by Y. SAKURAI
Faculty of Fisheries, Hokkaido University, Hakodate, Hokkaido, Japan (sakurai@pop.fish.hokudai.ac.jp)
Most commercially exploited squids live less than two years. In such short lived species, recruitment success probably depends on the physical and biological environments at the spawning and nursery grounds. Since the
"failures" or "collapse" of the Todarodes pacificus and Illex illecebrosusfisheries in the 1980s, the role of environmental change on squid stocks has become an increasingly important topic of research (Lipinski et al., 1998; Roberts, 1998).
Three populations with different peak pawning seasons (summer, autumn and winter ) migrates seasonally in the Sea of Japan and the Pacific Ocean, with most spawning occurring near Kyushu Island. T. pacificus annual catches have fluctuated widely since 1900s and increased markedly during 1986-97. However, the catch in 1998 decreased to about 150,000 tonnes.
Episodic "regime shifts" involving entire biological community structures occur worldwide (Lluch-Belda et al., 1992). In the western North Pacific, environmental conditions have shifted from a warm regime, which began in the late 1940s, to a cool regime in the late 1970s, and back to a warm regime in the late 1980s (e.g.
Minobe, 1997). These regime shifts coincide with variation in T. pacificus catches, particularly during the early 1980s catch decrease and the late
1980s catch increase. Paralarval abundances of the autumn spawning group have also been higher since the late 1980s than during the late 1970s to the mid-1980s. These stock fluctuations could be related to the effects of regime shifts on spawning and paralarval survival.
Paralarval abundance is also used to assess the stock-recruitment relationship. To analyze the stock-recruitment relationship, paralarval density index (PDI) values in the northern part of the East China Sea and the southwestern Sea of Japan during autumn, and annual CPUE values of the autumn-spawning group in the Sea of Japan were examined. Annual PDI values were relatively high in 1975, low during 1976-88, and increased remarkably after 1989, which corresponds to annual changes of CPUE values.
During 1973-84, positive relationships occurred between both adult catch and PDI of their offspring and between PDI and adult catch of the same generation for the autumn-spawning group (Murata, 1989). In the Sea of Japan during 1975-96, PDI was strongly correlated to CPUE in the summer of the same year. A positive relationship also existed between PDI and CPUE in the next summer. Annual changes of T. pacificus paralarvae density index (PDI) during fall in 1973 to 1993 in the northern East China Sea and the
24 Sesión 1: Estudios sobre reproducción y sobrevivencia de calamares y peces southwestern Sea of Japan. In fall, paralarval
distribution areas expand to south after 1989.
These results suggest that catch increases since the late 1980s could reflect a possible relationship between stock and PDI and between PDI and recruitment. Stock size of the winter-spawning group was the largest of the three groups through the late 1960s, but the autumn-spawning group has been the largest in the 1970-80s (Murata, 1989). The present results indicate that spawning areas of autumn and winter spawning groups have expanded and overlapped from offshore of the southwestern Sea of Japan in autumn to the East China Sea in winter (i.e. the Tsushima Strait).
This suggests that the recent stock increase may have occurred in both the autumn and winter spawning groups.
In my presentation, we review recent trends of annual variation in T. pacificus stock size and paralarval density, and propose a possible scenario for stock fluctuations based on two assumptions:
1. stock size increases with a shift to a warm regime, and
2. environmental conditions are favorable for reproduction, paralarval survival, and therefore recruitment during warm regimes.
We then estimate the relationship between the recent seasonal and annual changes in the inferred extent of the spawning grounds based on water temperatures around Japan.
Potential spawning areas were estimated using information collected from laboratory and field studies, which have shown that T. Pacificus spawns gelatinous nearly neutrally buoyant egg masses (Bower and Sakurai, 1996). Temperature range for normal embryonic development is 15-23oC (Sakurai et al., 1996). Most hatchlings collected off southern Japan occur where sea surface temperatures range is 17-23°C (Bower, 1997). The temperatures at 50-m depth were used to estimate the range of spawning grounds, since most paralarvae occur at 25-50 m depth (Watanabe, 1965). Days from egg mass produce to egg mass destroy by hatching. Egg mass will
be maintained from 3 to 7 days in the temperature ranges of 15 to 23°C. Spawning was assumed to occur above the continental shelf and slope around Japan, because captive females regularly sit on the tank bottom just before spawning (Bower and Sakurai, 1996), and bottom trawls often collect exhausted, spent (post-spawning) females on the continental shelf and slope at 100-500 m depth (Hamabe and Shimizu, 1966) (Fig.1).
