Anders Koed

Atlantic salmon ( Salmo salar ) are iconic for their homing capacity, with most populations migrating into the ocean and eventually returning to their natal river. However, straying is known to occur, with small portions of the population returning to different rivers. Here, we studied the seaward and return migration of Atlantic salmon kelts from wild and hatchery origin, in River Skjern, Denmark. Kelts were captured, tagged and released along River Skjern, or tagged at the local stocking facilities following gamete removal. Of the 199 tagged fish (134 wild and 65 stocked), 110 reached the ocean (55%). Wild‐spawned kelts captured and released along the river were the most likely to survive (63 out of 97, 65.6%) while kelts tagged after stripping and reconditioning at the stocking facilities had the lowest survival rates (5 out of 25, 20%). Only 10 kelts were recorded returning after spending more than 1 year at sea (five wild and five stocked fish). While this constitutes a limited sample size, seven of the 10 returning fish (four wild, three stocked) entered a neighbouring river instead (i.e., strayed). This high straying may be due to (1) a weakening of homing ability with age, (2) the returning fish encountering the neighbouring river first during their southward return‐migration from the Atlantic Ocean and mistakenly entering it given the similarity between the two river systems, (3) some fish actually originating from the neighbouring river, or (4) the influence of long‐term hatchery‐rearing practices, which are known to increase straying rates. Although the small sample size of repeat spawners limits firm conclusions, the observed high straying rates raise concerns for monitoring and management that depend on accurate adult return counts, particularly in systems with a history of stocking.

Atlantic bluefin tuna (Thunnus thynnus;ABFT) is one of the most iconic fish species in the world. Recently, after being very rare for more than half a century, large bluefin tunas have returned to Nordic waters in late summer and autumn, marking the return of the largest predatory fish in Nordic waters. By tagging 18 bluefin tunas with electronic tags (pop-up satellite archival tags), we show that bluefin tuna observed in Nordic waters undertake different migration routes, with individuals migrating into the western Atlantic Ocean, while others stay exclusively in the eastern Atlantic and enter the Mediterranean Sea to spawn. We additionally present evidence of possible skipped spawning inferred from behavioural analyses. In Nordic waters, ABFT are primarily using the upper water column, likely reflecting feeding activity. The results support the hypothesis that ABFT migrating to Nordic waters return to the same general feeding area within the region on an annual basis. These observations may have important implications for management because (1) tunas that come into Nordic waters might represent only a few year classes (as evidenced by a narrow size range), and thus may be particularly vulnerable to area-specific exploitation, and (2) challenge the assumption of consecutive spawning in adult Atlantic bluefin tuna, as used in current stock assessment models. Without careful management and limited exploitation of this part of the ABFT population, the species’ return to Nordic waters could be short-lived.

Repeatable individual variation in behaviour has been demonstrated in all taxa, though few studies have explored the broader ecological consequences of such consistent behaviour. We tagged and tracked 401 individual sea troutSalmo truttawith passive integrated transponder telemetry over two to four spawning seasons as they migrated between freshwater and marine environments to spawn and feed respectively. We found the timing (time of river entry) and duration of migration (time spent in freshwater and time spent at sea) to be repeatable within individuals across years. In addition, we identified two peaks in return migration timing; a group of summer‐migrating fish and a group of fall‐migrating fish. At the time of first spawning, sea trout differed in size depending on how long the first marine phase was, but summer‐migrating fish were smaller in all subsequent spawnings. Furthermore, summer‐migrating fish had significantly better marine survival than fall‐migrating fish. Our study suggests that the timing of arrival in freshwater is an important fitness‐relevant trait, such that summer‐migrating fish have decreased fitness but remain in somewhat safer conditions, while fall‐migrating fish have increased fitness but are exposed to more risky conditions. Taken together, our findings suggest that migration timing and duration is determined via a trade‐off between size and survival, which resulted in two evolutionary stable strategies. A freePlain Language Summarycan be found within the Supporting Information of this article.

Coexistence of ecotypes, genetically divergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod (Gadus morhua) occur as two such coexisting ecotypes. We applied a combination of acoustic telemetry, genotyping, and stable isotope analysis to 72 individuals to investigate movement ecology and food niche of putative local “Fjord” and putative oceanic “North Sea” ecotypes—thus named based on previous molecular studies. Genotyping and individual origin assignment suggested 41 individuals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype individuals left the study system during the study while 42% of North Sea individuals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among individuals. Fjord ecotype cod had significantly higher δ¹³C and δ¹⁵N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.

The status of Atlantic salmon, Salmo salar L., over the last decades has been of concern across its entire distribution area. Its anadromous nature exposes the species to human pressures in both freshwater and marine environments, and over long periods, thus exacerbating its decline. Given its value within the food industry, the recreational angling community as well as culturally, the status of Atlantic salmon is regarded as a matter of national and international conservation interest, providing great incentive for its management. The literature currently lacks specific examples of successful and unsuccessful management strategies and practices for Atlantic salmon populations at a broader scale. To address this, the present article describes how the multi‐faceted management approach taken for Danish Atlantic salmon, which included changes in legislation, stocking practices, habitat restoration, population genetics and barrier removals, successfully rehabilitated salmon populations in four major Danish rivers. Specific recommendations are provided for the successful management of Atlantic salmon elsewhere.

Juvenile salmonids experience high mortality when negotiating lentic waters during their downstream migration to the sea. The development of artificial lakes and wetlands in streams has become a widely used management tool to reduce nutrient load to coastal areas. Such wetlands may threaten anadromous populations. In this study we quantify net ground speed of downstream migrating brown trout Salmo trutta smolts in equally long stream and lake sections in a Danish lowland stream and artificial lake. This was done by passive integrated transponder telemetry in 2016 and 2017. Mean net ground speed in the stream section was 36.58 and 0.8 km day⁻¹ in the lake section. This decrease of net ground speed through the lake may lead to prolonged exposure to predators and probably contributes to high mortalities threatening anadromous populations.

The survival rates of three groups of seaward‐migrating Salmo salar smolts were investigated in 2005, 2016, and 2017 in the River Skjern and River Omme, as well as in the Ringkøbing Fjord using acoustic telemetry. Ringkøbing Fjord extends for approximately 300 km², and has a narrow, regulated outlet to the sea. Smolts of three different origins: (a) wild smolts, (b) hatchery‐reared smolts previously released at half‐year‐old, and (c) hatchery‐reared smolts previously released at 1‐year‐old were captured in rotary screw traps and surgically implanted with acoustic transmitters. The progress during seaward migration was monitored with a network of automatic listening stations deployed in the river estuary, fjord mouth and sea opening.The smolts' probability of survival in the river was related to their length, with larger smolts being more likely to reach the fjord. Once in the fjord, the probability of reaching the sea was related with the smolt's group, with smolts previously released at half‐year‐old being more likely to succeed than wild smolts. However, none of the biometric or behavioural variables explained the difference between the studied smolt groups, masking the potential reasons behind this difference in survival probability.Overall, approximately 47% of the tagged smolts were registered at the last array of automatic listening stations (i.e., entered the sea), demonstrating the early migration as a critical bottleneck for the local Atlantic salmon population. Ultimately, this limits the number of Atlantic salmon that survive to adulthood and return to River Skjern and River Omme for spawning.