Almost all data (1955-92) on offshore distribution and migration patterns of northeastern Pacific salmon (Oncorhynchus spp.) were obtained by the national research programs of the member nations of the former International North Pacific Fisheries Commission (INPFC). The results of much of the research through the early 1970s are summarized by species in five joint-comprehensive reports issued by INPFC on coho (O. kisutch; Godfrey et al. 1975), sockeye (O. nerka; French et al. 1976), chum (O. keta; Neave et al. 1976), chinook (O. tshawytscha; Major et al. 1978), and pink (O. gorbuscha; Takagi et al. 1981) salmon. More recently, reports were issued on offshore distribution and migration patterns of steelhead (O. mykiss; Burgner et al. 1992), and on the distribution and origins of salmon and steelhead in the area of the former Japanese land-based driftnet salmon fishery, south of lat. 46°N (Myers et al. 1993). The final Bulletin in the INPFC series included information on distribution of salmon and steelhead in the area of the former squid driftnet fisheries (e.g., Pella et al. 1993).
The primary objective of the historical U.S. and Canadian offshore research was to determine if the Japanese high seas driftnet fisheries were catching North American salmon. Before 1978, field research was concentrated in the times (primarily May-July) and areas of the Japanese mothership salmon driftnet fisheries, particularly in the vicinity of the INPFC abstention line along meridian 175°W, which was the eastern boundary of Japanese salmon fishing. With the implementation of U.S.S.R. and U.S. 200-mile zones in 1977-78, the times, areas, and fishing quotas of the Japanese fisheries were reduced. In 1978, the eastern boundary of the fishery was moved to long. 175°E, and research emphasis shifted to identifying the continent of origin of salmonids caught by the expanding Japanese land-based driftnet fishery in the offshore area south of 46°N. In the late 1980s and early 1990s, as the times, areas, and catch quotas of the salmon driftnet fisheries were further reduced, research emphasis shifted again to determining the bycatch of salmonids by the rapidly developing Asian squid driftnet fisheries in the area south of 46°N and west of 140°W.
The historical data are not adequate to provide a good understanding of the offshore distribution and migration patterns of northeastern Pacific salmonids. Most of the research in the Gulf of Alaska, which is the major offshore rearing area for many northeastern Pacific salmon populations, was done opportunistically during the 1950s and 1960s, primarily in spring and summer (April-August). Data from offshore areas south of the Gulf of Alaska (south of 50°N) and off the U.S. West Coast are particularly limited. Spatial and temporal distribution of sampling effort and the types of gear used to catch salmon varied. Almost all salmon catches were by surface gear (gillnet, longlines, and purse seines), and they do not provide information on vertical distribution of salmon. Sea surface temperature was often the only oceanographic data collected. There have been significant changes in abundance and composition of northeastern Pacific salmon populations since the mid-1960s (e.g., Rogers 1986). In addition, there have been major changes in North Pacific climate and oceanography since the mid-1960s (summarized by Trenberth and Hurrell 1995).
The historical data have been used to develop conceptual models of seasonal offshore distribution and migration patterns of northeastern Pacific salmon by combining observations from all gear types and years (e.g., Neave et al. 1976, French et al. 1976, Takagi et al. 1981). The models do not provide information on discrete populations or on interannual variation in distribution and migration patterns, and they need to be validated with new field data.
High seas tag recovery data (1955-95) provide limited evidence that offshore distribution and migration patterns of Pacific salmon are population-specific. The most extensive data are for sockeye salmon, which were the major focus of the historical U.S. and Canadian research programs. These data clearly show that Asian and North American sockeye have different ocean ranges (Fig. 1A). North American populations from large lake or river systems also have broadly overlapping but different ocean ranges (e.g., Bristol Bay and Fraser River, Fig. 1A). Analyses of the tag data with respect to expected returns indicated that Bristol Bay fish with different early life histories have different ocean distribution and migration patterns (Rogers 1986; Fig. 1B). The high seas tag data are not adequate to determine offshore distributions of discrete populations of northeastern Pacific salmon because the majority of recoveries were made in coastal, mixed-stock fisheries.
Many scientists have concluded from the historical data that offshore movements of Pacific salmon are not random and involve sophisticated orientation or true navigation (e.g., Quinn 1991). Tagging and tracking studies show that movements of individual adult Pacific salmon returning from offshore to coastal waters are rapid and direct (e.g., Ogura and Ishida 1995). Tagging and transplantation experiments on Atlantic salmon (Salmo salar) indicate that offshore patterns of migration and orientation of salmonids at sea are stock-specific traits (Kallio-Nyberg and Ikonen 1992, Hansen et al. 1993). Direction of homeward migration from offshore waters involves an inherited crude compass sense of direction (Hansen et al. 1993). Kallio-Nyberg and Ikonen (1992) hypothesized that stock-specific ocean feeding migration patterns result from natural selection of fish having the shortest possible route to sufficiently good feeding areas for growth and reproduction, and that intra-stock variation in these patterns is related to biological and environmental factors. Magnetite crystals in the brain tissues of salmon may provide an internal compass needle that aligns with the earth's magnetic field (e.g., Mann et al. 1988) or, as in pied flycatchers (Ficedula hypoleuca), geomagnetically controlled melatonin may transmit genetically encoded orientational data (Schneider et al. 1994). As in loggerhead sea turtles (Caretta caretta; Light et al. 1993), salmon may use inherited geomagnetic compass directions and angles of inclination and anomalies of the earth's magnetic field to navigate during their extensive offshore migrations.
