Science at scofield
Gill net surveys amass decades of data on fish populations, enabling better sport fishing
Here's the latest word on Scofield Reservoir: "The fish are fat and healthy, especially for just coming out of winter."
That's the comment from Calvin Black, regional fisheries biologist for the Division of Wildlife Resources and it is not a guess. It is a conclusion based on carefully collected data from the most recent gill net survey.
Though it is a time-consuming, labor-intensive process, each survey is a glimpse of science at its most basic: the collection accurate information.
It began the evening of May 8, when DWR crews submerged the nets at five locations. Each net is about as long as the Price indoor pool and made of plastic webbing.
Shortly after a steel-gray dawn lit Scofield Reservoir May 9, the crews hauled in the gill nets.
Every net was loaded with dead fish that had strayed into the plastic webbing overnight. The DWR researchers muscled each net into a big galvanized tub aboard their boat. Once ashore, they stretched the net out on the concrete boat ramp.
The catch was impressive: about 20 to 25 trout per net, mainly cutthroat and tigers. There were roughly ten times that number of chubs and red-sided shiners.
Next stop for the unlucky fish was the weight and measure station.
Nickole McCarty, Dan Keller and Justin Hart made up one team. McCarty would lay a fish out on a ruler and call out the species and length in centimeters to Hart, who made a note. She passed the fish to Keller, who put it on a scale and called out the weight in grams.
The count by species showed some impressive results. About 35 percent of the catch were cutthroat trout which were missing their adipose fins (the small fin on the back nearest the tail). "That means they were part of the 2009 planting," Black explained. That in turn means that they have been growing and thriving for five years, going from seven-inch planters to 20 inches or more.
Lots of chubs
The chub count is still high, and probably higher than proportion in the nets indicate. That is because the water was colder, about 45 degrees F. compared to 55 in years past. The DWR had to move its survey up a week because of a scheduled planting of more than 100,000 cutthroat during the next week.
Rainbow trout are not doing so well. The evidence showed only seven or eight of them in the spring survey nets. Black explained that the rainbows, unlike the predatory cutthroat and tigers, don't change their diet as they grow bigger.
They stick with insects and zooplankton, putting themselves in direct competition with the more numerous and efficient chubs.
The survey continues beyond the lake. Back in the shop in Price, the DWR team conducts necropsies on selected fish.
This is where the internal information gets exposed to the light of day. A long cut of the belly gives a look at the fatty deposits at have endured the long winter under the ice. Females are loaded with roe.
But the really good news is on stomach contents of the cutthroats and tigers. The predators are living up to expectations. "Ordinary these fish will eat prey up to 30 percent of their length. Now we're seeing some of them have swallowed prey up to 70 percent," Black said. "We found one 19 inch cutthroat that had two five inch chubs in the stomach," he added.
This means that the chub and shiner populations could be kept in check by the predators. In the past, Scofield has had to be treated every few decades with rotenone, the gill poison that kills the entire fish population in the lake and tributaries. It may be possible to avoid that measure with a purely biological approach.
The stomach content findings also explain why there is a slot limit in effect at Scofield. All cutthroat and tiger trout between 15 and 22 inches must be immediately released. This is the crucial size when the fish begin to feed on other fish and when their growth is most rapid, Black explained.
An angler can keep one of these fish over 22 inches, and two under 15 inches.
This year's spring survey, along with every other survey over the years, will become part of a growing data base for scientific research. "We have about 25 years of data and that helps us to look at trends," Black said. The information collected at the spring and fall gill net surveys is correlated with water depth and temperature to complete the picture.
Seeing all the netted dead fish spread out on the boat ramp may raise a question about the necessity of the method. In answer, Black said that the 100 or so fish sacrificed for the survey make up only a fraction of a percent of the population already in the lake, and probably less than a tenth of a percent of the number planted this year.