Ashokan Rainbow Trout Study Yields Unexpected Findings

Feb 11, 2019

What’s to blame for a decline in rainbow trout in the Ashokan watershed in the Catskills? The researchers behind a new study say there are some surprising findings.

A study of how invasive fish in the Ashokan Reservoir have affected rainbow trout populations was launched when the number of such trout in the upper Esopus Creek, a major tributary draining into the Ashokan Reservoir, showed a precipitous drop between 2009 and 2013. The principal investigator of the study, Scott George, with the U.S. Geological Survey, says he became engaged around 2009 while conducting surveys in the Esopus Creek and, when Tropical Storm Irene hit in 2011, surveying continued with the goal of looking at the effects of Irene.

“And, it was at that point we realized that many of the fish species had recovered pretty nicely following Tropical Storm Irene. And the rainbow trout population was already in decline, it seemed, prior to Tropical Storm Irene hitting and continued declining,” George says. “And we actually had a data set that showed about a 85 percent decline in small rainbow trout in the Esopus Creek spanning from about 2009 through 2013.”

One potential cause for the decline was the establishment of two invasive fish in the reservoir.

“You have these two invasive fish species that were established in the Ashokan Reservoir. You have alewife, which is a small herring species that was introduced in the 1970s, and then you have white perch, where were established sometime in the mid-2000s. “And they both, at times, occupied pretty similar habitats to rainbow trouts. They’re pelagic so they cruise around in the water column rather than just attaching to a rock or a log or something like that,” says George. “And it was hypothesized that they may be actually competing with rainbow trout for food, and alewife also were responsible for knocking out the emerald shiner population, which was believed to be the rainbow trout’s, their primary food source.”

But the findings showed something unexpected, including that the largest increases in both growth and condition of rainbow trout were observed after the introduction of white perch.

“I think the findings are very positive because if we had found maybe the opposite of what we found and found evidence that these invasive species in this 8,000 acre reservoir adversely affecting the rainbow trout, your management options are very limited,” says George.

And so the route to take is stream management. That’s where the Ashokan Watershed Stream Management Program of Cornell Cooperative Extension of Ulster County comes in. Leslie Zucker is program leader.

“The conclusions pointed to maybe taking a closer look at spawning areas and streams feeding into the reservoir and what we can do to make sure spawning habitats, they, present, healthy and abundant and that that may be able to sustain the rainbow trout population, other trouts, in the long term,” says Zucker.

The Ashokan Watershed Stream Management Program funded researchers to complete an analysis of fish caught in the Ashokan Reservoir over the past 70 years. And much like rings on trees, fish scales can indicate age as well as rate of growth. George says the study’s aim underscores the value of having long-term data sets.

“Discussions really took off when the New York state DEC Region 3 fisheries folks, Mike Flaherty in particular, came forward and said, hey, I’ve got a box of over 500 rainbow trout scale samples at the office,” George says. “And that was when realized we had a neat opportunity to try to get at this question.”

The Department of Environmental Conservation’s Michael Flaherty, the primary DEC collaborator and co-author of an article on the study, says the DEC’s data set goes back to the 1920s and scale sample data began in around the 1950s. Flaherty sometimes eliminating theories can be helpful, as in the case of the rainbow trout study.

“Even though there had been these disturbances with different species coming in, we weren’t able to show categorically that there was a big difference in the, at least the growth rate of the rainbow trout," Flaherty says. "That doesn’t really answer all the questions, however. It just looks at one small little component of this. And if we can then move on to maybe some other ideas and then focus our attention there, that’s where it can be really helpful.”

Again, Cornell Coop’s Zucker:

“This trout fishery is so locally, it’s economically important and it’s also historic and a part of the culture here,” Zucker says. “So I think we’ll continue looking at ways to make sure the trout fishery stays here and stays healthy in the Catskills.”

An article on the study was published in the North American Journal of Fisheries Management.