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.