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Hydroacoustic assessment of mono‐ and polymorphic Coregonus density and biomass in subarctic lakes

Malinen, Tommi, Tuomaala, Antti, Lehtonen, Hannu, Kahilainen, Kimmo K.
Ecology of freshwater fish 2014 v.23 no.3 pp. 424-437
Coregonus, autumn, biomass, diet, ecosystems, food webs, habitats, lakes, microsatellite repeats, morphs, pelagic fish, surveys
Density and biomass estimates of pelagic fish are essential to understand food web interactions and ecosystem functioning. We conducted surveys of six subarctic lakes for assessing both mono‐ and polymorphic whitefish Coregonus lavaretus (L.) populations. Monomorphic whitefish lakes were inhabited by a habitat and diet generalist, large sparsely rakered (LSR) morph, whereas polymorphic whitefish lakes had a littoral benthivorous LSR morph, a pelagic planktivorous densely rakered (DR) morph and in two cases a benthivorous small sparsely rakered (SSR) morph inhabiting the profundal zone. In addition, an introduced specialist zooplanktivore, vendace Coregonus albula (L.), inhabited one of the monomorphic lakes. Hydroacoustics was found to be an appropriate method for estimating coregonid densities and biomasses in large and deep polymorphic lakes occupied by the planktivorous DR morph or vendace, but only during dark nights in autumn. The suitability of hydroacoustic assessment for benthivorous LSR and SSR morphs was low, especially in polymorphic whitefish lakes due to their preference for near‐bottom habitat or shallow areas not sampled with hydroacoustics. The pelagic density of DR morph varied from 330 to 1780 fish·ha⁻¹and biomass from 1.4 to 13.3 kg·ha⁻¹in polymorphic whitefish lakes, whereas corresponding estimates for LSR morph were 10–320 fish·ha⁻¹and 0.5–8.4 kg·ha⁻¹in monomorphic whitefish lakes. In general, polymorphism tended to increase the density and biomass of whitefish in the pelagic area compared with monomorphic systems.