Size, connectivity and edge effects of stream habitats explain spatiotemporal variation in brown trout (Salmo trutta) density

Ecological theory postulates that size and isolation of habitat patches impact the colonization/extinction dynamics that determine community species richness and population persistence. Given the key role of lotic habitats for life history completion in rheophilic fish, evaluating how the distribution of swift-flowing habitats affects the abundance and dynamics of subpopulations is essential. Using extensive electrofishing data, we show that merging island biogeography with meta-population theory, where lotic habitats are considered as islands in a lentic matrix, can explain spatiotemporal variation in occurrence and density of brown trout (Salmo trutta). Subpopulations in larger and less isolated habitat patches had higher average densities and smaller between-year density fluctuations. Larger habitat patches also had lower predicted risk of excessive zero catches, indicative of lower extinction risk. Trout density further increased with distance from the edge of adjacent lentic habitats with predator (Esox lucius) presence, suggesting that edge- and matrix-related mortality contributes to the observed patterns. These results can help reduce the negative impacts that habitat loss and fragmentation have on biodiversity, by stressing the importance of suitable habitat size and connectivity, and aid in prioritization of habitat restoration, dam removal and reintroduction programs aimed at vitalizing declining and locally extinct riverine fish populations.

Methods
Data on fish presence and density was generated through electrofishing, a non-lethal fish sampling method mainly conducted in streams where it is possible to wade. Electric current (DC) is used to attract fish to swim towards a hand-held anode where they are caught with a dipping net. It is an established and reliable method for quantifying fish density. See Bohlin et al (1989) and the Swedish and European Standard (SIS. SS‐EN 14011:2003: Water quality – Sampling of fish with electricity. Stockholm: Swedish Standard Institute; 2003.) for detailed descriptions on the method.

Bohlin, T., Hamrin, S., Heggberget, T. G., Rasmussen, G., & Saltveit, S. J. (1989). Electrofishing - Theory and practice with special emphasis on salmonids. Hydrobiologia, 173(1), 9–43. https://doi.org/10.1007/BF00008596