MECHANISMS STRUCTURING STREAM FISH ASSEMBLAGES - ARE THERE LESSONS FROM INTRODUCED SPECIES
Stream fish faunas are being increasingly subjected to non-native fishes. Successful establishment of non-native forms varies widely between geographic regions (38-77%); but is generally greater in areas that are either altered by man or initially depauperate in fish species. While such introductions have generally had undesirable effects on native fish species, they show no signs of decreasing. Thus, this essay is an attempt to determine if the introduction of non-native fishes may provide insights into processes structuring stream fish assemblages. There are, unfortunately, few detailed ecological studies documenting the response of stream fish assemblages to introduced fishes. Of the 31 studies included in my analysis, the majority (77%) documents a decline in native fishes following the introduction of exotic or transplanted forms. Only 10 studies examined shifts in resource use, of which 5 documented or suggested shifts in habitat. Overall, the responses of native stream fish assemblages to non-native fishes suggest that biotic interactions are important in structuring the impacted assemblages, being implicated in 62% of the studies. Proposed or demonstrated mechanisms are divided equally between competition and predation. Unfortunately, pre-disturbance data are generally lacking so that processes important in structuring native fish assemblages cannot be determined.