SPATIAL VARIATION AND HISTORICAL CHANGE IN FISH COMMUNITIES OF THE SCHUYLKILL RIVER DRAINAGE, SOUTHEAST PENNSYLVANIA

The use of fish communities to evaluate stream habitat typically requi res that the community at a target site be compared with concurrently or recently sampled reference communities at nearby sites. For this ap proach to be effective, spatial and temporal variation in the referenc e communities of a region must both be known. This study relates 307 r ecent (1976-1994) fish survey records in the Schuylkill River drainage , southeastern Pennsylvania, to four concurrent measures of water chem istry (pH, conductivity, alkalinity hardness), two measurements of str eam size (stream order, stream width) and three variables describing s ite position within the drainage network (elevation, link number and c link number defined herein). Based on canonical correspondence analysi s, stream size and position within the watershed influenced fish speci es composition more strongly than did water chemistry. Species richnes s, dominated by the families Cyprinidae (minnows) and Centrarchidae (s unfishes), increased from a median of four species in ist-order stream s to 21 species in 5th-order streams. Secondly, recent frequencies of occurrence are compared to frequencies inferred from historical collec tions using log-linear analysis. Nine species have increased in freque ncy, while the frequencies of eight species have declined. Eighteen sp ecies, or about 1/3 of the recent community, have been introduced. All but three of these introduced species were absent from collections be fore 1931, and many are now widespread. Several previously common spec ies associated with clear, vegetated streams are now uncommon or rare (e.g., Esox americanus americanus, Notropis bifrenatus); one species, N. chalybaeus, was not found in the recent dataset Substantial histori cal changes in the fish community thus accompany the influences of str eam habitat and location in determining species composition at sites w ithin the drainage.