Fishing for Answers in Island Park Reservoir

The fishery in Island Park Reservoir, just like the water level, has seen its ups and downs over the years. That's not surprising given the two are related. The fishery has suffered since the onset of the most recent drought in 2000, leaving many anglers wondering what can be done to improve it. To answer that question, the fisheries staff members at the Idaho Department of Fish and Game have been examining the fishery's history and identifying management options. The Island Park Reservoir fishery is not an easy one to understand. Combine the highly variable water levels with an inconsistent stocking program over the past several decades, and then throw in a few rotenone projects and it's pretty tough to tease out the recipe for a great fishery. Suckers and chubs have been fingered as the main impediments to a good fishery in the past. The reservoir has been treated with rotenone to kill the unwanted fish on several occasions, most recently in 1992. While non-game fish may have an affect on the fishery, it's not the driving factor, said Jim Fredericks, regional fishery manager for Fish and Game. Instead, Fredericks points to water levels and drought as being the bigger issue. "There's little question that the single most important factor in the fishery is the carryover, or the amount of water held in the reservoir after irrigation season," he said. On average, Island Park Reservoir is about 45 percent full going into the winter. But from 2001 through 2003, it averaged only 11 percent. Besides winter carryover, the stocking rates have confounded the picture. Over the past 50 years, Fish and Game has managed the reservoir with a wide range of stocking strategies. Varying numbers of fingerling rainbow trout have been stocked in the spring, summer, and fall, in addition to a lesser number of "catchable" or 10- to 12-inch fish. Furthermore, as part of a statewide effort to reduce risks of interbreeding with native fish, Fish and Game has been stocking only sterile triploid rainbow trout in the reservoir since 2003. "The triploid program really wasn't aimed at Island Park or the Henrys Fork," Fredericks said. "Obviously the vast majority of reproducing fish in the system are already rainbow trout, so the triploids were stocked more as a matter of consistency than a native fish concern." Upwards of 1 million fingerling rainbows were planted until the late 1990s, but that was decreased by about two thirds, coinciding with the drought. "The decreased stocking rates haven't helped the fishery in recent years," Fredericks said. "But planting a half million or a million fish into the reservoir when it gets taken down to 15 percent full in the winter would just be a waste." Fredericks believes now that water levels have improved there's an opportunity to rebuild the fishery. "The approach we're taking is to go back to the last time we had a good fishery and see if we can't duplicate what we were doing," Fredericks said. The biggest change would be to switch back to late spring fingerling stocking and increase the plants to at least 750,000 fish. Additionally, Fish and Game could stock reproductively viable diploid rainbow trout, instead of the sterile fish that have been stocked since 2002. In addition to going back to a stocking program reminiscent of the late 1980s, Fish and Game has identified other alternatives. One is to stock the reservoir heavily with a type of fish that would take advantage of the extensive nongame fish population. Splake, a cross between a brook trout and a lake trout, have been used widely in the eastern United States. They tend to live much longer than a brook trout, and have a lake trout's appetite for fish. They were stocked in a few waters in Idaho in the 1990's, including Island Park Reservoir with some success. In 2006, a 10-pound 12-ounce state record splake was caught in Ririe Reservoir. "There's some potential that they could be used to take advantage of the chubs and suckers and create a unique trophy fishery," Fredericks said. "But there's no evidence that they'd do anything to improve the rainbow trout fishery." One last management alternative is chemical treatment. Despite rotenone being used several times in the past, it hasn't lived up to expectations in the benefits to the fishery. In 1982, following a 1979 treatment, Fish and Game concluded that it had not improved the fishery, Fredericks said. Similar conclusions were drawn in 1994, two years after the 1992 treatment. "Rotenone isn't off the table," he said, "But from a biological, social, or economical perspective, it's not jumping out right now as the logical solution." Anyone interested in learning more about the fishery and management options may attend a meeting from 7 to 9 p.m. January 18 at the Madison County Senior Citizen Center at 40 South 2nd West in Rexburg. Individuals with disabilities may request meeting accommodations by contacting Jim Fredericks at Fish and Game at 208-525-7290, or jfredericks@idfg.idaho.gov or through the Idaho Relay Service at 1-800-377-3529(TDD).