The presence of navigational lock and dam (L/D) facilities can transform a free-flowing river into a series of interconnected impoundments. Fish communities adjacent to such installations may become isolated due to the physical barriers presented by the structures and attendant habitat modifications. Immediately upstream, a lentic environment contrasts with the downstream lotic character. The purpose of this study was to assess the effects of eight fixed-crest dam L/D structures on the Allegheny River in Pennsylvania on their nearby benthic fish communities. We performed three two-minute trawls within 1000 m upstream and downstream of each structure. Fishes collected from each set of three trawls were pooled to form a composite sample for each site. Species richness, relative abundance, and the Jaccard Coefficient of Community Similarity (JCS) were calculated to compare benthic communities above and below each L/D site.

Results/Conclusions

The logperch (Percina caprodes) was the most common member of the depauperate communities upstream of L/D structures; while channel darters (Percina copelandi) dominated the diverse downstream communities. Species richness and relative abundance of benthic communities were markedly depressed upstream of each L&D structure when compared with those below, resulting in values of JCS ranging from 0 to 0.330. This study suggests that navigational lock and dam systems on large rivers may harbor distinct and possibly isolated adjacent benthic fish communities interrupting avenues of biotic connectivity. The passage of benthic fish through these navigation locks is likely fortuitous, given the low attraction of these facilities for a diversity of fishes.