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A fundamental assumption of biotelemetry studies is that there are no adverse consequences from the surgical implantation or presence of the acoustic transmitter. In fisheries, most studies have evaluated this assumption over only short time periods (

Significant advances have been made towards surveying animal and plant communities using DNA isolated from environmental samples. Despite rapid progress, we lack a comprehensive understanding of the "ecology" of environmental DNA (eDNA), particularly its temporal and spatial distribution and how this is shaped by abiotic and biotic processes. Here, we tested how seasonal variation in thermal stratification and animal habitat preferences influences the distribution of eDNA in lakes. We sampled eDNA depth profiles of five dimictic lakes during both summer stratification and autumn turnover, each containing warm- and cool-water fishes as well as the cold-water stenotherm, lake trout (Salvelinus namaycush). Habitat use by S. namaycush was validated by acoustic telemetry and was significantly related to eDNA distribution during stratification. Fish eDNA became "stratified" into layers during summer months, reflecting lake stratification and the thermal niches of the species. During summer months, S. namaycush, which rarely ventured into shallow waters, could only be detected at the deepest layers of the lakes, whereas the eDNA of warm-water fishes was much more abundant above the thermocline. By contrast, during autumn lake turnover, the fish species assemblage as detected by eDNA was homogenous throughout the water column. These findings contribute to our overall understanding of the "ecology" of eDNA within lake ecosystems, illustrating how the strong interaction between seasonal thermal structure in lakes and thermal niches of species on very localized spatial scales influences our ability to detect species.

Construction of artificial channels to divert water is common in a variety of natural resource development projects; however, the length of time required for these stream channels to become productive fish habitat remains an understudied aspect. The creation of a bedrock channel (∼150 m) to drain a third-order boreal lake and its watershed (∼300 ha) offered the unique opportunity to study colonization by comparing habitat and invertebrate metrics with a reference stream. The amount of riparian vegetation on the banks of the diversion channel steadily increased, but remained much lower than the reference stream after 5 years. The channel was quickly colonized by benthic macroinvertebrates, which were of comparable abundance to the reference stream starting in the first year, and thereafter were greater in abundance. Taxa diversity and richness responded more slowly, becoming similar to the reference stream after 3 years. Results from this study suggest that newly created, lake-outlet channels can become productive small stream habitats in a relatively short time period (

Climate-driven declines in oxythermal habitat in freshwater lakes can impose prolonged constraints on cold-water fishes sensitive to hypoxia. How fish cope with severe habitat limitations is not well understood, yet has implications for their persistence. Here, we use acoustic-positioning telemetry to assess seasonal habitat occupancy and activity patterns of lake whitefish (Coregonus clupeaformis), a cold-water benthivore, in a small boreal lake that regularly faces severe oxythermal constraints during summer stratification. During this stratified period, they rarely ( 10 °C (interquartile range = 5.3–7.9 °C), which resulted in extensive use (> 90% of detections) of water with −1 dissolved oxygen (DO; interquartile range = 0.3–5.3 mg L−1). Lake whitefish were least active in winter and spring, but much more active in summer, when only a small portion of the lake (1–10%) contained optimal oxythermal habitat ( 4 mg L−1 DO), showing frequent vertical forays into low DO environments concurrent with extensive lateral movement (7649 m d−1). High rates of lateral movement (8392 m d−1) persisted in the complete absence of optimal oxythermal habitat, but without high rates of vertical forays. We found evidence that lake whitefish are more tolerant of hypoxia (−1) than previously understood, with some individuals routinely occupying hypoxic habitat in winter (up to 93% of detections) despite the availability of higher DO habitat. The changes in movement patterns across the gradient of habitat availability indicate that the behavioural responses of lake whitefish to unfavourable conditions may lead to changes in foraging efficiency and exposure to physiological stress, with detrimental effects on their persistence.

Anthropogenic releases of mercury (Hg)1–3 are a human health issue4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans5,6. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg7–9. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those in the watershed remained there. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38–76% of MeHg concentration in large-bodied fish populations in eight years. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers., In a 15-year whole-ecosystem, single-factor experiment, stopping experimental mercury loading results in rapid decreases in methylmercury contamination of fish populations and almost complete recovery within the timeframe of the study.