To estimate historical spawning areas, mean monthly temperature data at 50-m depth during 1900-72 (JODC, 1978) over the continental shelf around Japan were examined. The main inferred spawning areas, based on temperature at 50m, for the winter-spawning group occurred widely along the continental shelf and slope of the East China Sea from Kyushu to Taiwan, and small spawning areas occurred in the western and the southwestern coastal waters of Kyushu and Pacific coastal waters off Shikoku. During spring to summer, the estimated spawning areas gradually shifted northwards along the continental shelves of the Sea of Japan and Pacific Ocean. In autumn, the inferred spawning areas occurred around the Tsushima Strait and Noto Peninsula. Limited spawning could also have occurred along the coast of northern Japan.
These results suggest that the main spawning grounds occurred in the Tsushima Strait in autumn, and the East China Sea in winter, as reported by Murata (1989; 1990). Spawning areas of the three groups may seasonally shift and overlap in the southwestern Sea of Japan to the East China Sea. However, possible spawning sites along central and northern Japan are restricted due to the narrow continental shelf zone and low temperatures that occur during winter to spring.
The seasonal shifts in inferred spawning areas based on temperatures changes at 50 m depth and the range of continental shelf around Japan are similar to the reported changes in paralarval distribution (Murata, 1989).
The relationship between monthly and annual changes in spawning grounds and water
temperatures at 21-53° N and 121-153° E during 1984-95 was examined using three data sets:
monthly GMCSST (global multi-channel sea surface temperature), Levitus, climatological oceanographic and topography data, NOAA/NESDIS (National Environmental Satellite Data and Information) and the NODC (National Oceanographic Data Center) produced the GMC-SST and Levitus data sets.
We used a linear equation to relate temperatures at 0-m and 50-m depth for each grid station and then estimated the 50-m depth temperature for each month during 1984-95 using this equation and GMCSST data. The area were the 50-m depth temperature of 15-23°C occurred above bottom depths of 100-500m was then estimated. We used GIS data to examine the relationship between the recent seasonal and annual changes of the inferred spawning areas and the shift from a cold regime to a warm regime that occurred in the late 1980s by comparing the seasonal and annual shifts of inferred spawning areas during 1984 to 1995.
Kiyofuji et al. (1998) concluded that the inferred spawning grounds occurred year-round in Tsushima Strait and near the Goto Islands in 1989-91, which was the beginning of the warm regime when fall paralarval abundance and annual catch increased. From October to December, the inferred spawning areas did not vary geographically between 1985/86 and 1990/91. In February 1991, inferred spawning sites occurred in waters from the Tsushima Strait to the East China Sea along the continental shelf.
In 1986 inferred spawning areas were similar except that they did not occur in the Tsushima Strait (February).
We then compared the interannual variability of inferred spawning areas in February during 1984-94, which is the peak of spawning month of the winter-spawning group (Murata, 1989). In February of 1989, 1990, 1991 and 1994, the inferred spawning sites occurred widely from the Tsushima Strait to the East China Sea. Spawning areas in February of 1984, 1986, 1988 and in 1992 occurred above the continental shelf in the East China Sea and southwest of Kyushu, but not in the Tsushima Strait.
These results suggest that winter spawning areas in the East China Sea will shrink when adult stocks decrease during a cool regime, and that autumn and winter spawning areas will extend and overlap in the Sea of Japan and the East China Sea when adult stocks increase during a warm regime (i.e. 1989, 1990 and 1991). The annual series of warm winter may promote the stock increase. However, the inferred spawning sites varied annually (e.g. in 1992 and 1993, there were a reduction in size of inferred spawning areas in winter), which might be a cause of the historical annual catch fluctuations of T.
pacificus. To elucidate those fluctuations related to recruitment success, we must make a stock fluctuation model coupled with the retrospective data series of T. pacificus catch and the environment.
GIS in increasingly being used for oceanographic and fisheries studies and has potential to estimate and forecast the stock fluctuations of T. pacificus related to climatic regime shift by examining temporal and spatial distribution of the optimum temperature areas for reproduction. GIS will become an important tool in future studies of stock fluctuations in exploited squid species.
26 Sesión 1: Estudios sobre reproducción y sobrevivencia de calamares y peces
Fig. 1. Working hypothesis on natural reproduction of Todarodes pacificus, based on results of experimental studies.