1999 Survey of Juvenile Salmon in the Marine Waters of Southeastern Alaska
September 24, 1999
In 1999, biophysical data were collected along a primary marine migration corridor of juvenile (age-.0) Pacific salmon (Oncorhynchus spp.) in the northern region of southeastern Alaska at 24 stations during five sampling intervals. This survey marks the third consecutive year of systematic sampling and monitoring within the region, and was implemented to identify the relationships among the habitat use, marine growth, predation, stock interactions, year-class strength, and ocean carrying capacity of salmon. Stations were stratified into three different habitats—inshore (Taku Inlet and three stations near Auke Bay), strait (four stations each at Chatham Strait and Icy Strait), and coastal (four stations each at Cross Sound, Icy Point, and Cape Edward)—and were sampled from the National Oceanic and Atmospheric Administration ship John N. Cobb, May to October. At each station, fish, zooplankton and surface water samples, and temperature and salinity profile data were collected during daylight using a surface rope trawl, conical and bongo nets, and a conductivity, temperature, and depth profiler.
Surface (2-m) temperatures and salinities during the survey ranged from 6.5 to 13.6 °C and 13.9 to 32.0 PSU. A total of 7,204 fish and squid, representing 29 taxa, were captured with the rope trawl. Five species of juvenile Pacific salmon comprised 47% of the total catch. Of the 3,535 salmonids caught, > 95% were juveniles, and < 5% were immatures or adults. Non-salmonid species making up > 2% of the catch included capelin (Mallotus villosus), Pacific herring (Clupea harengus), and sablefish (Anoplopoma fimbria).
Temporal and spatial differences were observed among juvenile salmon species in their catch rates, size, condition, stock of origin, and in predation rates. The most frequently occurring species in the trawl catches (> 25%) were coho salmon (O. kisutch), chum salmon (O. keta), pink salmon (O. gorbuscha), sockeye salmon (O. nerka), and walleye pollock (Theragra chalcogramma). Catches of juvenile salmon were highest in June for coho and in July for other species. By habitat type, juvenile salmon were most abundant in strait and inshore habitats, with the exception of sockeye salmon in coastal habitats. In the coastal habitat, catch rates along the 65 km transect at Icy Point were the highest within 25 km of shore. Lengths of juvenile salmon increased seasonally; in the five time periods mean FL (mm) were: pink (97–115–134–168), chum (103–130–139–165), sockeye (123–145–122–163), coho (154–207–225–279), and chinook salmon (139–160–190–214). Coded-wire tags were recovered from 7 juvenile and immature chinook and 15 juvenile coho salmon; all fish were of Alaska origin. In addition, Alaskan stocks were identified from thermal-marked otoliths of 447 juvenile chum and 47 juvenile sockeye salmon; marked stocks comprised 71% of the chum salmon and 19% of the sockeye salmon examined. Onboard stomach analysis of 307 fish of thirteen species of potential predators of juvenile salmon indicated that 36% of sablefish, 18% of adult coho salmon, and 10% of spiny dogfish (Squalus acanthias) preyed on juvenile salmon.
Results from this study suggest that, in southeastern Alaska, juvenile salmon exhibit seasonal patterns of habitat use synchronous with environmental change, and display species-and stock-dependent migration patterns. Long term monitoring of key stocks of juvenile salmon, both on intra- and inter-annual bases, will enable researchers to understand how growth, abundance, and ecological interactions affect year-class strength and ocean carrying capacity for salmon.