Downstream migration and sea entry are periods of high mortality for sea trout smolts and migration timing is a critical aspect of survival. We aimed to investigate migration timing of PIT-tagged sea trout smolts across a latitudinal gradient in five freshwater systems across Europe: Norway, Sweden, Denmark, France and Portugal. In two systems, smolt migration was further examined to assess a) differences in sex, body size and condition between autumn and spring migrants; b) the influence of spatial origin within the stream; and c) relationships between individual body size and migration date. Tagged smolts were not detected migrating in the Portuguese watershed. Spring migration timing differed significantly between watersheds in Norway, Sweden, Denmark and France. Generally, there was a trend of earlier migration at lower latitudes. Autumn vs spring migration was examined in Gudsø-Denmark. Autumn migrants were larger in both length and mass, with no differences were found in sex ratios or body condition. In terms of spatial origin, fish originating from an upstream site were more likely to migrate in the autumn compared to the spring and vice versa. Size dependent migration was found in the Swedish system, Haga å-Sweden, with larger individuals migrating earlier in the spring than smaller individuals. Outward-migrating smolts were also more likely to originate from a downstream site than an upstream site. Overall, these results show both large-scale geographic and fine-scale individual influences on migration timing. Given that climate change may have large impacts on migration patterns in sea trout, understanding variability in migratory patterns across a latitudinal gradient is an important tool for predicting responses to environmental changes.

ABSTRACT 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.

Stocked Atlantic salmon (Salmo salar) used to supplement wild populations often exhibit lower survival than their wild counterparts. The causes of their reduced survival in nature are multifaceted and likely include morphological deformities that can arise in the rearing environment. Opercular shortening, characterized by a partially or completely missing gill cover, is a common deformity in hatchery-reared salmon. However, although the opercula are important for respiration and protecting the gills, the causes and consequences of opercular shortening are not well known. Here, we assessed whether shortened opercula influenced the seaward migration success of stocked salmon smolts using passive integrated transponder (PIT) telemetry. One-year-old smolts with intact opercula and varying degrees of opercular shortening were PIT-tagged and released into Guds & Oslash; stream in 2017 and River Omme in 2018, Denmark. Their downstream spring migration was subsequently tracked using stationary PIT antenna stations. The overall migration success was higher in Guds & Oslash; stream, where 61.9 % of smolts were classified as successful migrants, compared to 31.1 % in River Omme. However, the degree of opercular shortening did not influence the likelihood of successful seaward migration in either study system. Similarly, opercular shortening had no detectable effect on migration timing or bird predation risk of smolts. Despite these findings, it remains important for hatcheries to minimise the occurrence of opercular deformities to promote fish welfare during rearing. As the salmon must still undertake an extensive marine migration to reach their feeding areas, further studies are needed to assess marine survival before robust conclusions can be drawn about the long-term effects of opercular shortening.

Biotelemetry networks have transformed our capacity to study the movements and ecology of aquatic animals, yet their development has been geographically uneven over time. While North America and Australia (among others) already hosted mature, well-coordinated infrastructures in the early 2000s, Europe has long lacked a formal framework for collaboration. The European Tracking Network (ETN) emerged in 2017 as a grassroots response to this gap. Since then, ETN has grown tremendously and has become a sustainable network with a central data system and research infrastructure installed all across Europe, enabling collaborative research on a multitude of species and environments while maintaining a strong track record in capacity building. ETN operates through a decentralised model in which national and regional partners lead infrastructure deployment while contributing data to a shared data platform that adheres to FAIR principles and supports collaborative tools, interactive dashboards and a dedicated R-package. ETN has successfully enabled a growing body of transnational research on species movements, habitat connectivity and behavioural ecology, as well as meta-analyses that maximise data reuse and help reduce animal use. At the same time, ETN faces challenges common to large-scale telemetry initiatives, including the need for stable funding, potential equipment incompatibility, the risk of false-positive detections, a potential lack of community buy-in, and ongoing discussions around data ownership and proper attribution. By working proactively with the community and stakeholders, ETN could address most of these potential challenges through the development of open transmission protocols, by aligning data policies with community needs, and by enhancing stakeholder engagement. Looking ahead, ETN is positioning itself at the interface of digital twins, machine learning, multisensor integration and responsible stewardship. Together, these efforts are transforming ETN into a key component of a global aquatic animal observatory, supporting evidence-based management and conservation across European aquatic ecosystems and beyond.

