Fish assemblages in forest drainage ditches: Degraded small streams or novel habitats?

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Abstract Artificial drainage of forested wetlands to increase timber production has profoundly altered the hydrology of North-European landscapes during the 20th century. Nowadays, drainage ditches and small dredged streams can comprise most fluvial water bodies there, but the resulting ecological effects are poorly documented. In the current study, we explored, using fish as an indicator group, consequences of the transformation of natural stream networks to a mixture of natural and artificial watercourses. We asked whether the transformation results in impoverishment, enrichment or re-assembling of the communities both at watercourse and the landscape scales. We sampled fish in 98 sites in five well-forested regions in Estonia where ditches formed 83–92%, dredged streams 4–7%, and natural streams 3–10% of the total length of small watercourses. Based on a total of 6370 individual fish of 20 species, we found that, compared to natural streams, ditches had an impoverished fauna at both scales and both in terms of species richness and assemblage composition. Only natural streams hosted characteristic species (with Barbatula barbatula , Lampetra planeri and Lota lota emerging as significant indicators), while dredged streams had intermediate assemblages. The habitat factors explaining those drainage-related differences included a reduced flow velocity, loss of stream channel variability, less transparent water, and abundant aquatic vegetation. Hence, for stream-dwelling fish, drained forest landscapes represent degraded habitats rather than novel ecosystems, which contrasts with the transformation of terrestrial assemblages. Future studies should address whether that reflects the situation for whole aquatic assemblages, and how is the functioning of the hydrological systems affected. We suggest that the critical management issues for environmental mitigation of ditching effects on fish include basin scale spatial planning, protecting of the remaining natural streams, and rehabilitation of ditch channels in flat landscapes lacking beavers.