In systems with multiple piscivores, co-occurrence is dependent on resource partitioning. This is pronounced in oligotrophic northern lakes, which have simple food webs and short open-water seasons. We used acoustic telemetry and stable isotopes to quantify habitat and dietary partitioning during thermal stratification among three piscivores that commonly co-occur in Canadian sub-Arctic lakes-burbot (Lota lota), lake trout (Salvelinus namaycush), and northern pike (Esox lucius). Spatial core areas and core habitat niches (space and depth) did not significantly overlap among species. Although burbot and lake trout occupied similar mean daily depths (16.2 m and 13.4 m, respectively), and water temperatures (5.4°C and 6.9°C, respectively), they were spatially segregated. Burbot were closely associated with the lake bottom on steep drop-offs between the offshore and nearshore zone with moderate substrate complexity, whereas lake trout were located over deep offshore basins and suspended above the lake bottom. Northern pike occupied shallow depths (5.3 m) and warmer water (16.5°C) within the nearshore region and were closely associated with bottom substrate of highest complexity. Some significant overlap among spatial home ranges and broad habitat niches indicated that these species interact. However, dietary niches did not significantly overlap at either the core or broad levels, suggesting that species were utilizing spatially diverse food sources. Our results highlight the importance of including depth and space when quantifying resource partitioning among fishes and provide insight into the mechanisms that promote piscivore co-occurrence in northern lakes.

The diel vertical distribution of young-of-the-year (YOY) burbot Lota lota in the pelagic zone of Lake Constance was compared to light intensity at the surface and to the light intensity at their mean depth. Lota lota larvae inhabited the pelagic zone of Lake Constance from the beginning of May until the end of August. From early June, after the stratification of the water column, fish performed diel vertical migrations (DVM) between the hypolimnion and epilimnion. The amplitude of DVM increased constantly during the summer and reached 70 m by the end of August. Lota lota started their ascent to the surface after sunset and descended into the hypolimnion after sunrise. As the YOY fish grew from May to August, they experienced decreasing diel maximum light intensities: in May and early June L. lota spent the day at light intensities >40 W m À2 , but they never experienced light intensities >0Á1 W m À2 after the end of June. From this time, L. lota experienced the brightest light intensities during dusk and dawn, suggesting feeding opportunities at crepuscular hours. The present study implies, that YOY L. lota in the pelagic zone of Lake Constance increased their DVM amplitude during the summer to counteract a perceived predation risk related to body size and pigmentation.

To improve release techniques of Japanese flounder, Paralichthys olivaceus, for the purpose of stock enhancement, the substrate preference and burial behaviour over a range of sand grain sizes were investigated under laboratory conditions. Ten 12‐hr trials (i.e. 10 experimental groups), which, respectively, contained two different levels of substrates, demonstrated that juvenile flounder (55–78 mm total length) selected substrate of grain size (GS) < 2.00 mm, and the degree (from high to low) of their preference could be listed as Level 3 (0.50–2.00 mm) > Level 4 (0.25–0.50 mm) > Level 5 (0.05–0.25 mm) > Level 2 (2.00–4.00 mm) > Level 1 (4.00–6.00 mm). The average burying of fish in substrates of GS < 2.00 mm was more than 93.1%. Additionally, along with food deprivation time increased from 12 to 24 hr, some flounder showed random off‐bottom swimming behaviour, and the appearance of off‐bottom swimming behaviour was <3.2%. These results suggest that releasing sites with appropriate substrate grain size (GS < 2.00 mm) are suitable for conducting mass releases of hatchery‐reared flounder juveniles, and starvation prior to release should be minimized. The results of the present study are essential to improve stocking strategies of Japanese flounder and reduce the juvenile mortality after release.

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.

Northern form Dolly Varden is an anadromous char with significant ecological value found in high-gradient rivers of the Western Arctic. Because of declines in population abundance, Dolly Varden was recently designated as “Special Concern” under the federal Species at Risk Act. This species is also of great cultural and dietary significance to Indigenous Peoples of many communities in the Western Arctic; thus, expanding knowledge of the distribution, biology, and essential habitat is an important priority. We present results of a fisheries survey in the headwaters of the Arctic Red River, Northwest Territories, that focused on confirming the presence of Dolly Varden. Of 143 fish captured among 12 sampling locations, two were Arctic grayling (Thymallus arcticus), 33 were slimy sculpin (Cottus cognatus), and 108 were char identified using qualitative and quantitative morphological features. A subsample of 44 char voucher specimens were frozen whole and later identified using a linear discriminant function (LDF) based on meristic counts and morphological measurements, and a mitochondrial DNA genetic marker. LDF scores indicated that char collected in the Arctic Red River were northern form Dolly Varden. Genetic analysis showed that all but one char possessed mitochondrial DNA sequences common in northern form Dolly Varden from Canada. Our results confirm the presence of Dolly Varden in the Arctic Red River headwaters, extending the confirmed known distribution of this taxon in the Northwest Territories approximately 450 km south and 100 km east of previously delimited areas.