The diel vertical migrations (DVM) of Japanese eels were previously studied during their spawning migrations, but what determines their swimming depths is unknown. Ten silver-phase Japanese eels were released off the Japan east coast near the end of the Kuroshio Current with pop-up archival transmitting tags attached. Three tags transmitted the recorded data after the eels traveled various distances from the release location, ranging from nearby (Eel-H, 23.7 km), farther northeastward (Eel-I, 331.8 km), to far to the southeast (Eel-F, 1051.6 km). Two of three individuals performed diel vertical migration during tracking, but the others showed indistinct daily rhythms in bottom layers. The DVM of Eel-F showed a similar pattern as previously published Japanese eel DVM behaviors. Eel-I experienced gradually decreasing water temperatures with the progress of the tracking, and it was estimated to have arrived in the colder environment of the Kuroshio-Oyashio confluence region at around 3 January 2011. Eel-I experienced colder temperatures and swam at shallower depths in daytime and nighttime. These results indicate that the DVM of Japanese eels is likely affected by not only light but also water temperature. This study led to a hypothesis that a combination of motivations for DVM based on light and thermal incentives determines the swimming depths of Japanese eels during their oceanic spawning migrations.

The European eel (Anguilla anguilla L.) has a catadromous life cycle, with a single panmictic population that colonises continental Europe and northern Africa yet migrates 5000 to 9000 km to spawn in the Atlantic Ocean. It is unknown how this continental migration is organised so individual eels arrive in time for spawning with conspecifics. This meta-analysis combined tracking data from 18 water bodies in freshwater and transitional systems distributed along the southwest-northeast axis of Europe, resulting in a dataset of 2306 eels, making it the most comprehensive in terms of geographical coverage and number of eels tagged. The eels were tracked using acoustic telemetry and the Nedap Trail System and allowed us to study the eel's migration phenology at a continental scale. The findings reveal that the day when eels arrive at sea varies significantly with latitude, with northern eels arriving earlier. Migration speed differs between tidal and non-tidal habitats, suggesting that tidal currents facilitate faster movement. However, despite these patterns, we observed substantial variability in arrival at sea time and migration speed within water bodies, suggesting that the eel's migration phenology is considerably plastic. The presence of water regulating structures such as weirs, pumping stations and hydropower plants can impact migration timing and speed, potentially delaying eels, but is likely dependent on local hydrological conditions which can be water body specific.

Research on the direct effects of capture and tagging on post-release behaviour is typically limited to short-term deployments. To investigate the initial and longer-term behavioural responses to capture and tagging, we deployed eight Cefas G7 tags (1Hz depth and temperature, and 20 Hz triaxial acceleration) for 21–94 hours and 12 Wildlife Computers MiniPATs (depth, temperature, light and triaxial acceleration, each at 0.2 Hz) for 110–366 days on Atlantic bluefin tuna (ABT) in the English Channel. Post-release, ABT exhibited a strong, highly active initial swimming response, consistent with patterns reported in previous bluefin tuna, billfish and elasmobranch tracking studies. Accelerometry tags revealed that activity (VeDBA g), tailbeat amplitude (g) and dominant stroke frequency (Hz) were greater (2.4, 3.2 and 1.4 times respectively) within the first hour post-release than the subsequent 24 hours, stabilising at lower levels within 5–9 hours. However, lower resolution accelerometry data (0.2 Hz), obtained from longer periods from MiniPATs, revealed that fish then maintained this reduced activity for 11 ± 7.9 days (mean ± 1 SD; range: 2–26 days), during which they displayed disrupted diel patterns of activity and allocated on average 5 minutes of each day to burst energy events, compared to 14 minutes (max 74 minutes) during “recovered” periods. Subsequently, their activity levels increased again and were characterised by higher magnitude acceleration events (which may constitute feeding events) and became more active during the day than at night. Year-long deployments revealed that consistent diel vertical migration, diurnal patterns of activity, and increased time allocation to fast starts are normal for ABT off the British Isles in summer months, and their absence at the start of data collection may be related to the effect of capture and tagging, which may be longer lasting, and more complex than previously appreciated.

This study examines the dietary composition of the recently reoccurring Atlantic bluefin tuna (ABFT, Thunnus thynnus) in the Skagerrak-Kattegat-Sound area of the North-East Atlantic. We analysed the stomach contents of 44 ABFT, with lengths between 232 and 295 cm (CFL), captured in August and September annually from 2020 to 2022 via rod-and-reel fishing. Our analysis identified 19 species of teleost fish, four types of squid, and one crab species, although the diet was dominated by pelagic schooling fish such as garfish (percentage of index of relative abundance (%IRI) = 22.5), herring (%IRI = 6.8), mackerel (%IRI = 6.8), and various demersal gadoid species (combined %IRI = 42). Regional variation in dietary composition between Skagerrak and the Sound indicates that ABFT exhibit foraging flexibility, primarily targeting locally abundant, energy-rich schooling fish, while occasionally consuming other prey. The prevalence of garfish in the diet, which is neither commercially targeted nor managed or regulated, highlights the importance of maintaining healthy stocks of this species for supporting the continuous return of ABFT to the area.

