Population structure, dynamics, and food habits of northern pike and smallmouth bass in Lake Coeur d’Alene: potential effects on adfluvial cutthroat trout and management alternatives
June 2011 - December 2013
- Coeur d'Alene Tribe
Cutthroat trout have one of the broadest native distributions of any salmonid in North America; unfortunately, cutthroat trout have drastically declined across their distribution. A number of factors have been associated with cutthroat trout declines, including hybridization, habitat degradation, and interactions with nonnative competitors and predators. Interaction with predators is particularly relevant for adfluvial stocks of cutthroat trout. Not only must adfluvial cutthroat trout contend with habitat alterations and nonnative competitors (e.g., brook trout) in lotic systems, but they must also deal with habitat degradationand nonnative species in lakes. Nonnative species in lakes usually contain introduced salmonids (e.g., lake trout), but many lakes in western North America have been stocked with a variety of warmwater and coolwater fishes. Common species include smallmouth bass, largemouth bass, walleye, and northern pike. In systems with species of conservation concern or in systems with native species that have recreational value, northern pike and smallmouth bass can be particularly problematic. Northern pike are highly piscivorous and can play an important role in structuring fish assemblages. They have been used as a management tool for reducing densities of undesirable species and are an important recreational species due to their ubiquity, relative ease of capture, and ability to achieve trophy size. Consequently, they have been widely distributed, legally and illegally, beyond their native distribution and are now common across portions of the midwestern, southern, and western U.S. In Idaho, the Coeur d’Alene basin is the only system with a significant northern pike fishery. They were illegally introduced to the system in the early 1970s, but the fishery did not develop until the early 1980s. Like northern pike, smallmouth bass are a popular sport fish and have been widely introduced across North America and beyond. Smallmouth bass are opportunistic carnivores and consume a variety of invertebrate and vertebrate prey. As juveniles (i.e., < 80 mm), smallmouth bass diets are dominated by insects and other macroinvertebrates, and as they grow, large invertebrates (e.g., crayfishes) and fishes become an increasingly important component of smallmouth bass diets. Unlike northern pike, smallmouth bass are widely distributed across Idaho and provide important recreational opportunities in a variety of systems, particularly reservoirs. Smallmouth bass were illegally introduced to Lake Coeur d’Alene in the early 1990s and have since become highly abundant in the system. Adfluvial cutthroat spawn and rear in many of the small tributaries to Lake Coeur d’Alene. Over the last 10-15 years, the Coeur d’Alene Tribe has been implementing strategies in watersheds of the Coeur d’Alene Basin to recover depressed populations of adfluvial westslope cutthroat trout. Through this work, they have found that in-lake survival rates of juvenile cutthroat trout are two to three times lower than those that have been reported for juvenile cutthroat trout in other lake systems. The mechanism responsible for the high mortality of adfluvial cutthroat trout is unknown, but is thought to be largely a function of predation in Lake Coeur d’Alene. Northern pike use shallow, vegetated habitats throughout the year and are especially common in those habitats during spring when they are spawning. Similarly, smallmouth bass are common in shallow-water habitats, particularly during the spring. In Lake Coeur d’Alene, shallow-water habitats are almost exclusively located in bays where tributaries enter the lake, and represent areas of high spatial and temporal overlap between northern pike, smallmouth bass, and returning adult and outmigrating juvenile cutthroat trout. Thus, predation on cutthroat trout by northern pike and smallmouth bass may be high and occur over relatively short time periods. As such, the objectives of this project are to: 1) describe the population structure (i.e., size and age structure) and dynamics (i.e., density, growth, mortality) of northern pike; 2) determine the seasonal food habitats of northern pike, and 3) use bioenergetics modeling to evaluate consumption of cutthroat trout by northern pike in Lake Coeur d’Alene.