Paul M. Craig

Objective: Inuit subsistence fishers in Kugluktuk, Nunavut, Canada, have noticed changes in the timing and locations of their catches of Arctic Char Salvelinus alpinus and northern form Dolly Varden S. malma malma (collectively referred to by the community as “char”) in the past ∼20 years. In response, an ongoing research program was formed to study char in partnership with the Kugluktuk Hunters and Trappers Organization. The occurrence of sympatric populations of anadromous Arctic Char and Dolly Varden is rare but was recently identified in Coronation Gulf, Nunavut, where char populations were conventionally understood to be composed of only Arctic Char. To date, no studies have compared use of marine habitats between sympatric anadromous Arctic Char and Dolly Varden. Our objective was to identify high-use areas for each species, provide a broad comparison of habitat use (riverine, estuarine, nearshore marine, offshore island marine, and offshore marine) between species, and assess differences between species in minimum total distance traveled as calculated from observed movements between receiver stations. Methods: Between 2019 and 2023, we used acoustic telemetry to document the marine movements of sympatric anadromous Arctic Char and northern form Dolly Varden. Using network analysis, we investigated and compared the marine movements of individuals from each species that spent ≥28 d in the marine environment. Results: We observed highest use of estuarine and nearshore habitats by both Arctic Char and Dolly Varden. Dolly Varden had significantly larger mean corrected node strength and residency index values at offshore island habitats compared with Arctic Char in 2020, 2021, and 2023. Dolly Varden used the offshore marine environment more than Arctic Char; however, differences were not significant. Median standardized (per hour detected) minimum distances that Arctic Char and Dolly Varden were detected traveling in the marine environment were 107 and 122 m, respectively. Data are provided to enable reanalyses with different marine residency thresholds. Conclusions: Our findings indicate that stewardship of estuarine and nearshore marine habitats will benefit anadromous populations of Arctic Char and Dolly Varden near Kugluktuk. While we found that Dolly Varden had higher use of offshore marine habitats than Arctic Char, our results should be interpreted with caution because of low sample sizes and limited offshore receiver coverage. Our study provides a foundation for future research comparing the habitat use and ecology of sympatric anadromous forms of Arctic Char and Dolly Varden, which may support the development of species-specific restoration and stewardship measures.

Anadromous Arctic char ( Salvelinus alpinus ) and northern form Dolly Varden ( Salvelinus malma malma ) are highly valued by Indigenous communities in northern Canada. Rapid, ongoing environmental changes are affecting populations of both of these iteroparous species, and understanding variability in life‐history tactics is essential for assessment of current and future impacts. The Coppermine River, Nunavut, provides a unique opportunity to study anadromous life‐history types of Dolly Varden and Arctic char that occur in sympatry in fresh water and have documented inter‐ and intra‐specific variability in overwintering locations. We used acoustic telemetry and physical samples from community collection programs to infer that Arctic char overwintered in easily accessible habitats in the lower reaches of the Coppermine River, likely in non‐spawning years, and that spawning likely occurs in rivers other than the Coppermine. Younger and smaller Dolly Varden overwintered in the river below a substantial obstacle (a large cascade), along with some larger adults that we suggest skipped spawning. Larger Dolly Varden, including potential spawners, migrated above the cascade and overwintered in the upper reaches of the Coppermine River. Arctic char exhibited distinct patterns of return migration to fresh water that were consistent with intent to overwinter below the cascade. Dolly Varden that overwintered above the cascade exhibited distinct patterns of return migration that were consistent with directed movements. Patterns of return migration to fresh water for Dolly Varden that overwintered below the cascade suggested some individuals intended to overwinter below the cascade, whereas others may have attempted to overwinter in the upper reaches of the river but failed to ascend the cascade. The patterns and variability we observed in overwintering location and return migration to fresh water have important implications for community‐based management of the local fishery, provide insights into the ecology and adaptability of Arctic char and Dolly Varden, and highlight a need for additional information regarding spawning locations and spawning frequency in this system.

Shifts in migration timing in response to climate change have implications for fitness and survival of cold-adapted species. We used acoustic telemetry to investigate and compare environmental influences on return to fresh water between sympatric anadromous Arctic char ( Salvelinus alpinus) and Dolly Varden ( Salvelinus malma malma), and between overwintering locations (above or below an obstacle—a substantial cascade). Both Arctic char and Dolly Varden that overwintered below the cascade returned to fresh water later in the year, when sea surface temperature was warm and river temperature was cool, whereas Dolly Varden that overwintered above the cascade returned to fresh water earlier in the year, at cooler sea surface temperatures and during warm and likely stressful river temperatures. Ascension of the cascade by Dolly Varden (no Arctic char were observed to ascend the cascade) was likely assisted by high tide but unaffected by other environmental factors, including temperature. Fish demonstrated plasticity in response to inter-annual variation in environmental conditions, but may experience decoupling of environmental cues and suitable migratory conditions, which could impact persistence of these ecologically and culturally important species.

