ASSESSING OVERWATER STRUCTURE-RELATED PREDATION RISK ON JUVENILE SALMON: FIELD OBSERVATIONS AND RECOMMENDED PROTOCOLS

The purpose of this study was to evaluate whether conditions associated with overwater structures enhance predation pressure on juvenile salmon in Puget Sound. Washington State Ferry (WSF) terminals served as model overwater structures for exploring these issues. This document reports bird and mammal survey results from six north-central Puget Sound WSF terminals and paired reference sites over both "pre-" and >"peak" periods of outmigrating salmon fry abundance from April 1 to May 10, 2002. The authors conclude that potential salmon predators were slightly more abundant at WSF terminals as compared with unmodified shorelines, although large aggregations were not observed on any occasion. The spatial distribution patterns of both bird and fish predators rarely overlapped with juvenile salmon oriented in surface waters close to shore. The authors were unable to verify whether potential predators were more abundant during peak salmon outmigration, because salmon were available in these habitats throughout the duration of the study. The authors found no evidence that avian, marine mammal, or fish predators consumed more juvenile salmon near WSF terminals than along shorelines without overwater structures. Few species appeared to be targeting abundant fry in nearshore habitats, and the authors observed only two occasions in which predators (one tern sp., one staghorn sculpin) had consumed juvenile salmon. Several hypotheses are offered as to why the authors did not observe elevated rates of predation on juvenile salmon in the face of their greater relative availability to predators in nearshore habitats.