Assessing estuaries as stopover habitats for juvenile Pacific salmon

Habitats along migratory routes may provide key resources for migratory species (e.g. stopover habitat). For example, migratory juvenile salmon transit through estuaries on their way from freshwaters out to the ocean, but they may also reside and feed in these habitats. Here we examined the amount of time that juvenile salmon feed and reside in the estuary of the Skeena River (British Columbia, Canada), the second-largest salmon-bearing watershed in Canada. We implemented a novel application of stable isotopes of sulfur, carbon, and nitrogen as clocks to estimate the days since estuary entry. Salmon estuary residency varied across species; 25% of individuals spent at least 33, 22, 30, and 5 d in the estuary for Chinook Oncorhynchus tshawytscha, coho O. kisutch, pink O. gorbuscha, and sockeye salmon O. nerka, respectively. Larger pink and Chinook salmon resided in the estuary for longer durations, growing at an estimated 0.2 and 0.5 mm d(-1), respectively, evidence that estuary residency provides growth opportunities. A negative relationship between size and estuary residency in coho salmon suggests the potential existence of an estuary fry life history. Genetic stock assignment indicated that different populations of sockeye salmon may reside in the estuary for different amounts of time. Collectively, these results reveal that estuaries can represent stopover habitats for salmon, and that the extent varies across salmon species and populations. These data address a knowledge gap in assessment of environmental risks of proposed industrial developments. This study indicates the importance of considering the fundamental nature of habitats through which migratory species move.