2008 Annual Survey of Juvenile Salmon, Ecologically-Related Species, and Environmental Factors in the Marine Waters of Southeastern Alaska
Juvenile Pacific salmon (Oncorhynchus spp.), ecologically-related species, and associated biophysical data were collected from the marine waters of the northern region of southeastern Alaska in 2008. This annual survey marks 12 consecutive years of systematically monitoring how juvenile salmon interact in marine ecosystems, and was implemented to identify the relationships among biophysical parameters that influence habitat use, marine growth, predation, stock interactions, and year-class strength of juvenile salmon. This report summarizes findings from the 2008 survey year, and contrasts these findings to selected biophysical parameters of the prior 11 sampling years. Up to 13 stations were sampled in epipelagic waters over four time periods (20 sampling days) from May to August. Typically, 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, water sampler, and a conductivity-temperature-depth profiler. Surface (3-m) temperatures and salinities ranged from 6.8 to 11.6 ºC and 18.2 to 32.0 PSU from May to August. A total of 5,186 fish, representing 16 taxa, were captured in 56 rope trawl hauls from June to August. Juvenile salmon comprised about 97% of the total fish catch. Juvenile salmon occurred frequently in the trawl hauls, with pink (O. gorbuscha), chum (O. keta), sockeye (O. nerka), and coho salmon (O. kisutch) present in 6686% of the trawls, whereas juvenile Chinook salmon (O. tshawytscha) occurred less commonly, in about 39% of the hauls. Exceptionally few juvenile salmon were captured in June. Peak monthly catch rates of juvenile salmon differed by species: pink, chum, and coho were highest in July, whereas sockeye and Chinook were highest in August. Coded-wire tags were recovered from 11 juvenile coho salmon and three Chinook salmon (one juvenile and two immature). All fish were from hatchery and wild stocks originating in southeastern Alaska. Alaska enhanced stocks were also identified by thermal otolith marks from 39% of the chum and 4% of the sockeye salmon examined. Onboard stomach analysis of 20 potential predators, representing four species, did not provide evidence of predation on juvenile salmon. Biophysical measures from 2008 differed from prior years, in many respects. Integrated (20-m) temperatures and salinities were anomalously low and zooplankton densities were anomalously high in 2008. In addition, for most juvenile salmon species, unusual CPUE patterns, small fish size, and low condition residuals suggested that migration timing shifted to later than average. Long-term monitoring of key stocks of juvenile salmon, on seasonal and interannual time scales, will enable researchers to understand how growth, abundance, and ecological interactions affect year-class strength of salmon and to better understand their roles in North Pacific marine ecosystems.
The Southeast Coastal Monitoring Project (SECM), a coastal monitoring study focused 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 and to better understand effects of environmental change on salmon production. Salmon are a keystone species that constitute an important ecological link 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.
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; Downton and Miller 1998; Beauchamp et al. 2007; Taylor 2007). 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). Biophysical attributes of climate and habitat, such as temperature, salinity, and mixed layer depth (MLD), influence primary and secondary production (Bathen 1972; Kara et al. 2000; Alexander et al. 2001) and therefore influence the trophic links leading to variable growth and survival of salmon (Mann and Lazier 1991; Francis and Hare 1994; Brodeur et al. 2007). However, research is lacking in several 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 number of salmonids produced in the region have increased over the last few decades, understanding the consequences of these population changes on the growth, distribution, migratory rates, and survival of all salmon stock groups is important.
One SECM goal is to identify mechanisms linking salmon production to climate change using a time series of synoptic data that combines stock-group life history characteristics of salmon with the ocean conditions they experience. In the past, stock 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), a technological advance frequently implemented by enhancement facilities throughout Alaska, enables researchers to collect stock-specific data, including growth, survival, and migratory rates, in southeastern Alaska (Courtney et al. 2000). For example, two private non-profit enhancement facilities in the northern region of southeastern Alaska produced more than 150 million otolith-marked juvenile chum salmon (O. keta) in recent years. Consequently, a high proportion of these otolith-marked fish have been included in commercial harvests of adult chum salmon in the common property fishery of the region since the mid-1990s, and have contributed substantially to the average annual catch of 12 million fish and the ex-vessel commercial value of 27 million $U.S. (Alaska Department of Fish and Game [ADFG] 2008). 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.
The extent of interactions between hatchery and wild salmon stocks in marine ecosystems is also important to examine. Increased hatchery production of juvenile chum salmon has coincided with declines of some wild chum salmon stocks, suggesting the potential for hatchery and wild stock competition or other interactions in the marine environment (Seeb et al. 2004; Reese et al. 2009). A study using a bioenergetics approach and SECM data from Icy Strait concluded that hatchery and wild stocks of juvenile salmon consumed only a small percentage of the available zooplankton during their summer residence (Orsi et al. 2004a). Since feeding indices remained high for juvenile pink (O. gorbuscha), chum, and coho salmon throughout the diel cycle and summer season (Sturdevant et al. 2002, 2004, 2008), this suggests that growth of the fish was not food-limited. The bioenergetics study also suggested that vertically-migrating planktivores may have a greater impact on the zooplankton standing stock than hatchery stock groups of chum salmon, including abundant forage species such as walleye pollock (Theragra chalcogramma) and Pacific herring (Clupea pallasi) (Sigler and Csepp 2007). Companion studies in Icy Strait suggested that the amount of food consumed may be more important to survival of juvenile salmon con-specifics than the type of food consumed (Sturdevant et al. 2004; Weitkamp and Sturdevant 2008) and that predation events may affect salmon year class strength (Sturdevant et al. 2009). These findings stress the importance of consistently examining the entire epipelagic community of ichthyofauna in the context of trophic interactions.
In previous years, when NOAA vessel support was available, the SECM research scope also included sampling in the southern region of southeastern Alaska. This regional study component was added to the SECM project to support an increased emphasis on forecasting of adult pink salmon returns and to understand regional differences in prey, competitor, and predation dynamics. This study component supplements the core sampling of eight stations in the strait habitat of the northern region, and geographically broadens the monitoring to include the strait habitat in the southern region which encompasses a migration corridor at the opposite end of southeastern Alaska. A primary focus of this component is to explore the concordance of adult pink salmon harvests in both the southern and northern regions of southeastern Alaska in conjunction with biophysical parameters such as juvenile abundance, temperature, and zooplankton abundance in each region.
This document summarizes catches of juvenile salmon, ecologically-related species, and associated biophysical data collected by SECM scientists in 2008, and contrasts key parameters from 2008 with the entire 12-yr time series. This study has been partially funded by the Northern Fund of the Pacific Salmon Commission, and the Alaska Sustainable Salmon Fund of the ADFG.