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Municipal wastewaters are a complex mixture containing estrogens and estrogen mimics that are known to affect the reproductive health of wild fishes. Male fishes downstream of some wastewater outfalls produce vitellogenin (VTG) (a protein normally synthesized by females during oocyte maturation) and early-stage eggs in their testes, and this feminization has been attributed to the presence of estrogenic substances such as natural estrogens [estrone or 17β-estradiol (E2)], the synthetic estrogen used in birth-control pills [17α-ethynylestradiol (EE2)], or weaker estrogen mimics such as nonylphenol in the water. Despite widespread evidence that male fishes are being feminized, it is not known whether these low-level, chronic exposures adversely impact the sustainability of wild populations. We conducted a 7-year, whole-lake experiment at the Experimental Lakes Area (ELA) in northwestern Ontario, Canada, and showed that chronic exposure of fathead minnow (Pimephales promelas) to low concentrations (5-6 ng·L⁻¹) of the potent 17α-ethynylestradiol led to feminization of males through the production of vitellogenin mRNA and protein, impacts on gonadal development as evidenced by intersex in males and altered oogenesis in females, and, ultimately, a near extinction of this species from the lake. Our observations demonstrate that the concentrations of estrogens and their mimics observed in freshwaters can impact the sustainability of wild fish populations.

Climate change is anticipated to have multiple consequences for aquatic ectotherms. Warming temperatures, for example, are predicted to reduce body size in many fish species. Smaller sizes may be caused by physiological constraints associated with respiration, life-history responses to faster growth and concomitant earlier age at maturity, and interactive effects of fishing and climate change on evolution. Here, using a phylogenetically broad dataset of 100 marine species, we explore how natural mortality might respond to a 10% reduction in asymptotic length (L∞). We find that this decrease in size (predicted to be associated with a 1 °C ocean temperature increase) is likely to exacerbate natural mortality (M) for most marine fishes, albeit not all, on Canada’s Scotian Shelf. Across all bony fishes (Actinopterygii), the median proportional increase in natural mortality is 10.5%; limited data suggest that chondrichthyans are less affected. Smaller-bodied fish species experienced greater absolute increases in M than larger-bodied species. Among commercially exploited species for which sufficient data are available, M is predicted to increase by 0.14 for Atlantic mackerel (Scomber scombrus), 0.08 for Silver hake (Merluccius bilinearis), 0.04 for Atlantic herring (Clupea harengus), and 0.02 for Atlantic cod (Gadus morhua) and Pollock (Pollachias virens). The present study offers a simple means of exploring the mortality consequences of reduced body size hypothesized to result from globally warming water temperatures, thus contributing a potential tool for climate-change vulnerability assessments of marine fishes.

Background In lake ecosystems, predatory fish can move and forage across both nearshore and offshore habitats. This coupling of sub-habitats, which is important in stabilizing lake food webs, has largely been assessed from a dietary perspective and has not included movement data. As such, empirical estimates of the seasonal dynamics of these coupling movements by fish are rarely quantified, especially for northern lakes. Here we collect fine-scale fish movement data on Lake Trout ( Salvelinus namaycush ), a predatory cold-water fish known to link nearshore and offshore habitats, to test for seasonal drivers of activity, habitat use and diet in a subarctic lake. Methods We used an acoustic telemetry positioning array to track the depth and spatial movements of 43 Lake Trout in a subarctic lake over two years. From these data we estimated seasonal 50% home ranges, movements rates, tail beat activity, depth use, and nearshore habitat use. Additionally, we examined stomach contents to quantify seasonal diet. Data from water temperature and light loggers were used to monitor abiotic lake conditions and compare to telemetry data. Results Lake Trout showed repeatable seasonal patterns of nearshore habitat use that peaked each spring and fall, were lower throughout the long winter, and least in summer when this habitat was above preferred temperatures. Stomach content data showed that Lake Trout acquired the most nearshore prey during the brief spring season, followed by fall, and winter, supporting telemetry results. Activity rates were highest in spring when feeding on invertebrates and least in summer when foraging offshore, presumably on large-bodied prey fish. High rates of nearshore activity in fall were associated with spawning. Nearshore habitat use was widespread and not localized to specific regions of the lake, although there was high overlap of winter nearshore core areas between years. Conclusions We provide empirical demonstrations of the seasonal extent to which a mobile top predator links nearshore and offshore habitats in a subarctic lake. Our findings suggest that the nearshore is an important foraging area for Lake Trout for much of the year, and the role of this zone for feeding should be considered in addition to its traditional importance as spawning habitat.

