Sea trout

Sea trout. Photo: Øyvind Solem /NINA.

Sea trout

What are sea trout?

Sea trout are a form of trout (Salmo trutta) that choose to migrate out to sea to increase their food supply. Both sea trout and resident brown trout spawn in freshwater, and they can originate from the same parents. Immature sea trout can stay in the sea for several months or years, and some sea trout move between freshwater and the sea every year. Sea trout usually grow much faster and larger, and produce more eggs, than resident brown trout that stay in freshwater their whole life. This means that sea trout are important for maintaining good populations of large trout in Norwegian catchments.

The life history of sea trout and resident brown trout. The life history of sea trout and resident brown trout.

Why study sea trout?

Many Norwegian sea trout populations have experienced severe declines due to man-made problems in both freshwater and at sea. In order to ensure healthy sea trout populations for the future, it is important understand which problems threaten local sea trout populations.

Monitoring of sea trout

The Norwegian Institute for Nature Research uses several methods to monitor the status of sea trout populations. Current research activities include monitoring sea trout migration patterns, surveying juvenile stocks, assessing spawning stocks, evaluating age/growth rates, and modelling population dynamics.

Sea trout migration patterns

Since 2014, the Norwegian Institute for Nature Research has studied the migration patterns of sea trout in multiple catchments. Sea trout are marked with a PIT (passively integrated transponders) tags that record their movements out to sea and back to freshwater when they pass underwater antennas at river mouths. During their marine migrations, sea trout need to find food to maximize their growth, while also being exposed to multiple threats from parasites (e.g. salmon lice), predation (e.g. birds, large fish, marine mammals) and marine fishing. For many catchments, this results in only 25-40% of sea trout returning to freshwater each year.

Spawning stocks of sea trout

Read more on the page "Gytebestander av laks og sjøørret" (in Norwegian)

Scale sampling

Salmon lice

Modelling of sea trout population dynamics

Norway has several thousand sea trout populations, which are all influenced by human activity to some extent. As sea trout display several alternative life history strategies, and occupy a range of habitats with large natural fluctuations, it is not possible to collect information on critical life phases for every sea trout population. Here, a model-based approach provides the best solution for assessing population-level responses of sea trout to environmental changes and management actions. Our individual-based dynamic model for sea trout populations (IBTRUTTA; Hedger et al. 2021) provides realistic assessments of population-level responses to:

  • consequences of reduced time and growth in the sea. For a given population, we can quantify changes in population size and distributions, and their reproductive potential under different levels of salmon lice pressure (Hedger et al. 2021; Diserud et al. 2020). 
  • population-specific conservation goals. Spawning stock targets can be developed based on stock-recruitment relationships. 
  • long-term effects of different management and harvesting strategies. 
  • importance of smolt population characteristics for survival and growth (e.g. size, timing of emigration; Jensen et al. 2020). 

In order to build population-specific models, we examine life history parameters critical to population dynamics, and assess how they vary between population types, and in response to different human activities. Our long-term sea trout time-series, and national and international collaborations with other sea trout researchers are integral to our modelling activities. For instance, in conjunction with ICES working group WGTRUTTA, we coordinate data collection, fit stock-recruitment models over a large geographical and environmental scale, and quantify impacts on European sea trout populations. 

Hedger R, Diserud OH, Finstad B, Jensen AJ, Hendrichsen DK, Ugedal O, & Næsje TF. (2021). Modeling salmon lice effects on sea trout (Salmo trutta L.) population dynamics using an individual-based approach. Aquaculture Environment Interactions 13: 145-163 https://doi.org/10.3354/aei00397 

Diserud OH, Hedger R, Finstad B, Hendrichsen D, Jensen AJ, & Ugedal O. (2020). Salmon louse infestation in wild brown trout populations generates multi-modal mixture distributions. Aquaculture Environment Interactions 12:447-456. https://doi.org/10.3354/aei00374

Jensen AJ, Finstad B, Fiske P, Diserud OH, & Thorstad EB. (2020). Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences. Ecology & Evolution 10:11727–11738. https://doi.org/10.1002/ece3.6808

Contact

Publications

2021

Hedger R, Diserud OH, Finstad B, Jensen AJ, Hendrichsen DK, Ugedal O, & Næsje TF. (2021). Modeling salmon lice effects on sea trout (Salmo trutta L.) population dynamics using an individual-based approach. Aquaculture Environment Interactions 13: 145-163 https://doi.org/10.3354/aei00397 

Finstad B, Sandvik AD, Ugedal O, Vollset KW, Karlsen Ø, Davidsen JG, Sægrov H, & Lennox RJ. (2021). Development of a risk assessment method for sea trout in coastal areas exploited for aquaculture. Aquaculture Environment Interactions 13:133-144. https://doi.org/10.3354/aei00391

Paterson RA, Berntsen HH, Næsje TF, Berg M, & Finstad B. (2021). Factors influencing return rate and marine residence duration in sea trout populations in Central Norway. Journal of Fish Biology, 99( 3):875–887. https://doi.org/10.1111/jfb.14770 

2020 

Diserud OH, Hedger R, Finstad B, Hendrichsen D, Jensen AJ, & Ugedal O. (2020). Salmon louse infestation in wild brown trout populations generates multi-modal mixture distributions. Aquaculture Environment Interactions 12:447-456. https://doi.org/10.3354/aei00374

Eldøy SH, Ryan D, Roche WK, Thorstad EB, Næsje TF, Sjursen AD, Gargan PG & Davidsen JG. (2020). Changes in growth and migration patterns of sea trout before and after the introduction of Atlantic salmon farming. ICES Journal of Marine Science 77(7-8):2623-2634. 

Jensen AJ, Finstad B, Fiske P, Diserud OH, & Thorstad EB. (2020). Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences. Ecology & Evolution 10:11727–11738. https://doi.org/10.1002/ece3.6808 

Bergan MA, Stensland S, Lund SD, & Haugen T. (2020). Sjøørreten sliter, men gjør vi noe med det?. Tidsskriftet VANN nr 3-2020. Fagfellevurdert temakronikk

Sea trout research at Agdenes (brochure):

Sea trout at Vatne, Møre og Romsdal