Few population-specific estimates of offshore mortality rates have been published. Offshore mortality rates of maturing pink salmon, calculated directly from abundance estimates of fish from the Karaginskii region of eastern Kamchatka, Russia, show considerable annual variation (55.4-95.8%, brood years 1986-91), and in some years are higher than coastal mortality rates of juveniles (Karpenko 1995).
In the past, the Japanese high seas salmon driftnet fisheries were a major cause of offshore mortality. Differences in offshore distribution and migration patterns made some populations more vulnerable to high seas fishing mortality than others. For example, tag return data indicate that the ocean distribution of eastern Kamchatka sockeye salmon directly overlapped the area of the pre-1978 mothership salmon driftnet fishery. Konovalov (1985) estimated an exploitation rate of 90% by the mothership fishery on spring sockeye salmon from Lake Azabayachi. In 1977, the U.S.S.R. 200-mile zone was closed to fishing, and after 1977 no fishing was allowed in the area north of 46°N in May, when the traditional fishery targeted sockeye salmon. After these closures, populations of Lake Azabayachi spring sockeye recovered rapidly (estimated abundance of spawning adults increased by 25 times from 1977 to 1984; Konovalov 1985).
The last year of operation of the high seas salmon driftnet fisheries was 1991, and the last year of operation of the high seas squid driftnet fisheries, which had a bycatch of salmon, was 1992. The Convention for Conservation of Anadromous Stocks in the North Pacific Ocean (CCAS, signed by Canada, Japan, Russia, and the United States in 1992) prohibits all directed fishing for salmon in international waters of the North Pacific Ocean and Bering Sea. In addition, United Nations (UN) Resolution 46/215, passed in December 1991, called for a worldwide moratorium on all high seas large-scale driftnet fishing. Prior to the CCAS and UN moratorium, substantial "unauthorized" salmon driftnet fishing operations were being conducted on the high seas in violation of international treaties on salmon fishing (Pella et al. 1993). The new international agreements, combined with the current low market value of salmon, have effectively eliminated illegal offshore fishing for salmon.
Predation by other dominant epipelagic species (e.g., killer whales (Orcinus orca), northern fur seals (Callorhinus ursinus), and salmon sharks (Lamna ditropis)) is likely an important cause of natural mortality of salmon during offshore feeding and adult return migrations. Other factors such as disease, parasites, starvation, and unusual environmental events (e.g., El Niño) may also play a major role, but the direct causes of natural mortality in offshore waters remain largely unknown.
Environmental changes that adversely affect complex biological processes (e.g., feeding, growth, maturation, run-timing, competition, predation, and disease) are likely the major mechanisms underlying natural mortality in offshore waters. Figure 2 is a conceptual model of natural mortality in offshore waters. The ellipses represent the distribution of a discrete population at various critical life history stages. The Chinese yin-yang symbol within the ellipses represents the dynamic interaction between environment and biological processes. Within an area of distribution, changing environmental conditions affect various complex biological processes, which in turn may change environmental conditions. Through this iterative process, distributions of discrete populations expand, contract, or shift, and offshore survival increases or decreases.
Phases in offshore distribution and migration patterns critical to survival are 1) juvenile emigration, 2) summer feeding, 3) overwintering, and 4) adult migration. In conceptual models of migration patterns developed from historical data, offshore movement and distribution of northeastern Pacific juvenile salmon is inferred from the distribution of immature age 0.1 fish in the following spring. Early juvenile migrants may have moved well offshore by January, whereas late migrants may remain for the entire winter in protected inshore or coastal areas, and move directly offshore in the spring. Interannual variation in timing of juvenile outmigrations and environmental conditions and, subsequently, the distribution of juvenile salmon populations at the end of their first winter or beginning of the first spring at sea may play a critical role in migration patterns and run-timing of returning adults (Rogers 1986). Most offshore growth in salmon occurs during summer months. Summer movements from one offshore feeding area to another through coastal waters may result in increased predation or fishing mortality. Competition in offshore feeding areas may result in decreased growth and increased age at maturity, and mortality may increase because fish must remain at sea longer before maturing. Salmon are probably most susceptible to predation, starvation, and disease in winter, and some populations may move from offshore into coastal areas, where increased rates of marine mammal predation and fishing mortality may occur. Interannual variation in timing and offshore location of adult movements may affect location of entry into coastal areas, and, subsequently, interception in coastal fisheries and timing of runs to river mouths (Blackbourn 1987). New offshore research should focus on obtaining population-specific information on offshore distribution and migration patterns and associated biological processes and environmental conditions during critical life history phases.
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