Introductions or invasions of nonnative organisms can mediate major changes in the trophic structure of aquatic ecosystems. Here we document multitrophic level impacts in a spatially extensive system that played out over more than a century. Positive interactions among exotic vertebrate and invertebrate predators caused a substantial and abrupt shift in community composition resulting in a trophic cascade that extended to primary producers and to a nonaquatic species, the bald eagle. The opossum shrimp, Mysis diluviana, invaded Flathead Lake, Montana, the largest freshwater lake in the western United States. Lake trout had been introduced 80 y prior but remained at low densities until nonnative Mysis became established. The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout by providing a deep water source of food where little was available previously. Lake trout subsequently flourished on mysids and this voracious piscivore now dominates the lake fishery; formerly abundant kokanee were extirpated, and native bull and westslope cutthroat trout are imperiled. Predation by Mysis shifted zooplankton and phytoplankton community size structure. Bayesian change point analysis of primary productivity (27-y time series) showed a significant step increase of 55 mg C m −2 d −1 (i.e., 21% rise) concurrent with the mysid invasion, but little trend before or after despite increasing nutrient loading. Mysis facilitated predation by lake trout and indirectly caused the collapse of kokanee, redirecting energy flow through the ecosystem that would otherwise have been available to other top predators (bald eagles).

We found no relationship between fish growth rate and mercury concentration in a lake trout population and conditional support for an inverse relationship in a smallmouth bass population. A bioenergetics model indicated that mercury concentration was more responsive to dietary mercury intake than to growth rate. When biodilution is evident, it may bias contaminant versus fish size relationships.

To assess mercury contamination of fish in Maine, fish were collected from 120 randomly selected lakes. The collection goal for each lake was five fish of the single most common sport fish species within the si/e range commonly harvested by anglers. Skinless, boneless fillets of fish from each lake were composited, homogenized, and analyzed for total mercury. The two most abundant species, brook trout Salvelinusfontinalis and smallmouth bass Micropterusdolomieu. were also analyzed individually. The composite fish analyses indicate high concentrations of mercury, particularly in large and long-lived nonsalmonid species. Chain pickerel Esox niger, smallmouth bass, largemouth bass Micropterus salmoides, and white perch Morone aniericana had the highest average mercury concentrations, and brook trout and yellow perch Perca flavescens had the lowest. The mean species composite mercury concentration was positively correlated with a factor incorporating the average size and age of the fish. Lakes containing fish with high mercury concentrations were not clustered near known industrial or population centers but were commonest in the area within 150 km of the seacoast. reflecting the geographical distribution of species that contained higher mercury concentrations. Stocked and wild brook trout were not different in length or weight, but wild fish were older and had higher mercury concentrations. Fish populations maintained by frequent introductions of hatchery-produced fish and subject to high angler exploitation rates may consist of younger fish with lower exposure to environmental mercury and thus contain lower concentrations than wild populations.

Mercury levels in lake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis, and benthic invertebrates were investigated in Flathead Lake, Montana. For both fish species, mercury increased with size and age and showed a negative relationship with growth rate. No gender‐based differences in mercury levels were observed for either lake trout or lake whitefish. A positive relationship between mercury concentration and depth was documented for lake trout and the pooled invertebrate sample, suggesting that individual lake trout have some long‐term habitat preferences. In general, these findings underscore the need to consider biological attributes of organisms when conducting contaminant assessments and illustrate the usefulness of contaminants as food web tracers.

The establishment of the opossum shrimp Mysis relicta in Flathead Lake, Montana, was followed by a dramatic increase in the population of lake trout Salvelinus namaycush. Using otoliths, we compared lake trout growth before and after the arrival of M. relicta. The widths of annual increments 1-3 were similar, but width generally declined in increments 4-10 after the establishment of M. relicta. To calculate body size in the post-M. relicta fish, we used the relationship between body length and otolith radius. To retrospectively calculate body length for the pre-M. relicta fish (for which we lacked body sizes), we developed a simple empirical correction to account for the conservative nature of otolith growth rate relative to body growth rate.