Ludwig, G.X., Hokka, V., Sulkava, R. & Ylönen, H. 2002: Otter Lutra lutra predation on farmed and free-living salmonids in boreal freshwater habitats. - Wildl. Biol. 8: 193-199.
In this paper we aim to define whether fish farms or stocked trout streams substantially contribute to the diet of otters Lutra lutra living in freshwater habitats of Mid-Finland. Diet was assessed using spraint analysis. We especially focused on areas with fish farms and stocked trout Salmo trutta streams (salmonid-rich habitat) and used 'normal' (salmonid-poor) habitats as control. Frequency of salmonid remains in spraints was significantly higher in salmonid-rich habitats than elsewhere in all seasons, the difference increasing from summer to winter. In salmonid-rich habitats, however, salmonid consumption was not strongly seasonal. Data from spraint collection indicated a shift in activity towards salmonid-rich habitats in winter. Otters therefore seem to switch prey seasonally, due to seasonal variation in the availability of other prey categories, by choosing to forage in particular, predictable habitats. Furthermore, our results suggest that, in salmonid-rich habitats, the increase in salmonid frequency is to a larger extent due to the presence of fish farms than that of trout streams. Fish farms, and to a lesser extent stocked streams, may therefore constitute seasonally important feeding grounds for otters.
Key words: diet, fish farms, Lutra lutra, otter, predation, salmonids, spraint analysis
Gilbert X. Ludwig, Ville Hokka & Hannu Ylönen, University of Jyväskylä, Department of Biological & Environmental Science, P.O. Box 35, FIN-40350 Jyväskylä,
Finland - e-mail addresses: gilludw@cc.jyu.fi (Gilbert X. Ludwig); ville.hokka@welho.com (Ville Hokka); hylonen@dodo.jyu.fi (Hannu Ylönen)
Risto Sulkava, Aaponkaari 3, FIN-42800 Haapamäki, Finland - e-mail: risto.sulkava@pp.inet.fi
Corresponding author: Gilbert X. Ludwig
Received 4 April 2001, accepted 17 December 2001
Associate Editor: Miguel Delibes