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Importance of woody debris for stream dwelling brown trout (Salmo trutta L.)

Sundbaum, Karl


The work presented in the following thesis was made in an attempt to study and evaluate the importance of woody debris as a factor in natural stream habitats, influencing brown trout (Salmo trutta L.) abundance and behaviour. I also attempt to identify the mechanisms behind the trout’s behavioural response to the presence of woody debris. The work has been conducted both in natural woodland streams as well as in experimental stream channels. In the field the responses to increased amounts of woody debris was on the individual level on growth rate, and behaviour (mainly mobility and spatial distribution), and on the population level, in terms of changes in abundance. This was done at five different locations in small tributary streams to the River Ammerån in the county of Jämtland, Sweden. The field results indicate that increased amount of woody debris in the stream has had a positive impact on trout abundance during the summer period one year after habitat improvement. The fish also distributed themselves in relation to the woody debris, indicating an utilisation of the debris, probably for cover and /or feeding positions. I also found that there was less movement within the sections containing added woody debris compared to control sections. I did not detect any differences in growth rate between the sections. The results suggest that the increased habitat complexity, provided by large woody debris, leads to an increased number of feeding territories during summer, and as a consequence, an increase in trout abundance during this season. In the experimental studies paired stream channels were used to study the effects of woody debris on growth, aggression, movement, and feeding activity of brown trout. Two habitat types were used; woody debris placed on a gravel bed, and only the plain gravel bed (control). The studies were performed both outdoors in the summer, using natural stream water and natural food (drifting invertebrates), and indoors using natural stream water and commercial fish food (pellets). I found that growth rates were consistently higher in the channels containing woody debris. There were also differences in behaviour. In the channel containing woody debris there was less movement, less aggression and lower feeding activity than in the control channel

Published in

Rapport / Sveriges lantbruksuniversitet, Vattenbruksinstitutionen
2001, number: 32
Publisher: Vattenbruksinstitutionen, SLU

UKÄ Subject classification

Fish and Aquacultural Science

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