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Report, 2012

Fiskundersökningar i Vänerns strandzon - en test av två kvantitativa provtagningsmetoder

Sandström, Alfred; Bergquist, Björn; Ragnarsson Stabo, Henrik; Andersson, Magnus


Shallow, vegetated habitats have an important role as spawning and nursery areas for several species of fishes in lakes. Such habitats are often laborious to monitor with conventional sampling methods. We evaluate two quantitative methods for sampling of young fishes in shallow habitats; point abundance with electro-fishing and low-impact pressure wave sampling using small underwater detonations. The study was conducted in an extensive archipelago area in Lake Vänern, Sweden. Both methods rendered quantitative estimates on the most common fish species in the littoral zone. More species were observed using underwater detonations. This method also provided data with lower variation among samples and fish within a wider size interval. These differences are interpreted to mainly depend on the larger sample area of detonations. Species distribution and juvenile fish density estimates and size-structure were compared with results from a standardised multi-mesh gill-net programme running in the same area. The dominant fish species in the gill-net program were recorded with both methods although the relative composition differed, mainly as a consequence of differences in species and size selectivity. Detonations provided additional, small-bodied, species that were not recorded in the gill-net program. The estimated total fish density in shallow water in the investigated area was 2.6 individuals per m2 with small detonations and 10.9 with electrofishing. The corresponding estimates of biomass were 6 and 20 g per m2. These figures are valid only for fish in the covered size-interval of 2-15 cm total length. The differences between methods are mainly interpreted as a consequence of differences in the estimation of effective sample area. The estimated fish densities are slightly higher compared to similar measures on fish abundance in pelagic areas of Lake Vänern. The studied methods can provide information of particular relevance for identifying essential fish habitats, understanding ecosystem dynamics and assessing long-term changes in Lake Vänern, e.g. due to climate change or new water level regimes.

Published in

Rapport / Vänerns vattenvårdsförbund
2012, number: 69
Publisher: Vänerns Vattenvårdsförbund