2000 Survey of Juvenile Salmon in the Marine Waters of Southeastern Alaska
Biophysical data were collected along a primary marine migration corridor of juvenile Pacific salmon (Oncorhynchus spp.) in the northern region of southeastern Alaska at 20 stations in five, six-day sampling intervals from May to September 2000. This survey marks the fourth consecutive year of systematic monitoring, and was implemented to identify the relationships among biophysical parameters that influence the habitat use, marine growth, predation, stock interactions, year-class strength, and ocean carrying capacity of salmon. Habitats were classified as inshore (Taku Inlet and Auke Bay), strait (Chatham Strait and Icy Strait), and coastal (Cross Sound and Icy Point), and were sampled from the National Oceanic and Atmospheric Administration ship John N. Cobb. At each station, fish, zooplankton, surface water samples, and physical profile data were collected during daylight using a surface rope trawl, conical and bongo nets, and a conductivity-temperature-depth profiler. Surface (2-m) temperatures and salinities during the survey ranged from 6.6 to 14.1°C and 11.5 to 32.0 PSU. A total of 7,920 fish and squid, representing 30 taxa, were captured in 89 rope trawl hauls from June to September. Juvenile Pacific salmon comprised 86% of the total catch and were the most frequently occurring species: pink (O. gorbuscha; 60%), chum (O. keta; 55%), coho (O. kisutch; 49%), sockeye (O. nerka; 47%), and chinook salmon. Of the 6,846 salmonids caught, > 99% were juveniles. Non-salmonid species making up > 2% of total catch included walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasi), and soft sculpin (Psychrolutes sigalutes). Temporal and spatial differences were observed in the catch rates, size, condition, stock of origin, and predation rates of juvenile salmon species. Catches of juvenile chum, pink, and coho salmon were highest in July, whereas catches of juvenile sockeye and chinook salmon were highest in June and September, respectively. By habitat type, juvenile salmon except chinook were most abundant in straits; juvenile chinook salmon were most abundant in inshore habitat. In the coastal habitat, catches along the Icy Point transect were highest within 40 km of shore. Size of juvenile salmon increased steadily throughout the season; mean fork lengths (mm) in June and September were: pink (95 and 198), chum (106 and 218), sockeye (114 and 196), coho (166 and 285), and chinook salmon (157 and 264). Coded-wire tags (CWTs) were recovered from seven juvenile and one immature chinook; only one was of non-Alaska origin, a juvenile chinook from the Columbia River Basin recovered in September. CWTs were recovered from seven juvenile and two adult coho; all were of Alaska origin. In addition, otoliths of 1,260 juvenile chum and 401 juvenile sockeye salmon revealed that 59% and 27% of these fish were Alaska hatchery stocks represented by thermal marks. Onboard stomach analysis of 214 potential predators, representing eleven species, indicated that 11% of adult coho salmon, 4.5% of spiny dogfish (Squalus acanthias), and 1% of adult walleye pollock preyed on juvenile salmon. Our results 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 interannual bases, will enable researchers to understand how growth, abundance, and ecological interactions affect year-class strength and ocean carrying capacity for salmon.
Studies of the early marine ecology of Pacific salmon (Oncorhynchus spp.) in Alaska require adequate time series of biophysical data to relate climate fluctuations to the distribution, abundance, and production of salmon. Because salmon are keystone species and constitute important ecological links between marine and terrestrial habitats, fluctuations in the survival of this important living marine resource have broad ecological and socio-economic implications for coastal localities throughout the Pacific Rim. Increasing evidence for relationships between production of Pacific salmon and shifts in climate conditions has renewed interest in processes governing salmon year-class strength (Beamish 1995). In particular, climate variation has been associated with ocean production of salmon during El Niño and La Niña events, such as the recent warming trends that benefitted many wild and hatchery stocks of Alaskan salmon (Wertheimer et al. 2001). However, research is lacking in areas such as the links between salmon production and climate variability, the links between intra- and interspecific competition and carrying capacity, and the links between stock composition and biological interactions. Past research has not provided adequate time-series data to explain such links (Pearcy 1997). Since the numbers of Alaskan salmonids produced in the region have increased over the last few decades (Wertheimer et al. 2001), mixing between stocks with different life history characteristics has also increased. The consequences of such changes on the growth, survival, distribution, and migratory rates of salmonids remain unknown.
To adequately identify mechanisms linking salmon production to climate change, synoptic data on stock-specific life history characteristics of salmon and on ocean conditions must be collected in a time series. Until recently, stock-specific information relied on labor-intensive methods of marking individual fish, such as coded-wire tagging (CWT; Jefferts et al. 1963), which could not practically be applied to all of the fish released by enhancement facilities. However, mass-marking with thermally induced otolith marks (Hagen and Munk 1994) has provided technological advances. The high incidence of these marking programs in southeastern Alaska (Courtney et al. 2000) offers an opportunity to examine growth, survival, and migratory rates of specific stocks during the current record production of hatchery chum salmon and wild pink salmon in the region. For example, two private non-profit enhancement facilities in the northern region of southeastern Alaska have produced over 100 million otolith-marked juvenile chum salmon (O. keta) annually in recent years. Consequently, since the mid1990s, average annual commercial harvests of about 14 million adult chum salmon have occurred in the common property fishery in the region (ADFG 2000), mostly comprised of otolith-marked fish. In addition, sockeye salmon (O. nerka) are marked by some state of Alaska facilities. Examining the early marine ecology of these marked stocks provides an unprecedented opportunity to study stock-specific abundance, distribution, and species interactions of the juveniles that will later recruit to the fishery.
This coastal monitoring study in northern southeastern Alaska, known as Southeast Coastal Monitoring Project (SECM), was initiated in 1997 and repeated in 1998 and 1999 (Orsi et al. 1997, 1998, 2000), to develop our understanding of the relationships between annual time series of biophysical data and stock-specific information. Data collections from prior years have been reported in several documents (Murphy and Orsi 1999; Murphy et al. 1999; Orsi et al. 1999; 2001). This document summarizes data collected by SECM scientists on biophysical parameters from May-September 2000 in southeastern Alaska.