Behavioural responses of a cold-water benthivore to loss of oxythermal habitat.

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 (< 15% of detections) occupied depths with water temperatures > 10 °C (interquartile range = 5.3-7.9 °C), which resulted in extensive use (> 90% of detections) of water with < 4 mg L-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 (< 10 °C and > 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 (< 2 mg L-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. Supplementary Information: The online version contains supplementary material available at 10.1007/s10641-022-01335-4.

Experimental evidence for recovery of mercury-contaminated fish populations.

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.

Thermal stratification and fish thermal preference explain vertical eDNA distributions in lakes.

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.

Mercury elimination rates for adult northern pike Esox lucius: evidence for a sex effect.

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.

Mercury elimination by a top predator, Esox lucius.

Top-level piscivores are highly sought after for consumption in freshwater fisheries, yet these species contain the highest levels of the neurotoxin monomethylmercury (MMHg) and therefore present the greatest concern for MMHg exposure to humans. The slow elimination of MMHg is one factor that contributes to high levels of this contaminant in fish; however, little quantitative information exists on elimination rates by top predators in nature. We determined rates of MMHg elimination in northern pike (Esox lucius) by transferring fish that had naturally accumulated isotope-enriched MMHg (spike MMHg) through a whole-lake Hg loading study to a different lake. Over a period of ~7 y, pike were periodically recaptured and a small amount of muscle tissue was extracted using a nonlethal biopsy. Spike total mercury (THg) persisted in muscle tissue throughout the entire study despite discontinuing exposure upon transfer to the new lake. Spike THg burdens increased for the first ~460 d, followed by a decline to 65% of original burden levels over the next 200 d, and subsequently reached a plateau near original burden levels for the remainder of the study. We estimated the half-life of muscle THg to be 3.3 y (1193 d), roughly 1.2- to 2.7-fold slower than predicted by current elimination models. We advocate for further long-term field studies that examine kinetics of MMHg in fish to better inform predictive models estimating the recovery of MMHg-contaminated fisheries.

Dietary and waterborne mercury accumulation by yellow perch: a field experiment.

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.