This study presents the first investigation into the oceanic spawning migrations of Australian long-finned eels (Anguilla reinhardtii), a critical and previously undocumented stage in their life cycle. In autumn 2024, twenty adult eels were collected from two estuaries in southern Australia and tagged with pop-up satellite archival tags. Individuals were successfully tracked up to five months, with some migrating as far as ~2,500 km from their release sites to the tropical Coral Sea. Migration trajectories revealed relatively consistent and direct movement pathways, with eels remaining several hundred kilometers offshore for much of their journey before venturing farther offshore during later stages. Tag data provided clear evidence of predation events, indicating that escapement to the sea does not guarantee successful spawning and highlighting the potential ecological role of eels within marine food webs. Approximately 40% of migrations were prematurely terminated due to predation, with sharks likely responsible in nearshore environments and marine mammals in offshore regions. Additionally, consistent observations of diel vertical migration, characterized by nocturnal ascents toward the surface and diurnal descents to deeper waters, offer valuable insights into behavioral adaptations during the oceanic migration phase of this enigmatic life history. Tag endpoints and reconstructed migratory trajectories generally aligned with a region between the East Australian Current and the Lord Howe Seamount Chain. Several endpoints clustered within a small region of the northeastern Coral Sea, south of the Solomon Islands. These locations align with historical leptocephali collection sites, strengthening the hypothesis that the spawning area of long-finned eels lies in the northeastern Coral Sea.

Restoring wetlands is often used by management to boost ecosystem services like improving downstream water quality, but it may create ecological traps for migrating salmonids by increasing migration time and predation rates, potentially compromising self‐sustaining populations. In River Gudenaa, Denmark, the wild Brown trout ( Salmo trutta ) population has declined over the past one to two decades, and it remains unclear whether this decrease is linked to higher mortality due to restored wetlands in the river's lower reaches. This study investigated the progression rates and survival of migrating wild Brown trout smolts through River Gudenaa and Randers Fjord before and after the wetland restoration using acoustic telemetry. In 2020 and 2021, 150 smolts were tagged and released, and their movements and survival were compared with those of 61 smolts tagged and released before the restoration, in 2003 and 2005. Smolt progression rates were significantly slower in the river and fjord after the restoration, with the greatest reduction in the river. Despite slower progression rates, restoration did not impact survival, suggesting the wetlands did not act as an ecological trap for smolts. However, it remains unknown whether the slower migration had carryover effects on sea survival by increasing energy expenditure and delaying sea arrival. The retention of a main river channel, with a directed albeit slower flow, likely kept smolts from venturing into the adjacent wetland lakes, where predation may be higher. By incorporating measures that support migrating fish, wetland restoration can remain a valuable management tool to secure ecosystem services while sustaining fish populations.

Sea trout, Salmo trutta, display a wide range of migratory behaviours, and one aspect of variation comes from freshwater migration distance. The overall aim of this study was to determine if offspring of long- and short-distance migrants exhibited phenotypic differences relating to parental migration distance. For that purpose, we conducted several behavioural tests (dyadic contest, boldness scoring and open field test) and morphological analysis (relative pectoral-fin length) in multiple freshwater systems across the distribution range of the target species in Europe. It was expected that offspring of long-distance migrants would be more active, bold and dominant than those of short-distance migrants and would have longer pectoral fins relative to body length. Additionally, we investigated if boldness varied in relation to latitude. We showed that offspring of long-distance migrants were more dominant in two cases and more active in one case than those of short-distance migrants; however, there was no difference in swimming distance or velocity. Boldness and relative pectoral-fin length were significantly related to site of origin; however, the direction of this relationship differed between systems. Generally, we detected a decrease in boldness with declining latitude. In summary, we have detected variation among juveniles related to location within a stream; however, the drivers and processes behind these are likely more complex than purely parental migratory strategy. Our results can inform suitable management and conservation efforts directed to anadromous Salmo trutta. For example, habitat restoration and removal of migration barriers can increase the possible range of migration distances helping maintain the phenotypic diversity of offspring.

