Kokanee Oncorhynchus nerka and Lake Trout Salvelinus namaycush are stocked for sportfishing in lakes and reservoirs throughout the western United States and Canada. However, where the two species co‐occur, unsustainable levels of predation by Lake Trout can lead to declines in kokanee abundance and declines in Lake Trout growth and body condition. Such declines occurred in Blue Mesa Reservoir, Colorado. In 2009, managers began removing Lake Trout (<740 mm TL) in an attempt to sustain the hatchery‐dependent kokanee population while still providing a trophy Lake Trout fishery. To evaluate this and other strategies for achieving the dual management goals, we developed age‐structured kokanee and Lake Trout population models and linked them to a bioenergetics model of Lake Trout predation. We found that the existing level of Lake Trout removal (0.23; ages 4–9) is insufficient to prevent further decline and ultimately the extirpation of the kokanee population. If removal of age‐4–9 Lake Trout is intensified to 0.38, the kokanee population would persist; however, removal would have to be increased to 0.63 to allow a return to historic kokanee abundance. Focusing removal on age‐4 Lake Trout (0.78) would allow for persistence of kokanee and would leave more trophy Lake Trout for anglers, suggesting that the two goals are compatible under some circumstances. However, management costs of balancing kokanee with trophy Lake Trout are high and put both fisheries at risk unless Lake Trout abundance is controlled. Received October 31, 2013; accepted May 5, 2014

Energy density (ED) is an indicator of fish nutritional status, physiological status, and fitness. Estimates of ED of predators and prey are also needed for bioenergetics modeling, but direct measurements of ED are difficult to obtain. Hence, investigators often borrow published values from the same species or related species. Alternatively, models have been developed that predict ED from dry matter content (DM). The scarcity of published ED values makes data borrowing and the generality of predictive modeling difficult to evaluate. We report new ED measurements derived from bomb calorimetry for six freshwater fish species (Arctic Char Salvelinus alpinus, Brown Trout Salmo trutta, kokanee Oncorhynchus nerka, Lake Trout Salvelinus namaycush, Rainbow Trout O. mykiss, and White Sucker Catostomus commersonii) and compared them to previously published data. We used our data to validate existing ED:DM models and to fit new ones. We also quantified bias that could result from borrowing inappropriate ED values for bioenergetics modeling. We collected a range of fish sizes from two reservoirs of differing productivity and measured whole‐body ED (wet‐mass basis) and DM. Our data substantially increase the range of ED data for Arctic Char, kokanee, and White Suckers. Two multispecies ED:DM models predicted the ED of our samples accurately (mean root mean square error [RMSE] < 500 J/g), even at the extremes for the range of prediction where data used for model development were limited. Taxon‐specific models performed less well (mean RMSE = 775 J/g), and some appeared highly biased (RMSE 956−1,900 J/g). Bioenergetics model simulations showed that Lake Trout prey consumption could be overestimated by as much as 22% when using borrowed EDs for Lake Trout and their prey, but prey consumption estimates fell within ±2% of observed when ED was predicted from DM. When direct measures of ED are unattainable, measurement of DM and prediction of ED from one of the published multispecies models offer a practical and accurate method for bioenergetics modeling and other studies requiring information on fish energy content.

Altering food web structure has been shown to influence mercury (Hg) concentrations in sport fish. Here, we describe a whole-system manipulation designed to assess the effectiveness of stocking relatively high-quality, low-Hg prey (rainbow trout, Oncorhynchus mykiss ) as a means of increasing northern pike ( Esox lucius ) growth to reduce Hg concentrations. A replicated pond experiment served as a reference for the lake experiment and provided information to parameterize bioenergetics simulations. Results indicate that stocking relatively high-quality, low-Hg prey is a rapid and effective method to reduce sport fish Hg concentrations by up to 50% through an increase in individual northern pike biomass. Large northern pike, the fish that tend to be the most contaminated, were affected most by the manipulation. The observed declines in northern pike Hg concentrations indicate that stocking might be used to reduce Hg concentrations in sport fish prior to harvest. However, after 1 year, northern pike Hg concentrations rebounded, suggesting that reductions would be temporary without continuous stocking. Thus, perhaps the most effective method of perpetually reducing sport fish Hg concentrations would be to manage for the development of a naturally reproducing forage fish population with relatively high energy content and low Hg concentrations.

The spread of Mysis diluviana, a small glacial relict crustacean, outside its native range has led to unintended shifts in the composition of native fish communities throughout western North America. As a result, biologists seek accurate methods of determining the presence of M. diluviana, especially at low densities or during the initial stages of an invasion. Environmental DNA (eDNA) provides one solution for detecting M. diluviana, but building eDNA markers that are both sensitive and species-specific is challenging when the distribution and taxonomy of closely related non-target taxa are poorly understood, published genetic data are sparse, and tissue samples are difficult to obtain. To address these issues, we developed a pair of independent eDNA markers to increase the likelihood of a positive detection of M. diluviana when present and reduce the probability of false positive detections from closely related non-target species. Because tissue samples of closely-related and possibly sympatric, non-target taxa could not be obtained, we used synthetic DNA sequences of closely related non-target species to test the specificity of eDNA markers. Both eDNA markers yielded positive detections from five waterbodies where M. diluviana was known to be present, and no detections in five others where this species was thought to be absent. Daytime samples from varying depths in one waterbody occupied by M. diluviana demonstrated that samples near the lake bottom produced 5 to more than 300 times as many eDNA copies as samples taken at other depths, but all samples tested positive regardless of depth.

The species-dependencies and variability of freezing effects on aquatic organisms suggest that more research is needed to understand the mechanisms responsible for freezing-related fractionation before standardized protocols for freezing as a preservation method can be adopted.