FRESHWATER METADATA JOURNAL

Title

Life-history data on Hunder brown trout (Salmo trutta) from Lake Mjøsa, Norway

Author(s)

Per Aass, Atle Rustadbakken, S. Jannicke Moe, Espen Lund & Tore Qvenild

Abstract

The dataset contains individual data for almost 8,000 brown trout (Salmo trutta L., 1758) captured during their spawning migration from Lake Mjøsa to the main tributary River Gudbrandsdalslågen in Norway during the period 1966 to 2005. These individuals belong to the large-sized piscivorous population of brown trout named Hunderørret (Hunder brown trout). A majority of these trout spawn upstream the waterfall Hunderfossen. Ascending this large waterfall, the migration length and characteristics of the spawning areas, are probably selection drivers for the large body sizes this population achieves compared to any other populations of piscivorous brown trout spawning in other rivers draining to Lake Mjøsa. A hydropower dam was established at Hunderfossen between 1961 and 1964, causing a migration barrier for the Hunder brown trout. A fish ladder was also built, making the ascent possible from 1966, but not without negative effects (Aass 1990). The functionality seems to vary due to water temperature and water flow (Jensen & Aass 1995). The body size seems to affect the success of both entering and ascending the ladder. Additionally, the dam reduced survival of both smolt and kelt, due to predation in the dammed area and turbine passage mortality during downstream migration. The damming also reduced the areas of productive fish habitats in the river both upstream and downstream the dam (Aass et al. 1989). When the fish ladder was opened, allowing the spawners to pass the dam, a capture-mark-recapture program was initiated. Trap capture, individual measurements, tagging, scale sampling and registration of recaptured repeated spawners in the fish ladder, were then implemented in a monitoring program. All spawners caught in the trap were individually measured and tagged with numbered Carlin tags (Carlin 1955). To abate the reduced natural production of Hunder brown trout in the regulated river, a stocking program was initiated during the mid-1960s. The stocked fish are mainly released as 2-years old both in the river and directly into the lake (15,000-20,000 individuals per year). The stocked fish also returns to the river on spawning runs as mature adults. This long-time data series has been continued more or less unchanged until it was terminated in 2016. All stocked brown trout were tagged by cutting the adipose fin prior to release. To evaluate and optimize the stocking strategy, a large number of stocked smolt individuals were also tagged. Hence more than 30,000 individuals (both wild and stocked, spawners and smolts) have been marked individually with Carlin tags before release both upstream and downstream the Hunderfossen dam during 1966-2015. For a subset of these fish, approximately 8,000 individuals who climbed the fish ladder between 1966 and 2005, information on age, growth, time of migration and spawning history has been obtained from schlerochronological analysis of the sampled scales. This data has also been used to back-calculate yearly growth and to identify important life-history information such as hatching year, growth in river, age and size at smoltification, growth in lake, age and size at sexual maturation, and number of spawning events (e.g. Haugen et al. 2008). Based on the 1966-2005 data the Hunder brown trout typically spend their first three to five years as parr in the river before migrating downstream to Lake Mjøsa (corresponding to smoltification and seaward migration in anadromous salmon and trout). In the lake they typically prey on fish for two to four years before maturation and migration back to the river to spawn. The Hunder brown trout typically perform biennial spawning runs and average age at first spawning run is ca. 7 years. Average size at first spawning run is ca. 3.5 kg and 65 cm. The Hunder brown trout seem to have a potential maximum life span of 15-20 years and a potential maximum size of 15-20 kg and >100 cm. However, less than 10% of the spawners reach age above 10 years, and less than 1% of the spawners reach weight above 10 kg.

Keywords

brown trout, age, growth, spawning, capture-mark-recapture, hydropower dam, fish ladder, migration, smoltification, fish scale

How to cite this article

Aass P., Rustadbakken A., Moe S.J., Lund E. & Qvenild T., 2017. Life-history data on Hunder brown trout (Salmo trutta) from Lake Mjøsa, Norway. Freshwater Metadata Journal 25: 1-11. https://doi.org/10.15504/fmj.2017.25