Title Impact of climatic changes on the distribution, migration pattern and stock abundance of the Japanese common squid, Todarodes pacificus in the Sea of Japan
Authers Hideaki KIDOKORO
Keywords Ommastrephid, Regime Shift, Stock management, Todarodes pacificus
Citation Bull. Fish. Res. Agen. No.27, 95-189, 2009
Abstract
Japanese common squid (Todarodes pacificus) is one of the most important fisheries resources in Japan and South Korea. In recent years, the total annual landing of Japan and South Korea has been about 500,000 tons. Since 1998, the stock of Japanese common squid has been managed in Japan by Total Allowable Catch (TAC) quota system.
 The annual landings of Japanese common squid have varied with oceanographic conditions in decadal or inter decadal cycles, therefore, it was pointed out that the stock size of Japanese common squid may be influenced by climatic regime shifts (CRS) which are the abrupt changes in oceanographic conditions with decadal or inter-decadal cycles. The latest CRS was considered to have happened between 1988 and 1989 (88/89CRS). Sea surface temperature in the northwestern Pacific was basically lower tendency before 88/89CRS (colder regime), but it turned to warmer tendency after 88/89CRS (warmer regime).
 Considering the influence of oceanographic conditions on the variability of the stock size of Japanese common squid, it needs not only regulation by the fishing mortality for the proper management of this stock but also the understanding of the influence of CRS on the trend of this stock size.
 In this study, for a better understanding of the influence of CRS on the trend of this stock size, the following subjects were examined, 1) Clarification of the stock structure in the Sea of Japan, 2) Changes in the distribution and migration pattern that coincided with CRS, 3) Variability in the stock size and sustainable production with CRS, and a hypothesis on the processes regulating the stock fluctuation with CRS is discussed.
1) Stock structure of Japanese common squid in the Sea of Japan
 In order to clarify the stock structure of Japanese common squid in the Sea of Japan, specimens were caught in three areas: offshore area (offshore cohort) , inshore area (inshore cohort)and off northern Hokkaido (northern cohort)were examined using statolith microstructure analysis. Furthermore, the spawning migration patterns of these three cohorts were examined based on tagging experiments conducted in June and July 1994-2006.
 The average mantle length of offshore cohort was 222.5 mm, inshore cohort was 197.8 mm, and northern Hokkaido cohort was 174.2 mm. No significant differences between estimated average ages of offshore and inshore cohorts (about 210-260 days)were found, and they were estimated to hatch during mainly October and November. However, the daily growth rate (mm/day) of the offshore cohort was significantly higher than that of the inshore cohort. The estimated age of the northern cohort was about 180-220 days and estimated to hatch mainly in December which was approximately one month later than those of the offshore and the inshore cohorts.
 Most recapture records reported during July and September were obtained in the same areas where they were released. In October, while recapture records of the offshore cohort and the inshore cohort were obtained in the spawning area (northern part of the East China Sea and southwestern part of the Sea of Japan), recaptures of the northern cohort were obtained in the offshore area.
 Based on the differences in hatch date distribution and migration pattern, the autumn spawning stock could be divided into the three sub-stocks.
2) Changes in the distribution and migration pattern in relation to CRS
 Changes in the distribution pattern of Japanese common squid with CRS were examined by comparing the distribution maps of CPUE of the results of experimental jigging surveys conducted in June and July 1979-2007. Changes in the migration pattern between before 88/89CRS and after 88/89CRS were examined based on the tagging experiments conducted in the offshore area of the Sea of Japan in 1984 and 1987-1991.
 Since the 88/89CRS, a clear increase of CPUE has been detected off northern Hokkaido. This means that the northern cohort increased drastically after 88/89CRS. Significant differences were detected between before 88/89CRS and after 88/89CRS in the stock index of mantle length less than 21 cm composed mainly of the inshore and northern cohorts. However, no significant difference was obtained between before 88/89CRS and after 88/89CRS in the stock index of mantle length larger than 21 cm composed mainly of the offshore cohort.
 The direction of spawning migration in the offshore area was southward before 88/89CRS: that is, from the central part of the Sea of Japan to the inshore area of middle and western Honshu, however it changed into southwest-ward after 88/89CRS: that is, from the central part of the Sea of Japan to the east off the Korean Peninsula. Although the spawning migration started in mid-September before 88/89CRS, it was delayed to October after 88/89CRS. These changes in spawning migration pattern were consistent with the shift of spawning ground which was observed by a shift in the larval distribution.
3)Variability in the stock size and sustainable production with CRS
 The stock size of the Japanese common squid was estimated by a stock index which is the average CPUE of experimental jigging survey by research vessels in June ? July and assumed catchability parameter. Stock size was quantified by supposing that the stock index is proportional to the stock size. Variation in the maximum sustainable production with the CRS was examined by comparing the spawner-recruit relationships estimated before 88/89CRS (colder regime)and after 88/89CRS (warmer regime).
 The number of stock individuals was estimated to be about 2 billion individuals (500 thousand tons) in the colder regime. It increased in the warmer regime, and reached about 7 billion individuals (2000 thousand tons)in the beginning of the 2000s. Exploitation rate was 30 % - 40 % in the 1980s, it decreased to 20 % - 30 % in the 1990s and was less than 20 % in the beginning of the 2000s. The maximum sustainable production estimated in each regime by spawner-recruit relationship was 170 thousand tons in the colder regime and 380 thousand tons in the warmer regime.
4)Conclusions
 Stock size of Japanese common squid has increased after 88/89CRS, in particular, the northern Hokkaido cohort. Moreover, not only the stock size but also the hatch date composition, migration route and spawning grounds of Japanese common squid have also changed with CRS. These changes in ecological properties are considered to play important roles in the changing stock size of Japanese common squid.
 We can predict the trend in stock size of Japanese common squid by observing these changes in ecological properties mentioned above, and this can be used to implement appropriate management strategy plans. However, we should be aware that the average yield may doubled or be reduced to half depending on CRS events even if appropriate fishing mortality management strategies are adopted for this stock.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull27/kidokoro.pdf