Tiger Muskellunge Growth, Condition, Diet, and Effect on Target Prey Species in Two Eastern Washington Lakes: Progress Report 2001-2006

Tiger muskellunge Esox masquinongy x E. lucius growth, condition, and diet, as well as the effect of stocking on forage populations, was studied in two eastern Washington lakes from 2001 to 2006. Curlew Lake (373 ha), located in Ferry County, was stocked with tiger muskellunge in 1998 to reduce an overabundant Northern Pikeminnow Ptychocheilus oregonensis population and to create a unique trophy fishery. Historically, Curlew Lake had provided good fishing opportunity for stocked Rainbow Trout Oncorhynchus mykiss, as well as naturally reproducing Largemouth Bass Micropterus salmoides and Smallmouth Bass M. dolomieu. The quality of trout fishing, however, had declined throughout the 1990’s, commensurate with anecdotal observations of increased numbers of Northern Pikeminnow in the sport catch. Silver Lake (197 ha), located in Spokane County, was stocked with tiger muskellunge in 2002 with similar management goals of reducing an overabundant Tench Tinca tinca population and to create a unique trophy fishery. Historically, Silver Lake had been managed as a put-grow-and-take Rainbow and Brown Trout Salmo trutta fishery, which relied on periodic rotenone rehabilitation to keep competing species in check. Prior to the introduction of tiger muskellunge, rotenone treatment had been abandoned as a management option for the lake even though the fish community had become dominated by naturally reproducing Tench and a variety of centrarchid species. To monitor changes in species relative abundance, each lake was sampled annually in the spring and fall with standardized boat electrofishing, gill netting and fyke netting surveys. Additionally, each lake was sampled by boat electrofishing monthly, from spring through fall, to collect tiger muskellunge diet samples gastric lavage. Tiger muskellunge growth and condition were greater in Curlew Lake than in Silver Lake. Rainbow Trout and Northern Pikeminnow were the most important prey species in Curlew Lake, while Largemouth Bass were a distant third. In Silver Lake, compressiform centrarchids were the most important prey, followed closely by Rainbow Trout. Largemouth Bass and Tench were only somewhat important. Diet varied seasonally, with Rainbow Trout being the most important prey during spring months in both lakes. Average consumed prey length was larger in Curlew Lake, where preferred-type prey (soft-rayed and fusiform) predominated. Average prey length in Silver Lake (dominated by compressiform, spiny-rayed prey) was smaller, which may account for slower tiger muskellunge growth in that community. The relative abundance of Northern Pikeminnow in Curlew Lake significantly declined over the duration of the study. The high proportion of Northern Pikeminnow observed in the tiger muskellunge diet analysis seems to indicate that the reduction can be attributed to the added presence of tiger muskellunge to the community. Considering this, the biological control goal of this tiger muskellunge introduction has been successful. Continued biannual monitoring of the fish community to assess Northern Pikeminnow abundance should provide the necessary data to refine future Tiger Muskellunge stocking rates in Curlew Lake. At Silver Lake, the impact of tiger muskellunge predation on Tench is less clear. This may be attributed to gape-limitation of the majority of tiger muskellunge inhabiting the lake to date, which had not yet grown to a size capable of consuming the average sized Tench (300-400 mm TL) in the lake. To date, Tench appear to be of minimal importance in the diet of tiger muskellunge, and their relative abundance within the fish community only appeared to decline in the last survey conducted. Continued monitoring of the Silver Lake fish community, as well as tiger muskellunge diet sampling, would likely provide a better understanding of the results of this particular introduction by allowing additional time for the tiger muskellunge population to more fully develop, both in abundance and individual fish growth.