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

There is a pressing need to understand how ecosystems will respond to climate change. To date, no long-term empirical studies have confirmed that fish populations exhibit adaptive foraging behavior in response to temperature variation and the potential implications this has on fitness. Here, we use an unparalleled 11-y acoustic telemetry, stable isotope, and mark–recapture dataset to test if a population of lake trout (Salvelinus namaycush), a coldwater stenotherm, adjusted its use of habitat and energy sources in response to annual variations in lake temperatures during the open-water season and how these changes translated to the growth and condition of individual fish. We found that climate influenced access to littoral regions in spring (data from telemetry), which in turn influenced energy acquisition (data from isotopes), and growth (mark–recapture data). In more stressful years, those with shorter springs and longer summers, lake trout had reduced access to littoral habitat and assimilated less littoral energy, resulting in reduced growth and condition. Annual variation in prey abundance influenced lake trout foraging tactics (i.e., the balance of the number and duration of forays) but not the overall time spent in littoral regions. Lake trout greatly reduced their use of littoral habitat and occupied deep pelagic waters during the summer. Together, our results provide clear evidence that climate-mediated behavior can influence the dominant energy pathways of top predators, with implications ranging from individual fitness to food web stability.

Background Mature walleye ( Sander vitreus ) that spawn in the Trent River conduct long-distance annual migrations into eastern Lake Ontario that begin and end in the Bay of Quinte. This scale of movement likely reflects seasonal spawning activity in the spring and a combination of temperature and foraging preferences at other times of the year. This study used a combination of acoustic transmitters and pop-off data storage tags to collect high-resolution data on temperature, depth, rate of vertical movement (ROVM), and rate of horizontal movement (ROHM) during these migrations. We tested the theory that post-spawn fish migrating to Lake Ontario experience colder water temperatures than those remaining in the upper Bay of Quinte, and offset this cost with greater foraging, as indicated by ROVM. We also documented the trends in these variables seasonally at the daily and hourly level. Results Temperature experienced by walleye in the lake (11.56 °C; SE ± 0.1) was on average 5.33 °C colder than in the upper bay (16.89 °C; SE ± 0.3), and there was a 15.5% increase in ROVM for fish in the lake. All the measured variables had significant seasonal trends, while only temperature, depth, and ROVM had significant hour of day trends. Sex based differences were limited to males having greater annual ROVM than females. Conclusions There were differences in thermal habitat selection and vertical activity measures between the upper bay and Lake Ontario, which supported the current conceptual model of post-spawn walleye migration from the Bay of Quinte. Vertical activity peaked during crepuscular periods during the summer and fall when water temperatures promoted growth. This study demonstrates the value of combining tagging techniques to collect high-resolution data across multiple aspects of annual fish migrations.

Background In large, interconnected systems such as the Laurentian Great Lakes, little is known about how smallmouth bass ( Micropterus dolomieu ) activity varies across contrasting habitat types. As an important and widespread predator in southern Boreal waterbodies, understanding how smallmouth bass allocate energy toward movement is critical for linking behaviour to ecosystem processes, particularly when habitat types differ in the energetic costs they impose. Eastern Lake Ontario and the St. Lawrence River form a natural flow gradient across a large geographic region and are supported by an extensive acoustic telemetry network, providing a unique opportunity to quantify how contrasting habitats influence the behaviour of free-swimming fish across diel, seasonal, and spatial scales. To address these knowledge gaps, we used acoustic telemetry and accelerometer data to investigate how smallmouth bass activity patterns and metabolic rate vary across this flow-driven habitat gradient within the Laurentian Great Lakes-St. Lawrence Seaway system. Results We modelled seasonal and diel activity patterns, estimated space use during high- and low-activity states, and applied established metabolic calibration curves to predict field-based metabolic rate at three distinct sites. Smallmouth bass exhibited strong, location-specific activity patterns across seasonal and diel cycles that were broadly consistent with predicted seasonal changes in metabolic rate. Diel models showed elevated daytime activity, with evidence of crepuscular behaviour, particularly during summer months. Spatial analyses revealed extensive overlap between high- and low-activity states. Smallmouth bass occupying the non-flowing habitats of Lake Ontario typically incurred higher metabolic rates than those residing in the St. Lawrence River for much of the year. Conclusions This study quantified how the activity and metabolic rate of smallmouth bass can vary between distinct habitats within large, heterogeneous ecosystems. By integrating multi-year behavioural and physiological data, these findings provide new insights into how environmental variables can influence the behaviour of native smallmouth bass.

