Kelts – individuals of anadromous fish species which have successfully spawned and may return to sea to repeat the cycle – are perhaps the least studied life stage of iteroparous fish species. To date, our understanding of what makes them successful in their return migration to sea is limited. We investigated the relationship between three physiological parameters (baseline cortisol, baseline glucose and low molecular weight antioxidants) and the timing and success of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) kelt migration. To do so, we combined blood samples obtained within 3 minutes of capture and acoustic telemetry to track 66 salmon and 72 sea trout as they migrated out of rivers, into fjords and out at sea. We show that baseline cortisol may be a good predictor of migration success. Individuals with high baseline cortisol levels exited the river earlier but were less likely to successfully reach the sea. Similar relationships were not observed with glucose or antioxidants. We provide the first evidence to support the role of physiological status in migration success in Atlantic salmon and sea trout kelts. Our findings contribute to our understanding of the relationship between physiology and fitness in wild animals. Further, we suggest that migration timing is a trade-off between stress and readiness to migrate.

The development of small acoustic transmitters has enabled researchers to monitor earlier life stages and smaller fish species than was previously possible. The underlying assumptions of any telemetry study are minimal tag loss and negligible effects on the behavior, survival, and growth of tagged individuals. To that end, tag retention, healing, survival, specific growth rates, and behavior were evaluated for 96 age-0 Muskellunge Esox masquinongy (TL [mean ± SD] = 205 ± 10 mm) from three treatment groups. Tagged fish were compared to untagged controls and sham fish (fish that had undergone anesthesia and laparotomy but not transmitter implantation). Thirtytwo fish (tagged group) were implanted with one of the smallest commercially available acoustic transmitters (Juvenile Salmon Acoustic Telemetry tag; 12.0 × 5.3 × 3.7 mm, 0.217 g in air, >120-d tag life) and monitored in a 4-month, overwinter tank experiment. Tricaine methanesulfonate was used for anesthesia, incisions were closed with a synthetic absorbable monofilament, and all surgeries were conducted by a single trained researcher. All tags were retained throughout the experiment; surgical wounds healed within 30 d, 32% of sutures were retained at 120 d postsurgery, and survival did not differ between treatments. No biologically significant effects of tagging on mean relative growth rates (percent change in weight/d) were observed among the three groups (tagged, untagged, and sham fish) at 4 months postprocessing. The reaction of tagged fish to a moving object within 15 minutes after tagging was slower than the reaction at 7 d postsurgery, reiterating the importance of testing appropriate sedation methods prior to releasing fish in field studies. Results validate the utility of surgical implantation of small acoustic transmitters in juvenile Muskellunge for future studies, although immobilization methods for early life stages require further study.

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Understanding the spatial ecology of juvenile freshwater fish beyond summer months is an essential component of their life history puzzle. To this end, declines in the natural populations of sympatric Muskellunge (Esox masquinongy) and Northern Pike (Esox lucius) in the upper St. Lawrence River prompted study of spatiotemporal patterns and habitat requirements associated with earlier life stages of these congeneric, freshwater predators in fall and overwinter periods. Over 75 age‐0 esocids were tagged and passively monitored using acoustic telemetry in four nursery embayments in fall and winter months from 2015 and 2017 months to elucidate spatiotemporal ecology and test hypotheses related to emigration. Presence, residency, space and habitat use were assessed and modelled against key environmental (i.e. water temperature and level) and biological (total length) covariates using mixed effect models. Muskellunge were found to spend more time in deeper, littoral regions with canopy‐forming, submerged aquatic vegetation while Northern Pike aggregated in the deepest, highly vegetated region of their nursery embayment. Results suggest fish may exhibit transitionary movements in fall months and may span outwards into nearshore regions along the main river channel. Studies informing coastal restoration initiatives to increase Muskellunge production are encouraged to assess sympatric habitat use relative to prominent embayment structures and further explore depth partitioning by these young predators. With a substantial influence from water‐level regulation on use of nursery habitat, future studies must work in concert with management plans aimed at producing more natural riverine cycles and thus increased recruitment of Esox species.