Predation on Juvenile Salmon (Oncorhynchus spp.) In Downstream Migrant Traps in Prairie Creek, California

Downstream migrant traps are a widely applied fishery management tool for sampling anadromous salmon and steelhead smolts along the west coast of North America and elsewhere, yet predation on juvenile salmonids in traps has not been studied quantitatively.  We assessed the frequency of occurence and abundance of  juvenile salmonids in stomachs of coastal cutthroat trout (Oncorhynchus clarkii clarkii), coho salmon (O. kisutch), steelhead (O. mykiss), and prickly sculpin (Cottus asper) >70 mm FL captured in traps and in nearby stream habitats.  All four predator species took juvenile salmonids with much greater frequency when in traps than in stream habitats.  Among free-swimming predators, only coastal cutthroat trout were observed with salmonid fry in their stomachs, but they took much fewer salmonid prey and appeared to rely more heavily on insect prey than did cutthroat trout captured in traps.   Predators consumed up to 25% of the available prey over a broad range of prey abundances.   Over the course of the study, predators consumed 2.5% of all salmonid fry captured in traps, but this fraction ranged from < 1% to >10% in any given year.  The number of prey taken in traps increased with predator length and prey abundance in traps and peaked during the period of most intense downstream migration by salmon fry.   Live box design and trap location had little or no effect on the total number of prey taken by individual predators.  We estimate that predators in traps caused an additional 0.5% to 1% mortality of juvenile salmon (a 9–10 % relative increase over natural predation rates). We found no evidence that predators selected for prey on the basis of species.  These results should motivate additional research on methods that reduce or eliminate predation in trap live-boxes and protocols for efficiently measuring predation associated with downstream migrant trapping.