Background Over the last decade, pharmaceutical pollution in aquatic ecosystems has emerged as a pressing environmental issue. Recent years have also seen a surge in scientific interest in the use of behavioural endpoints in chemical risk assessment and regulatory activities, underscoring their importance for fitness and survival. In this respect, data on how pharmaceuticals alter the behaviour of aquatic animals appears to have grown rapidly. Despite this, there has been a notable absence of systematic efforts to consolidate and summarise this field of study. To address this, our objectives were twofold: (1) to systematically identify, catalogue, and synthesise primary research articles on the effects of pharmaceuticals on aquatic animal behaviour; and (2) to organise this information into a comprehensive open-access database for scientists, policymakers, and environmental managers. Methods We systematically searched two electronic databases (Web of Science and Scopus) and supplemented these with additional article sources. The search string followed a Population–Exposure–Comparison–Outcome framework to capture articles that used an aquatic organism (population) to test the effects of a pharmaceutical (exposure) on behaviour (outcome). Articles were screened in two stages: title and abstract, followed by full-text screening alongside data extraction. Decision trees were designed a priori to appraise eligibility at both stages. Information on study validity was collected but not used as a basis for inclusion. Data synthesis focused on species, compounds, behaviour, and quality themes and was enhanced with additional sources of metadata from online databases (e.g. National Center for Biotechnology Information (NCBI) Taxonomy, PubChem, and IUCN Red List of Threatened Species). Review findings We screened 5,988 articles, of which 901 were included in the final database, representing 1,739 unique species-by-compound combinations. The database includes data collected over 48 years (1974–2022), with most articles having an environmental focus (510) and fewer relating to medical and basic research topics (233 and 158, respectively). The database includes 173 species (8 phyla and 21 classes). Ray-finned fishes were by far the most common clade (75% of the evidence base), and most studies focused on freshwater compared to marine species (80.4% versus 19.6%). The database includes 426 pharmaceutical compounds; the most common groups were antidepressants (28%), antiepileptics (11%), and anxiolytics (10%). Evidence for the impacts on locomotion and boldness/anxiety behaviours were most commonly assessed. Almost all behaviours were scored in a laboratory setting, with only 0.5% measured under field conditions. Generally, we detected poor reporting and/or compliance with several of our study validity criteria. Conclusions Our systematic map revealed a rapid increase in this research area over the past 15 years. We highlight multiple areas now suitable for quantitative synthesis and areas where evidence is lacking. We also highlight some pitfalls in method reporting and practice. More detailed reporting would facilitate the use of behavioural endpoints in aquatic toxicology studies, chemical risk assessment, regulatory management activities, and improve replicability. The EIPAAB database can be used as a tool for closing these knowledge and methodological gaps in the future.

Endocrine-disrupting chemicals (EDCs) in municipal effluents directly affect the sexual development and reproductive success of fishes, but indirect effects on invertebrate prey or fish predators through reduced predation or prey availability, respectively, are unknown. At the Experimental Lakes Area in northwestern Ontario, Canada, a long-term, whole-lake experiment was conducted using a before-after-control-impact design to determine both direct and indirect effects of the synthetic oestrogen used in the birth control pill, 17α-ethynyloestradiol (EE2). Algal, microbial, zooplankton and benthic invertebrate communities showed no declines in abundance during three summers of EE2 additions (5–6 ng l−1), indicating no direct toxic effects. Recruitment of fathead minnow (Pimephales promelas) failed, leading to a near-extirpation of this species both 2 years during (young-of-year, YOY) and 2 years following (adults and YOY) EE2 additions. Body condition of male lake trout (Salvelinus namaycush) and male and female white sucker (Catostomus commersonii) declined before changes in prey abundance, suggesting direct effects of EE2 on this endpoint. Evidence of indirect effects of EE2 was also observed. Increases in zooplankton, Chaoborus, and emerging insects were observed after 2 or 3 years of EE2 additions, strongly suggesting indirect effects mediated through the reduced abundance of several small-bodied fishes. Biomass of top predator lake trout declined by 23–42% during and after EE2 additions, most probably an indirect effect from the loss of its prey species, the fathead minnow and slimy sculpin (Cottus cognatus). Our results demonstrate that small-scale studies focusing solely on direct effects are likely to underestimate the true environmental impacts of oestrogens in municipal wastewaters and provide further evidence of the value of whole-ecosystem experiments for understanding indirect effects of EDCs and other aquatic stressors.

The littoral zone of lakes consists of a mosaic of habitat types, which support fish communities that contribute greatly to biodiversity and food web stability. Yet, relationships between habitat and littoral fish abundance, species composition, and the extent of movement among habitats are not well understood. We carried out a habitat-specific mark-recapture study of small littoral fishes in two small boreal lakes in central Canada. We found that the abundance of littoral fishes was lowest in the habitat type with the least amount of structural complexity, while areas containing coarse woody material were associated with high fish abundance and biomass. We found little difference in species composition among habitat types, although smaller individuals tended to prefer habitats with high structural complexity. Mark-recapture data provided evidence for fidelity to the initial marking habitat type, though this was higher in the lake with more open habitat areas, which may have acted as barriers to movement. Our data also indicated that these small-bodied species can move appreciable distances (up to ~ 1 km) throughout the nearshore zone. This research suggests that maintaining structurally complex habitats is important for littoral zone fish productivity by supporting growth and survival, and by facilitating movement among habitat patches.