2004 Survey of Juvenile Salmon and Associated Epipelagic Ichthyofauna in the Marine Waters of Southeastern Alaska
September 24, 2004
Juvenile Pacific salmon (Oncorhynchus spp.), ecologically-related species, and associated biophysical data were collected along a primary marine migration corridor in the northern region of southeastern Alaska. Thirteen stations were sampled over six time periods (31 sampling days) from May to August 2004. This survey marks the eighth consecutive year of systematic monitoring on how juvenile salmon interact in marine ecosystems, and was implemented to identify the relationships among biophysical parameters that influence the habitat use, marine growth, predation, stock interactions, and year-class strength of salmon. Habitats sampled included stations in inshore (Auke Bay and Taku Inlet), strait (four stations each in Chatham Strait and Icy Strait), and coastal (four stations off Icy Point) localities. At each station, fish, zooplankton, surface water samples, and physical profile data were collected using a surface rope trawl, conical and bongo nets, water sampler, and a conductivity-temperature-depth profiler, usually during daylight. Surface (3-m) temperatures and salinities ranged from 6.9 to
17.4 ºC and 9.5 to 31.6 PSU from May to August. A total of 13,460 fish and squid, representing 29 taxa, were captured in 75 rope trawl hauls from June to August. Juvenile salmon comprised 48% of the total catch and occurred frequently in the trawl hauls, with pink (O. gorbuscha) occurring in 75% of the trawls, sockeye (O. nerka) in 73%, chum (O. keta) in 72%, coho (O. kisutch) in 51%, and chinook salmon (O. tshawytscha) in 19%. Of the 6,552 salmonids caught, over 99% were juveniles. Walleye pollock (Theragra chalcogramma) and Pacific herring (Clupea pallasi) 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. Catch rates of juvenile salmon were generally highest in June for all species except coho that had catch rates highest in August. Between habitat types, juvenile salmon catch rates were almost always highest in the strait habitat for each species and in each time period. Size of juvenile salmon increased steadily throughout the season; mean fork lengths in June, July, and August were, respectively: 98, 129, and 163 mm for pink; 104, 139, and 166 mm for chum; 111, 137, and 165 mm for sockeye; 170, 203, and 246 mm for coho; and 199, 228, and 279 for chinook salmon. Coded-wire tags were recovered from 14 juvenile coho, three juvenile and six immature chinook salmon; all but one were from hatchery and wild stocks of southeastern Alaska origin. The non-Alaska stock was a juvenile chinook originating from Oregon. Alaska hatchery stocks were also identified by thermal otolith marks from 74% of the chum, 18% of the sockeye, 9% of the coho, and 45% of the chinook salmon. Onboard stomach analysis of 199 potential predators, representing 10 species, revealed four predation instances on juvenile salmon: three by adult coho salmon and one by an immature chinook salmon. This research suggests 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, on both intra- and interannual bases, will enable researchers to understand how growth, abundance, and ecological interactions affect year-class strength and to better understand the role salmon play in North Pacific marine ecosystems.
The Southeast Coastal Monitoring Project (SECM), a coastal monitoring study in the northern region of southeastern Alaska, was initiated in 1997 to annually study the early marine ecology of Pacific salmon (Oncorhynchus spp.) and associated epipelagic ichthyofauna to better understand effects of environmental change on salmon production. Salmon are a keystone species that constitute important ecological links between marine and terrestrial habitats, and therefore play a significant, yet poorly understood, role in marine ecosystems. 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 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 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.
One SECM goal is to identify mechanisms linking salmon production to climate change using a time series of synoptic data that combines stock-specific life history characteristics of salmon and their ocean conditions. 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) is a technological advance implemented in many parts of Alaska. 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 salmon stocks during high levels regional hatchery production of chum salmon (O. keta) and historically high returns of wild pink salmon (O. gorbuscha). For example, in recent years, two private non-profit enhancement facilities in the northern region of southeastern Alaska annually produced more than 150 million otolith-marked juvenile chum salmon. Consequently, since the mid-1990s, commercial harvests of adult chum salmon in the common property fishery in the region have averaged about 12.5 million fish annually (ADFG 2003), including a high proportion of otolith-marked fish from regional enhancement facilities. In addition, sockeye salmon (O. nerka), coho salmon (O. kisutch), and chinook salmon (O. tshawytscha) are otolith-marked by some enhancement facilities. Therefore, examining the early marine ecology of these marked stocks provides an opportunity to study stock-specific abundance, distribution, and species interactions of juvenile salmon that will later recruit to the fishery.
Increased hatchery production of juvenile salmon in southeastern Alaska has raised concern over potential hatchery and wild stock interactions during their early marine residence. A recent study using a bioenergetics approach and SECM data from Icy Strait concluded that hatchery and wild stocks consumed only a small percentage of the available zooplankton (Orsi et al. 2004); this study also suggested that abundant vertically migrating planktivores have a greater impact on the zooplankton standing stock than hatchery stock groups of chum salmon. These findings stress the importance of examining the entire epipelagic community of ichthyofauna in the context of trophic interactions.
This document summarizes catches of juvenile salmon, ecologically-related species, and the associated biophysical data collected by SECM scientists in 2004.