Aquaculture is a growing global industry; freshwater aquaculture has significant potential for expansion in Canada, but growth of the freshwater sector has been slow due to concerns over potential environmental impacts and a lack of information on potential impacts to native fish communities. To provide guidelines and target variables for evaluating aquaculture impacts on freshwater fish communities, we operated an experimental aquaculture farm as a whole‐lake experiment where 10,000 rainbow trout (Oncorhynchus mykiss) were raised annually from 2003 to 2007. Impacts were assessed using up to 8 years of pre‐impact and 8–10 years of post‐impact data. Prey fish abundance increased dramatically during aquaculture but declined sharply following the experiment. High abundance of littoral minnows in autumn was not observed during spring and, combined with size distribution data, suggests high overwinter mortality of adult minnows. White sucker (Catostomus commersonii) abundance and body condition declined during and after aquaculture, with evidence of overwinter juvenile recruitment failure in the last 2 years of operation, although size‐at‐age increased. The adult abundance of lake trout (Salvelinus namaycush) doubled during aquaculture, due to a combination of (a) increased growth rates of young trout and (b) earlier age and larger sizes at maturation. Within 2 years following aquaculture, lake trout abundance declined by nearly 50% to background levels, suggesting a large increase in lake trout mortality once operations ceased. These changes were not observed in nearby reference lakes. While aquaculture appeared to benefit some species (slimy sculpin [Cottus cognatus], minnows, lake trout), prolonged declines in white sucker abundance and condition and continued depression of Mysis densities and optimal oxythermal habitat availability nearly a decade following operations suggest potentially long‐term impacts at this magnitude. Importantly, this experiment highlights important indicator species and life history traits for monitoring of freshwater aquaculture impacts on native fish communities.

We examined the effect of sex on mercury elimination in fish by monitoring isotope-enriched mercury concentrations in the muscle tissue of three adult female and three adult male northern pike Esox lucius, which had accumulated the isotope-enriched mercury via a whole-lake manipulation and were subsequently moved to a clean lake. Mercury elimination rates for female and male northern pike were estimated to be 0.00034 and 0.00073 day(-1), respectively. Thus, males were capable of eliminating mercury at more than double the rate than that of females. To the best of our knowledge, our study represents the first documentation of mercury elimination rates varying between the sexes of fish. This sex difference in elimination rates should be taken into account when comparing mercury accumulation between the sexes of fish from the same population. Further, our findings should eventually lead to an improved understanding of mechanisms responsible for mercury elimination in vertebrates.

It is well accepted that the majority of monomethylmercury (MMHg) in fish originates in their food; however, the additional contribution of water as a source to fish MMHg levels remains unclear. We used isotope enriched mercury (Hg) in a controlled field experiment to quantify the uptake of Hg from ingested and aqueous sources by young-of-year yellow perch (Perca flavescens). Water and zooplankton from a lake that had received (202)Hg-enriched additions (called spike Hg) for 7 y during a whole-ecosystem loading study (METAALICUS) provided natural, low-level Hg exposure. We achieved separation of exposure pathways by housing perch in one of four treatments: clean water + clean food; clean water + Hg spiked food; Hg spiked water + clean food; Hg spiked water + Hg spiked food. Fish accumulated MMHg directly from water, and this source accounted for at least 10% of MMHg in fish during the 27-d trial. Accumulation of spike Hg from water and food was additive, with food providing the majority of spike MMHg taken in by fish. Predictions from a bioenergetics model that excludes water as a source underestimated Hg in perch by 11%. This study illustrates the importance of acknowledging both food and water as sources of Hg to fish and suggests that aqueous Hg should be included as a source of contamination in bioaccumulation models and experiments.

Background It has been stated that there is a certain amount of intrinsic error inherent in all remote sensing methods, including acoustic telemetry, which has gained popularity in both freshwater and marine environments to record fine-scale movements over small spatial scales. We performed stationary tag trials on three freshwater lakes where we placed transmitters at known locations around the lakes and used radio-acoustic positioning and telemetry (RAPT) system-derived location data to assess the measurement and systematic biases of the system. We used a geostatistical technique called ordinary kriging to deal with the systematic errors and a state-space model to represent the measurement error of the data. Furthermore, we applied the kriging correction and a continuous-time correlated random walk model in a state-space framework to predict locations of a lake trout. Results The stationary tagging trials produced a complex pattern of spatial error within each lake that could not properly be accounted for by a simple filtering process. Using fivefold cross-validation, positioning error was reduced from 93 to 99 % in three small lakes. We also identified tag depth as a potential source of measurement error. The application of a state-space model resulted in the contraction of home ranges of lake trout by 10–32 % and a 3–32 % reduction in total distance travelled. Conclusions Our results indicate that the systematic biases were a greater source of error than the measurement errors using a RAPT system. Consequently, the addition of a state-space model had relatively little effect on the quality of the spatial correction compared with the kriging method. The kriging method was able to compensate for the systematic biases produced by the RAPT systems and in turn increased the quality of data returned.