Understanding metabolic responses to temperature elevations is critical for determining how fish populations will be impacted by the increased occurrence of extreme heat events. Here, we characterized the thermal tolerance limits and metabolic functions of three closely related darter species native to the Grand River of Southern Ontario: Fantail darter (Etheostoma flabellare; FTD), Rainbow darter (Etheostoma caeruleum; RBD) and Johnny darter (Etheostoma nigrum; JD). Brain and heart activity of enzymes associated with cellular respiration were analysed for each species at 15°C baseline and following a Critical Thermal Maximum (CTmax) test. Additionally, aerobic scope (AS) was determined for each species while exposed to four heat ramps designed to mimic previously recorded heatwaves. CTmax significantly differed between species with FTD displaying the highest at 33.3°C, JD second at 31.8°C and RBD the lowest at 30.7°C. In darters not exposed to heat stress, FTD possessed higher brain enzymatic activity rates, specifically in pyruvate kinase (PK), citrate synthase (CS) and malate dehydrogenase (MDH). These patterns shifted slightly after exposure to CTmax, with JD displaying a substantial elevation in PK, lactate dehydrogenase, CS and MDH activity, suggesting they had greater enzymatic capacity at temperature extremes. Within heart tissue, we observed no interspecific differences at baseline temperatures; however, RBD had lower enzyme activity than FTD or JD in all enzymes but cytochrome c oxidase following CTmax. Metabolically, FTD exhibited the highest AS following exposure to 10 and 15°C temperature elevations. Our findings demonstrate that FTD may be the best equipped to respond to temperature-induced increases in metabolic demand due to their elevated baseline enzymatic activity and broader AS. These insights may contribute to future darter conservation efforts by informing predictions on species population shifts, particularly in the context of climate change.

Venlafaxine (VFX), a commonly prescribed antidepressant often detected in wastewater effluent, and acute temperature elevations from climate change and increased urbanization, are two environmental stressors currently placing freshwater ecosystems at risk. This study focused on understanding if exposure to VFX impacts the agitation temperature (T _ag ) and critical thermal maximum (CT _max ) of zebrafish ( Danio rerio ). Additionally, we examined the interactive effects of VFX and acute thermal stress on zebrafish heat shock and inflammatory immune responses. A 96 hour 1.0 μg/L VFX exposure experiment was conducted, followed by assessment of thermal tolerance via CT _max challenge. Heat shock proteins and pro-inflammatory immune cytokines were quantified through gene expression analysis by quantitative PCR (qPCR) on hsp 70, hsp 90, hsp 47, il-8, tnfα , and il-1β within gill and liver tissue. No significant changes in agitation temperature between control and exposed fish were observed, nor were there any differences in CT _max based on treatment. Unsurprisingly, hsp 47, 70, and 90 were all upregulated in groups exposed solely to CT _max , while only hsp 47 within gill tissue showed signs of interactive effects, which was significantly decreased in fish exposed to both VFX and CT _max . No induction of an inflammatory response occurred. This study demonstrated that environmentally relevant concentrations of VFX have no impact on thermal tolerance performance in zebrafish. However, VFX is capable of causing diminished function of protective heat shock mechanisms, which could be detrimental to freshwater fish populations and aquatic ecosystems as temperature spikes become more frequent from climate change and urbanization near watersheds. Summary Statement: This study predicts the effects that climate change and anthropogenic pollutants may have on fish ability to tolerate elevated temperatures, and examines the physiologic challenges these stressors may introduce.

The rise in presence of pharmaceuticals in wastewater effluent has had behavioural effects on fish, which this study aimed to investigate.⁴ The pharmaceutical studied was venlafaxine (VEN), a commonly prescribed antidepressant.⁴ Previous studies have shown that venlafaxine contributes to oxidative stress.³ Notably, oxidative stress has been linked to neurodegeneration and plays a role in the pathogenesis of neurodegenerative diseases, such as Alzheimer’s, and Parkinson’s in humans.¹ This study applied a novel approach through in vitro methods to directly expose zebrafish brains to varying VEN concentrations (0.01, 0.1, and 1 μg/L) and quantify oxidative stress. Zebrafish brains were used due to their neuroanatomical similarity to humans.¹⁴ Results demonstrated decreases in cell viability, and increases in antioxidant enzyme activity (catalase and superoxide dismutase) with rising concentrations of VEN. These findings demonstrate increases in oxidative stress, which was indirectly measured by antioxidant enzyme activity, as well as the effects of VEN on cell viability and protein amounts in the brain cells. These results indicate a possible link between venlafaxine exposure and increased risk of neurodegeneration in humans. Further investigation should be done to examine the association between venlafaxine concentrations in water and increased incidence of neurodegenerative disease in mammalian brains.