Tracking and monitoring brown trout migrations in Norway

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Davidsen’s team downloading receivers in Tosenfjorden, Norway.

OTN principal investigator, Jan Davidsen (Norwegian University of Science and Technology), has been tracking the movements of brown trout in Norway for the past ten years with the goal of providing insight into how trout make use of the marine habitat and how individual characteristics might lead to different ocean migratory strategies.

In early April 2017, the team, which includes Dalhousie University/OTN PhD student, Xavier Bordeleau, began their field season by externally tagging 41 brown trout in Åbjøra, Norway, with special acoustic tags that can record ambient conductivity and temperature. The team also serviced 50 OTN receivers and deployed 18 new receivers in Norwegian waters to aid in their work studying the marine habitat use and migratory strategies of brown trout.

A curious and complicated migratory phenomenon

Norwegian brown trout have a complicated and puzzling life cycle. Beginning their journey hatching from eggs in freshwater rivers, the young fish go through physiological changes during development, the most important of which is when they become smolts, after about two years. At this stage, brown trout become anadromous, meaning they become tolerant to salt water and can migrate to the ocean.

While some brown trout leave the freshwater habitats they were born in and spend other parts of their lives at sea (sea trout), others will instead remain in fresh waters (resident trout). Telemetry researchers are interested in looking at the individual differences between sea trout and resident trout, as well as the difference in sea trout with respect to their marine migrations (e.g. short-distance migrants vs. long-distance migrants).

Davidsen says sea trout numbers in central Norway have declined as much as 60 per cent over the past several decades, partly due to a decline in marine survival. However, little scientific knowledge exists on the physiological mechanisms (e.g. nutritional status) leading to different migratory strategies — either travelling long distances at sea, or staying within relatively small areas of the coastal marine ecosystem. This makes risk assessment and fisheries management practices difficult.

The value and productivity of sea trout

Researchers hypothesize that brown trout migrating to the sea have increased reproductive contributions playing an important role in population success. While travelling the ocean, sea trout have access to abundantly more food than resident trout and grow faster than their freshwater counterparts. When sea trout return to freshwater, females are generally larger, producing more offspring.

In spite of how valuable sea trout and their environments are to population sustainability, not much is known about these fish once they enter the sea. Information on the whereabouts of sea trout during certain times throughout the year, as well as their preferred habitat, is limited.

Future plans for brown trout research

A map of the acoustic array in Tosenfjorden, Norway (updated Feb, 2017).

Over the coming weeks, Davidsen plans to tag 50 brown trout smolts, and 60 adults in the Tosenfjorden river (65°N), as well as 115 adults in Skjerstadford (67°N), an area where receiver deployments are also planned to increase from 50 to 90 in the near future.

In addition to investigating migratory strategies, Davidsen and his team are also interested in how these strategies might be negatively impacted by intensive Atlantic salmon harvesting.

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