Electronic tags are often used to track the freshwater-marine migrations of smolts, where smolts are captured for tagging pre-migration (e.g., via electrofishing) or during-migration (e.g., via traps). Pre-migration capture allows smolts to initiate and complete their downstream migration unhindered, but risks smolt loss before the migration commences. The contrary is the case for during-migration trap-caught smolts, but trapping smolts temporarily halts their seaward journey which may negatively impact their progress. This study investigated the effect of trapping on the behaviour and survival of migrating Atlantic salmon (Salmo Salar) smolts using acoustic telemetry. We compared the movements and survival of smolts tagged before the smolt run captured by electrofishing ("comparator") with smolts trapped and tagged during the smolt run ("trapped"). A total of 478 smolts were tagged and released in River Skjern (2020 and 2022), Denmark, and 82 smolts in River Ballycastle (2022), Northern Ireland, and their seaward movements were monitored using acoustic receivers deployed in the river, fjord, and coastal area. In River Skjern in 2022, comparator smolts migrated earlier than trapped smolts, likely because these constituted more of the larger-sized, earlier migrating individuals. We found no differences in descent trajectories, diel patterns, progression rates, or survival between trapped smolts and comparator smolts in any of the rivers or study years. Thus, our results support the use of during-migration trapping as a low-impact method to capture smolts for telemetry studies, with trapped samples (if held <24 h) yielding comparable results in terms of behaviour and survival with non-delayed pre-migration tagged fish.

We provide a review of what we consider to be grand research challenges for fish science in an era of human-induced rapid environmental change.

Counting departing smolts and returning adults in index rivers is essential to estimate marine survival and track population trends of Atlantic salmon (Salmo salar). However, mortalities between counting facilities and a river mouth can skew survival estimates. We used acoustic and radio telemetry to investigate survival, mortality sources and behaviour of wild salmon smolts in the River Erriff, Ireland's index river, and Killary Fjord. Smolts were tagged with acoustic tags in 2017 (n = 40) and 2018 (n = 35) and radio tags in 2018 (n = 30). Survival was low for acoustic-tagged smolts in 2017 (26%) and 2018 (47%), mainly due to riverine mortality. Terrestrial or avian predators consumed 65% of acoustic-tagged smolts in 2017 and 67% of radio-tagged smolts in 2018. Nocturnal migration and ebb tide transportation likely contributed to high estuary survival. High predation on smolts emphasised the importance of assessing freshwater mortality for effective salmon management.

Knowledge about sex-specific difference in life-history traits-like growth, mortality, or behavior-is of key importance for management and conservation as these parameters are essential for predictive modeling of population sustainability. We applied a newly developed molecular sex identification method, in combination with a SNP (single nucleotide polymorphism) panel for inferring the population of origin, for more than 300 large Atlantic bluefin tuna (ABFT) collected over several years from newly reclaimed feeding grounds in the Northeast Atlantic. The vast majority (95%) of individuals were genetically assigned to the eastern Atlantic population, which migrates between spawning grounds in the Mediterranean and feeding grounds in the Northeast Atlantic. We found a consistent pattern of a male bias among the eastern Atlantic individuals, with a 4-year mean of 63% males (59%-65%). Males were most prominent within the smallest (< 230 cm) and largest (> 250 cm) length classes, while the sex ratio was close to 1:1 for intermediate sizes (230-250 cm). The results from this new, widely applicable, and noninvasive approach suggests differential occupancy or migration timing of ABFT males and females, which cannot be explained alone by sex-specific differences in growth. Our findings are corroborated by previous traditional studies of sex ratios in dead ABFT from the Atlantic, the Mediterranean, and the Gulf of Mexico. In concert with observed differences in growth and mortality rates between the sexes, these findings should be recognized in order to sustainably manage the resource, maintain productivity, and conserve diversity within the species.

Abstract Background The lack of compatibility between acoustic telemetry equipment from different manufacturers has been a major obstacle to consolidating large collaborative tracking networks. Undisclosed encrypted signal coding protocols limit the use of acoustic telemetry to study animal movements over large spatial scales, reduce competition between manufacturers, and stifle innovation. The European Tracking Network, in collaboration with several acoustic telemetry manufacturers, has worked to develop new transparent protocols for acoustic tracking. The results are energy-efficient transmission protocols accessible to all researchers and manufacturers. Today, the Open Protocols (OP) are already available to manufacturers and developers, and the first transmitters and receivers to implement them are already in the water. Results The main objective of this study was to confirm the compatibility between devices from different manufacturers using OP, characterise the acoustic range of each transmitter–receiver manufacturer combination, compare the detection efficiency to the standard protocols used at present (R64K and encrypted protocols), and assess its robustness against spurious detections. An international collaborative effort was made to conduct acoustic range tests in four main aquatic habitats: a river, a coastal lagoon, a coastal habitat, and the open sea. Receivers and transmitters from different manufacturers were deployed at increasing distances from each other using the same experimental design at each location. The decay of detection probability with distance was modelled for each transmitter–receiver manufacturer combination by applying logistic regression using a Bayesian approach. Furthermore, to thoroughly assess performance differences in an applied research context, we conducted a direct field comparison between groups of smolts tagged with OP and R64K tags, tracking their migration to the sea. Conclusions Our results confirm full compatibility between the tested devices, with negligible differences in the measured acoustic ranges between OP manufacturers and when compared to encrypted protocols. The OP was also robust against spurious detections, and the field comparison between OP and R64K showed equal performance. We hope these novel insights will encourage international research groups to promote OP-based studies to ensure compatibility and maximise the benefits of acoustic telemetry networks.

