Potential Influences of Individual Mobility and Habitat Use on Vulnerability of Stream Fish Populations to Habitat Fragmentation

The adaptive significance of inter-habitat movements by fishes generally is linked to seeking resources needed to complete life cycles; essential habitat-types include those facilitating survival, growth, and reproduction. Inter-habitat movement can satisfy a fish’s need to find complementary (distinct habitat-type) or supplementary (replicate habitat-type) habitats. Species vary greatly in the habitat configurations they need, while habitat-types vary greatly in their size and distribution across catchments. For example, the replication of reach-scale habitats decreases downstream through a catchment while the absolute size of, and distance between, channel units increases downstream. We expect variation in fish life-history and habitat distribution to drive variation in the frequency-distribution of species-specific movement distances (a metric of mobility). Factors selecting for greater mobility include body size, distance between habitat patches, and environmental instability. Anthropogenic fragmentation can affect populations by severing connections between supplementary or complementary habitats but we expect its effects to be strongest when it inhibits access to complementary habitats (needed to complete life cycles). However, the complex interface of fish movement and habitat use has precluded the emergence of a comprehensive conceptual framework for making predictions about fish mobility in a given habitat template or about fragmentation effects on populations due to impaired individual mobility. This situation leads to contradictory but plausible expectations regarding patterns of  stream-fish movement. For example, we might expect headwater species to be more mobile than riverine species because the former occupy widely dispersed reach-scale habitats and are adapted to environmental instability. Alternatively, we might expect headwater species to be less mobile (than riverine species) because they are smaller and their channel–unit habitats are closer together. To explore these expectations, we surveyed the recent literature on movement by non-migratory, non-salmonid stream fishes of North America. We found movement data for >50 species, varying widely in ecological traits. We will present our analysis of empirical relations between species traits and mobility, and discuss implications for population vulnerability to fragmentation.