Maximum Size of Chinook Salmon Consumed by Juvenile Coho Salmon

Abstract

We evaluated predation of coho salmon Oncorhynchus kisutch on wild fall chinook salmon O. tshawytscha by conducting feeding trials in partitioned fiberglass troughs and in outdoor enclosures. Our goal was to determine the maximum size of fall chinook salmon that juvenile coho salmon could, or would attempt to, consume. In one experiment, coho salmon (135–171 mm) stocked in each of eight cells (41 cm × 41 cm × 32 cm deep) with flowing water for 29 d consumed fall chinook salmon that were up to 40–46% of their length. In a second experiment, coho salmon (129–149 mm) ate fall chinook salmon stocked into a cell (328 cm × 41 cm × 32 cm deep) that were up to 41–47% of their length. In a third experiment, coho salmon attacked imitation fall chinook salmon lures ranging in size from 35 to 115 mm at rates that decreased with increasing lure size. Our results suggest that risks of predation on fall chinook salmon caused by stocking coho salmon can be reduced by stocking at times when fall chinook salmon have a size refuge from predation.