Winter is an energetically challenging period for many animals in temperate regions because of the relatively harsh environmental conditions and reduction in food availability during this season. Moreover, stressors experienced by individuals in the fall can affect their subsequent foraging strategy and energy stores after exposure has ended, referred to as carryover effects. We used exogenous cortisol manipulation of wild juvenile brown trout (Salmo trutta) in the fall to simulate a physiological stress response and then investigated short-term (2 weeks) and long-term (4 months) effects on condition metrics (hepatosomatic index and water muscle content), diet (stomach contents and stable isotopes), and morphology during growth in freshwater. We revealed some short-term impacts, likely due to handling stress, and long-term (seasonal) changes in diet, likely reflecting prey availability. Unfortunately, we had very few recaptures of cortisol-treated fish at long-term sampling, limiting detailed analysis about cortisol effects at that time point. Nonetheless, the fish that were sampled showed elevated stable isotopes, suggestive of a cortisol effect long after exposure. This is one of few studies to investigate whether cortisol influences foraging and morphology during juvenile growth, thus extending the knowledge of proximate mechanisms influencing ecologically-relevant phenotypes.

The use of Pop-up Satellite Archival Tags (PSATs) has provided considerable new information about the behaviour of migrating (silver) eels (Anguilla spp.) at sea, with 9 of 19 recognised species or subspecies tagged to date. However, such studies often reported premature tag detachment and relatively high rates of predation. Since 2000, several PSAT-tagged New Zealand longfin eels, Anguilla dieffenbachii, were released to understand migratory behaviour and location of potential spawning grounds. Of 20 PSAT-tagged eels released in 2019, all tags detached prematurely, but 7 (58%) were likely eaten or injured by predators 1-81 days after release. Two eels were probably eaten by sperm whales, whereas other tagged eels were likely eaten by lamniform and non-lamniform sharks, and southern bluefin tuna. Because of the high rate of predation, we recommend that future releases should be from smaller catchments or offshore.

Many organisms rely on migrations between habitats to maximize lifetime fitness, but these migrations can be risky due to a suite of factors. In anadromous salmonids, the smolt migration from fresh water to sea is a critical life stage, during which smolts can experience high mortality from multiple sources. This study investigated the migratory behavior and survival of Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) smolts during their seaward migration using acoustic telemetry between March and May 2021. Due to the extinction of wild salmon in the River Gudenaa after the construction of the Tange hydropower plant, this study used hatchery-reared salmon originating from a nearby Danish river. A total of 75 hatchery-reared salmon smolts, 75 hatchery-reared trout smolts, and 75 wild trout smolts were tagged with acoustic transmitters and released into River Gudenaa, Denmark. The downstream movements of tagged fish were monitored using acoustic receivers deployed in the river and fjord. Hatchery-reared trout initiated migration first, followed by hatchery-reared salmon, with wild trout being the last to migrate. There was no difference in riverine progression rates among the three smolt groups, but noticeable differences emerged once in the fjord: trout (wild and hatchery) slowed down, whereas hatchery-reared salmon maintained their speed. Riverine migration was predominantly nocturnal for all smolts; however, daytime migration increased at the fjord arrays. Day-of-year significantly influenced diurnal patterns in the river and fjord, where daytime migration increased later in the year. Hatchery-reared salmon and wild trout had reasonably good overall survival from river to sea entry (≥66%), whereas hatchery-reared trout had poor survival (c.26%). The fjord was the major bottleneck for survival of hatchery-reared trout. We found no strong evidence for differences in progression rate or diurnal patterns between wild and hatchery-reared trout to explain the lower survival. This study demonstrates that salmon and trout differ in their life-history strategy already in the post-smolt phase, and that stocking is a sub-optimal strategy to aid wild populations.