2002 Survey of Juvenile Salmon in the Marine Waters of Southeastern Alaska
September 24, 2002
Biophysical data were collected along a primary marine migration corridor of juvenile Pacific salmon (Oncorhynchus spp.) in the northern region of southeastern Alaska. Data were collected at 13 stations in four sampling intervals (25 d total) from May to August 2002. This survey marks the sixth 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 juvenile salmon. Habitats were classified as inshore (Auke Bay), strait (four stations each in Chatham Strait and Icy Strait), and coastal (four stations off 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 using a surface rope trawl (fish), conical and bongo nets (zooplankton), and a conductivity-temperature-depth profiler (physical profile), usually during daylight. Surface (2-m) temperatures and salinities ranged from 6.1 to 13.9EC and 17.4 to 32.2 PSU from May to August. A total of 8,665 fish and squid, representing 21 taxa, were captured in 75 rope trawl hauls from June to August. Juvenile salmon comprised 61% of the total catch and occurred frequently in the trawl hauls, with coho (O. kisutch) occurring in 65% of the trawls, pink (O. gorbuscha) in 57%, chum (O. keta) in 55%, sockeye (O. nerka) in 47%, and chinook salmon (O. tshawytscha) in 21%. Of the 5,336 salmonids caught, more that 98% were juveniles. Walleye pollock (Theragra chalcogramma) and crested sculpin (Blepsias bilobus) were the only non-salmonid species that comprised more than 1% of the total catch. Temporal and spatial differences were observed in the catch rates, size, condition, and stock of origin of juvenile salmon species, and in predation rates on them. Catches of juvenile chum, pink, sockeye, and coho salmon were generally highest in July, whereas catches of juvenile chinook salmon were highest in June. By habitat type, juvenile salmon catches were highest in straits. In the coastal habitat, catches were highest within 40 km of shore. Size of juvenile salmon increased steadily throughout the season; mean fork lengths in June and August were respectively: 86 and 143 mm for pink, 96 and 145 mm for chum, 121 and 139 mm for sockeye, 153 and 235 mm for coho, and 201 and 235 mm for chinook salmon. Coded-wire tags were recovered from 20 juvenile and immature salmon; most were from hatchery and wild stocks of southeastern Alaska origin; however, juvenile chinook and coho salmon from the Columbia River Basin were also recovered. In addition, otoliths were examined from four species of juvenile salmon: 1,525 from chum, 248 from sockeye, 363 from coho, and 18 from chinook salmon. Alaska hatchery stocks were identified by thermal marks from 44% of the chum, 17% of the sockeye, 5% of the coho, and 61% of the chinook salmon. Onboard stomach analysis of 135 potential predators, representing nine species, indicated five predation instances on juvenile salmon in August, including both of the age 1+ sablefish (Anoplopoma fimbria) and 3 of 12 (25%) adult coho 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 an intra- and interannual basis, 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 NiZo and La NiZa events, such as the recent warming trends that benefited 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, between intra- and interspecific competition and carrying capacity, and between stock composition and biological interactions. Past research has not provided adequate time-series data to explain such links (Pearcy 1997). Because 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 (O. keta) and wild pink salmon (O. gorbuscha) in the region. For example, two private non-profit enhancement facilities in the northern region of southeastern Alaska have produced more than 100 million otolith-marked juvenile chum salmon annually in recent years. Consequently, since the mid-1990s, average annual commercial harvests of about 14 million adult chum salmon mostly comprised of otolith-marked fish, have occurred in the common property fishery in the region (ADFG 2000). In addition, sockeye salmon (O. nerka), coho salmon (O. kisutch), and chinook salmon (O. tshawytscha) are otolith marked by some enhancement 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 the northern region of southeastern Alaska, known as the Southeast Coastal Monitoring Project (SECM), was initiated in 1997 and repeated from 1998 to 2002 (Orsi et al. 1997, 1998, 2000, 2001, 2002) 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, 2002). This document summarizes biophysical data collected by SECM scientists from May to August 2002 in southeastern Alaska.