34.
"It's fun," Carlon added. "With the coho coming back a year after they're tagged, you get this instant gratification and rapid feedback of information. It's great."
Carlon said by the time the weir is pulled from the Moose River in July, the previous year's tagged run begins to return.
"We sample the adult return in the river and we're looking for the tagged-to-untagged ratio in adults," Carlon said. "And we are working on two objectives: the smolt abundance and how many were taken in the commercial harvest."
He said with the marked-to-unmarked ratio, biologists can estimate how many smolt leave the Kenai River every spring. The smolt project just finished its ninth year.
"In the second step, we can use the marked-to-unmarked ratio to estimate the number of Kenai River fish that are commercially harvested," Carlon said. "The catch is usually examined at the processor, and when we see the adipose fin missing, we know it's a tagged fish."
He said when a clipped adipose fin is discovered, the fish's head is taken off and sent to Juneau, where the tag is analyzed, in this case for the Kenai River code.
Carlon said the coho sport harvest is handled through periodic creel surveys as well as the state's annual "postal survey" of anglers statewide. Those results lag a year behind, Carlon said, and probably will not be available until late next summer.
"The reason we're doing all this -- the driving force -- is that we're operating under a management plan with the word 'conservation' in the title," Carlon said, pointing to the current state Kenai River Coho Salmon Conservation Manage-ment Plan.
He also offered some simple formulas.
"The total adult return is equal to the sport harvest plus the commercial harvest plus the spawning escapement," Carlon said of the overall formula, "while the exploitation rate equals the sport plus commercial harvest divided by the total return.
"In the short term, we're trying to obtain an exploitation rate," Carlon said. "In the long term it's more of a look at a population estimate.
"This is a big project -- no question about it," Carlon said. "As for the life span of the project -- at least the adult work -- depends on the exploitation rate that occurred this year. If it is high, we will continue monitoring that for a few years. If it is extremely low, we might spend half the number of years monitoring.
"What's important, I think, is that the exploitation rate is going to vary with abundance," he added. "What we really want to know is what the exploitation is in years of low abundance."