Interannual–interdecadal variations of spear squid Loligo bleekeri abundance in the southwestern Japan Sea during 1975–2006: Impact of the trawl fishing and recommendations for management under the different climate regimes

A climatic regime shift, characterized by an abrupt change from cool to warm conditions, occurred in the Tsushima Warm Current (TWC) region in the Japan Sea in the late 1980s. The abundance of spear squid Loligo bleekeri in the southwestern part of the Japan Sea responded strongly to the changing thermal regime. Spear squid are widely distributed in Japanese coastal waters and form one of the most important target species of pair trawlers in the southwestern Japan Sea. Catch from the southwestern Japan Sea has fluctuated from a minimum of 16 tons in 2003 to a maximum of 13,700 tons in 1977. Catch has decreased to less than 100 tons in recent years, and consequently the stock is at the point of collapse. The abundance index of spear squid shows a decadal pattern of variation with a step change from positive to negative annual anomalies that occurred around 1990; this pattern corresponds closely with the changing regime in water temperature in the TWC, strongly indicating that the decadal variability in spear squid was largely affected by the climatic regime shift in the late 1980s. Significant negative correlations between water temperature and an abundance index of spear squid (R2 = 0.39, p < 0.01) indicate that increasing ambient water temperature reduced spear squid abundances during the 1990s. Monthly CPUE (catch per unit effort) decreases sharply during the fishing season from autumn through winter, a patterns that was well described by a DeLury model. DeLury estimates of apparent mortality showed that the fishing mortality coefficient was relatively stable during 1975–1987, but has increased greatly since the late 1980s. The average fishing mortality coefficient averaged during the warmer 1990s was about 2.1 times higher than during the colder late 1970s and 1980s, indicating that the fishing mortality has intensified during the 1990s and has accelerated the collapse of the stock. This suggests that the decadal pattern of variation of spear squid was largely forced by the late 1980s climatic regime shift of the TWC, while high fishing magnified interannual variation of spear squid and attributed to the subsequent collapse of the stock. Reducing fishing pressure on juvenile squid during the early part of the fishing season in autumn is considered to be particularly important for the recovery of this stock under the current unfavorable thermal regime.