Technical University of Denmark, National Institute of Aquatic Sciences, Vejlsøvej 39; 8600; Silkeborg, Denmark.ABSTRACT. Partial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater.In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT tags in the river Villestrup (Denmark) to study the initial post-smolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord while others migrate to the sea suggesting that partial migration occurs even in the marine environments.Overall, a total of 53 %of the tagged smolts migrated from the fjord to the sea, and 47 % stayed (or potentially died) in the fjord. The ratios of fjord-resident vs. sea-migrating post-smolts were consistent at the study times and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout post-smolts encounter a fjord system. At this point, post-smolts will assess the possibility of migration versus the alternative of fjord residency. AbstractPartial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater. 2/28In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT tags in the river Villestrup (Denmark) to study the initial post-smolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord while others migrate to the sea suggesting that partial migration occurs even in the marine environments.Overall, a total of 53 %of the tagged smolts migrated from the fjord to the sea, and 47 % stayed (or potentially died) in the fjord. The ratios of fjord-resident vs. sea-migrating post-smolts were consistent at the study times and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout post-smolts encounter a fjord system. At this point, post-smolts will assess the possibility of migration versus the alternative of fjord residency.
The seaward migration of wild (n = 61) and hatchery-reared (n = 46) sea trout smolts was investigated in the Danish River Gudenaa and Randers Fjord (17.3 and 28.6 km stretch, respectively) using acoustic telemetry. Their riverine and early marine migration was monitored by deploying automatic listening stations (ALS) at 4 locations in the river and fjord. Migration speeds were approximately 3 to 11 times faster in the river than in the early marine environment. Hatchery-reared smolts migrated faster than wild smolts, but the difference was small, especially compared with the large differences in migration speeds among habitats. There was no difference in the diurnal activity pattern between wild and hatchery-reared smolts. Both the riverine and early marine migration activity were primarily nocturnal, although some individuals were also recorded by the ALSs during the daytime. The survival of the wild smolts from release in the river to the outermost marine ALS site, 46 km from the release site, was 1.8 and 2.9 times higher than that of the hatchery-reared smolts in the 2 study years, respectively. Overall, survival from release to the outermost ALS site was 79% for wild and 39% for hatchery-reared smolts. Since the lower survival of the hatchery-reared compared with the wild smolts could not be explained by differences in migration speeds or diurnal migration patterns, behavioural differences on a smaller scale than those recorded in the present study may explain the difference in survival. KEY WORDS: Downstream • Ground speed • Telemetry • Postsmolt • Brown trout Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Tracking fitness in marine vertebrates'
Background: Understanding fish movements and migrations are paramount for management and conservation efforts. By applying Hidden Markov Models (HMMs) on records from electronic tags, migration routes of tagged fish can be reconstructed and new insights to the movement ecology of a species can be gained. We demonstrate the usability of HMMs on a widespread, iteroparous salmonid (sea trout, Salmo trutta) in a complex marine area with highly variable temperatures and salinities within small geographic distances. Understanding how the highly adaptable sea trout cope with these complex conditions could shed new light on factors driving the movement ecology of salmonids. Migration tracks of fish migrating at sea are reconstructed by applying an HMM on temperature and depth records from eight wild post-spawned sea trout from four Danish rivers. Results: The fish migrated at sea for 47-142 days. Estimated positions of all fish were close to the coast (< 100 km) throughout the marine period, but migrations along coastlines up to 580 km away from the natal river occurred. Seven of eight individuals resided in or actively migrated into stratified or shallow marine areas that heat up fast during spring, while all eight individuals resided in deeper and more heterogeneous areas that heat up slow during summer. All fish entered the Skagerrak (located between Denmark and Norway) at some stage during summer. Migrations were directed into less saline areas during the first 15 days at sea for all individuals. Mean linear progression of the fish was 16 km day −1 (range 0-58 km day −1). Conclusions: The results corroborate the expectation that sea trout are more coastally orientated than other salmonids, but also suggest that longer migrations occur in the seas surrounding Denmark compared to elsewhere. This could be a consequence of the fish seeking out habitats with optimal conditions (e.g. salinity, temperature, predation and foraging options) for growth in different parts of the year. The coinciding movement from shallow or stratified marine areas that heat up fast during spring to deeper, more well-mixed areas that heat up slow during summer suggested that some habitat selection had occurred. These results shed new light on factors influencing marine migrations in salmonids and demonstrate how HMMs can expand our knowledge on behaviour and movement ecology of marine fishes.
Spring migrating sea trout juveniles can be classified as parr, pre-smolt or smolt based on body morphology and osmoregulatory capacity. In this respect, parr are assumed to be less prepared for a marine life and to have lower survival at sea than pre-smolts and smolts. However, the behaviour and survival of these trout phenotypes upon entering the sea is not well known. Using passive integrated transponder telemetry, this study found that the return rate from the sea to the natal river was higher for parr compared to pre-smolts and smolts. Additionally, trout classified as parr generally migrated earlier to the sea and a larger proportion returned to the river after less than one year at sea. The daily mortality rate at sea was comparable among the different phenotypes of trout, suggesting that the higher proportion of returning parr to the river was linked to their shorter duration at sea. These results provide evidence of different life-history strategies for seaward-migrating juvenile sea trout, ultimately affecting their return rate to the natal river. Investigations failing to consider downstream migrating parr and pre-smolts risks neglecting a large part of the anadromous population and may result in inaccurate assessments of sea trout stocks in rivers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.