Effects of regional dispersal on local fish-environment relations

If organismal movements extend beyond the boundaries of sampling sites, “regional” dispersal may confound the effects of “local” environmental conditions. We compared fish-environment relations within streams that flow into rivers (>250 km2 upstream basin area) within 10 fluvial km against streams that lack riverine connectivity within this distance (i.e., mainstem tributaries and headwater tributaries, respectively). We predicted that riverine dispersal would tend to diminish fish-environment relations in mainstem tributaries but not in headwater tributaries. We evaluated fish and habitat data from the US Environmental Protection Agency's Regional Environmental Monitoring and Assessment Program in West Virginia, USA (n = 100). Non-metric multidimensional scaling ordinations revealed that mainstem tributaries and headwater tributaries supported similar environmental quality gradients but dissimilar fish-environment relations: fish metrics in headwater tributaries generally showed greater correspondence to local environmental conditions than in mainstem tributaries. Riverine connectivity affected the performance of trophic metrics more than other metric categories (i.e., taxonomy, reproductive strategy, and degradation tolerance). Our results suggest that intra-species variation in the spatial structure of fish metapopulation dynamics or life history expression may influence the strength of local fish-environment relations.