Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Decadal Changes in Abundance of Dominant Pelagic Fishes and Squids in the Northwestern Pacific Ocean Since the 1970s and Implications on Fisheries Management
Akihiko Yatsu, Kazuya Nagasawa, Tokio Wada
Low-frequency stock fluctuations of small pelagic fishes derived from changes in ocean climate known as “regime shifts” are a central issue in fisheries science and management (Kawasaki 1983; Baumgartner et al. 1992; Beamish 1995; Belayev and Shatilina 1995; MacCall 2002). Since the 1950s, 2 million to 6 million tons of small pelagic fishes have been harvested annually by Japanese commercial fisheries in the northwestern Pacific Ocean, and quasi-decadal alternations in dominant species known as species replacement have been evident: chub mackerel Scomber japonicus in the 1970s, Japanese sardine Sardinopus melanostictus in the 1980s, Pacific saury Cololabis saira, anchovy Engraulis japonicus, and Japanese common squid Todarodes pacificus in the early 1990s (Figure 1; Matsuda et al. 1991; Wada 1997).
Stock fluctuations of these species in the northwestern Pacific also have been detected in prey compositions in major predatory fishes and minke whale Balaenoptera auctorostrata (Wada and Honda 1992; Kasamatsu and Tanaka 1992; Savinykh 1994; Tamura et al. 1998; Nagasawa 1999), although responses in stock abundance of the predators have never been reported. Despite the need for ecosystem or multi-species stock management with low-frequency stock variability (MacCall 2002), an adaptive stock management plan has been proposed (Matsuda et al. 1991; Shuntov 1999). Therefore, we present a management strategy for small pelagic stocks in the northwestern Pacific on the basis of the alternation of dominant small pelagic species and its effect on predatory fishes and minke whale from the 1970s through the 1990s.
We used three data sets:
• catch and effort data from non-size-selective driftnet operations during 1978–1999 by Hokkaido University’s training ship, Hokusei maru (Hokkaido University 1980–2000);
• catch and effort data and stomach content data from predatory fishes obtained from large-mesh driftnet operations during 1983–1990 by the Tohoku National Fisheries Research Institute (TNFRI; Fisheries Agency of Japan 1985–1992); and
• stomach content data obtained from large-meshed driftnet operations during 1978– 1983 by the Japan Marine Fishery Resources Research Center (JAMARC 1980–1986 and unpublished data).
The distribution of sampling sites is summarized in Table 1 and in Figures 2 and 3. Predatory fishes examined were blue shark Prionace glauca, salmon shark Lamna ditropis, albacore Thunnus alalunga, pomfret Brama japonica, and neon flying squid Ommastrephes bartramii, all of which are dominant nekton in the northwestern Pacific (Brodeur et al. 1999).