2007 Annual Survey of Juvenile Salmon and Ecologically Related Species and Environmental Factors in the Marine Waters of Southeastern Alaska
September 24, 2007
Juvenile Pacific salmon (Oncorhynchus spp.), ecologically-related species, and associated biophysical data were collected along primary marine migration corridors in the northern and southern regions of southeastern Alaska in 2007. Up to 17 stations were sampled in epipelagic waters over four time periods (27 sampling days) from May to August. This survey marks 11 consecutive years of systematically monitoring 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. Typically, 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 during daylight. Surface (3-m) temperatures and salinities ranged from 7.7 to 15.3 ºC and 12.3 to 30.6 PSU from May to August. A total of 48,170 fish and squid, representing 17 taxa, were captured in 97 rope trawl hauls from June to August. Juvenile salmon comprised about 7% of the total fish and squid 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 51-92% of the trawls in the southern and northern regions, whereas juvenile Chinook salmon (O. tshawytscha) occurred in about 23% of the hauls. Of the 3,412 salmonids caught, over 97% were juveniles. Only two non-salmonid species represented catches of >30 individuals in either region: Pacific herring (Clupea pallasi) in the southern region (n = 44,637) and crested sculpin (Blepsias bilobus) in the northern region (n = 34). Catch rates of juvenile salmon in both regions were generally highest in June for all species except pink salmon. However, in the more extended, 11-yr time series in the northern region, juvenile pink salmon catches were among the lowest observed in June and July 2007, suggesting a poor adult return in the subsequent year. Mean size of juvenile salmon generally increased from June to July; however, condition residuals were lower than the longterm average for most species. Coded-wire tags were recovered from 14 juvenile coho salmon and five Chinook salmon (1 juvenile and 4 immature). All but one fish were from hatchery and wild stocks originating in southeastern Alaska. The non-Alaskan stock was a Chinook salmon that originated from the Upper Columbia River. Alaska enhanced stocks were also identified by thermal otolith marks from 67% of the chum and 4% of the sockeye salmon examined. Onboard stomach analysis of 95 potential predators, representing 8 species, did not provide evidence of predation on juvenile salmon. This research suggests that in southeastern Alaska, juvenile salmon exhibit seasonal patterns of habitat use and display species- and stock-dependent migration patterns. This third season of comparing biophysical parameters between the northern and southern regions of southeastern Alaska suggests that summer conditions differ between the regions. 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 role 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 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 with the ocean conditions they experience. In the past, 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), a technological advance implemented in many parts of Alaska by high numbers of enhancement facilities, enables researchers to collect stock specific data, including growth, survival, and migratory rates, in southeastern Alaska (Courtney et al. 2000). 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 (O. keta). Consequently, since the mid-1990s, commercial harvests of adult chum salmon in the common property fishery in the region have averaged about 12 million fish annually, with an ex-vessel commercial value of 27 million $U.S. (ADFG 2008), and have included 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.
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 interactions in the marine environment (Reese et al. In Review; Seeb et al. 2004). 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); and 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), suggesting 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 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 (Sturdevant et al. 2004; Weitkamp and Sturdevant 2008). These findings stress the importance of examining the entire epipelagic community of ichthyofauna in the context of trophic interactions.
This is the third year that the SECM research scope has 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. This study is currently proposed for continued funding over a 3-year period by the Northern Fund of the Pacific Salmon Commission. A primary focus is to explore the concordance of adult pink salmon harvests in both the southern and northern regions of southeastern Alaska with biophysical parameters such as juvenile abundance, temperature, and zooplankton abundance in each region.