2001 Survey of Juvenile Salmon in the Marine Waters of Southeastern Alaska

Introduction

Studies of the early marine ecology of Pacific salmon 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. In particular, climate variation has been associated with ocean production of salmon during El Nino and La Nina events, such as the recent warming trends that benefitted many wild and hatchery stocks of Alaskan salmon. 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. Since the numbers of Alaskan salmonids produced in the region have increased over the last few decades, 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, which could not practically be applied to all of the fish released by enhancement facilities. However, mass-marking with thermally induced otolith marks has provided technological advances. The high incidence of these marking programs in southeastern Alaska offers an opportunity to examine growth, survival, and migratory rates of specific stocks during the current record production of hatchery churn 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 churn salmon annually in recent years. Consequently, since the mid-l 990s, average annual commercial harvests of about 14 million adult churn salmon have occurred in the common property fishery in the region (ADFG 2000), mostly comprised of otolith-marked fish. In addition, sockeye salmon, coho salmon, and chinook salmon are also mass 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 northern southeastern Alaska, known as Southeast Coastal Monitoring Project (SECM), was initiated in 1997 and repeated from 1998 to 2001, 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. This document summarizes data collected by SECM scientists on biophysical parameters from May-September 2001 in southeastern Alaska.

Summary Of Results

Biophysical data were collected along a primary marine migration corridor of juvenile Pacific salmon in the northern region of southeastern Alaska at 13 stations in five sampling intervals (27 d total) from May to September 2001. This survey marks the fifth consecutive year of systematic monitoring, and was implemented to identify the relationships among biophysical parameters that influence the habitat use, marine growth, predation on, 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, conical and bongo nets, and a conductivity-temperature-depth profiler, respectively, usually during daylight. Surface (2-m) temperatures and salinities ranged from 7.0 to 14.1 °C and 18.0 to 32.2 PSU from May to September. A total of 52,156 fish and squid, representing 24 taxa, were captured in 108 rope trawl hauls from June to September. Juvenile Pacific salmon comprised 11 % of the total catch and were the most frequently occurring species: chum (0. keta; 78%), pink (0. gorbuscha; 73%), sockeye (0. nerka; 71 %), coho (0. kisutch; 65%), and chinook salmon (0. tshawytscha; 43%). Of the 5,979 salmonids caught,> 97% were juveniles. Non-salmonid species making up > 1 % oftotal catch included walleye pollock, eulachon, and soft sculpin. 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 September. 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 (mm) in June and September were: pink (93 and 203), chum (96 and 201), sockeye (119 and 178), coho (164 and 259), and chinook salmon (202 and 255). Coded-wire tags were recovered from 40 juvenile, immature, and adult salmon; all were of Alaska origin. In addition, otoliths were examined from four species of juvenile salmon: 1,157 chum, 383 sockeye, 407 coho, and 69 chinook salmon; Alaska hatchery stocks were identified by thermal marks from 30%, 12%, 11 %, and 74% of these species, respectively. Onboard stomach analysis of 235 potential predators, representing ten species, indicated juvenile salmon predation by 27% of the adult spiny dogfish, 14% of the adult coho salmon, and 8% of the adult Pomfret. 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 inter-annual bases, will enable researchers to understand how growth, abundance, and ecological interactions affect year-class strength and ocean carrying capacity for salmon.