Effects of non-native brook trout (Salvelinus fontinalis) on threatened juvenile Chinook salmon (Oncorhynchus tshawytscha) in an Idaho stream

Macneale KH, Sanderson BL, Courbois J-YP, Kiffney PM. Effects of non-native brook trout ( Salvelinus fontinalis ) on threatened juvenile Chinook salmon ( Oncorhynchus tshawytscha ) in an Idaho stream.
Ecology of Freshwater Fish 2010: 19: 139–152. © 2009 John Wiley & Sons A/S
Abstract –  Non-native fishes have been implicated in the decline of native species, yet the mechanisms responsible are rarely apparent. To examine how non-native brook trout may affect threatened juvenile Chinook salmon, we compared feeding behaviours and aggressive encounters between these sympatric fish in Summit Creek, Idaho. Snorkelers observed 278 focal fish and examined diets from 27 fish in late summer 2003. Differences in feeding behaviours and diets suggest that there was minimal current competition for prey, although individual Chinook feeding activity declined as their encounter rate with other fish increased. While difference in size between fish generally determined the outcome of encounters (larger fish 'winning'), it was surprising that in some interspecific encounters aggressive Chinook displaced slightly larger brook trout (≤20 mm longer). We suggest that in late summer, frequent intraspecific interactions may be more important than interspecific interactions in potentially limiting Chinook growth in Summit Creek and perhaps in other oligotrophic streams where they co-occur.