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Doctoral thesis, 2008

Using GIS to predict landscape-scale establishment and extinctions of lacustrine salmonids

Spens, Johan


Understanding the rigorously demanding habitat requirements of salmonids is directly related to the needs of both conservation and management of freshwaters. In this thesis, I used large-scale temporal and spatial data to evaluate which key predictors determine the distribution of self-sustaining lake-living salmonids. Surveys were conducted among more than a thousand lakes in northern Sweden. Multivariate analysis tools were used to model effects of native species, exotics, chemical and physical factors that may govern establishment and extinctions. Historical sources such as archaic lake names, fishers’ knowledge and documentary evidence combined with more recent limnological surveys provided estimates and validations of pre-industrial baseline distributions and extinction rates. Among native brown trout (Salmo trutta), the extinction rate increased from an insignificant level at pre-industrial times, up to exceeding 3 % lost populations per decade between 1920-2000. Using these data sources, I could also link extinctions to specific detrimental human impact. Long-term impact from introduction of the exotic brook charr (Salvelinus fontinalis) was linked to a 10-fold increase of native brown trout extinctions. Variables from Geographic information system (GIS) data were the best predictors of landscape-scale distributions of the keystone piscivore northern pike (Esox lucius), which in turn largely governed salmonid incidence among lakes. Landscape-scale patterns and retrospective whole-lake interventions involving stocking, removal and (re)colonization of pike and salmonids, clearly demonstrated that pike prevented self-sustaining populations of salmonids in these lakes. Set in the context of hierarchical filters, this approach provides a novel, highly predictable explanation to why these northern boreal lakes hold the specific fish communities they do. That is, keystone-specific connectivity is the fundamental determinant that either generates salmonid (low connectivity) or non-salmonid (high connectivity) assemblages and thus dictates fish species community compositions among these lakes. In contrast, water-quality (lakes ranging from highly acid-neutral, clear-murky and ultraoligotrophy-eutrophy) and lake morphometry (ranging from shallow small ponds to larger 53 m deep lakes) did not relate well to either salmonid or non-salmonid waters. In conclusion, I recommend that natural connectivity, e.g. SVimax = maximum stream slope measured in fixed vertical intervals, in drainage networks should be considered as standard characterization of lakes.


dispersal barriers, fish community composition, predation, species diversity, historical, abiotic factors, biotic factors, fish passage, fish species distribution, introduced species, lake isolation, local ecological knowledge, migration barriers, natural barriers

Published in

Acta Universitatis Agriculturae Sueciae
2008, number: 2008:71
ISBN: 978-91-86195-04-5
Publisher: Dept. of Wildlife, Fish, and Environmental Studies, Swedish University of Agricultural Sciences

Authors' information

Spens, Johan
Swedish University of Agricultural Sciences, Department of Wildlife, Fish and Environmental Studies

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