| The neon flying squid (Ommastrephes bartramii) is an important component in the North Pacific Ocean ecosystem, playing an important role in linking the food chains. And the squid also surports a major fishery in the North Pacific Ocean and is mainly exploited and utilized by China (including Taiwan province), Japan and other countries. The west stock of winter-spring cohort as one of four stocks, accounting for 70% to 80% of the total annual catch in the high fishing season of August to October in the last decade in the Northwestern Pacific Ocean, is the traditional target species of Chinese jigging-fleets. However, in recent years, the way how to efficiently predict the central fishing ground of O. bartramii urgently needs to be solved. The formation of central fishing ground of O. bartramii was not only impacted by sea surface temperature (SST), marine fronts, and etc, but also affected by vertical temperature structure in terms of diel vertical migration of the neon flying squid. Therefore, further analysis on the relation between fishing ground and vertical temperature structure is conducive to reveal the formation mechanism of main fishing ground, for providing the basis for scientifically searching for the fishing ground of O. bartramii.This paper was to analyze the law of the spatial and temporal distribution of squid fishing ground grativity, and discuss fishing ground distribution relation to vertical temperature structure, and the reasons for declines in catch in 2009 using deep sea water temperature in the Northwstern Pacific Ocean based on the historical statistical data from Chinese mainland squid fishery and marine environment of squid feeding ground and spawning ground including Sea Surface Temperature( SST), deep sea water temperature(at 50m, 100m, 200m and 300m depths), etc. The suitability index (SI) was established by habitat theory and method using SST, temperature gradient of 0-50m, deep water tempeture (at 200m and 300m depths) data. The integrated habitat suitability index (HSI) model was created by arithmetic mean model (AMM) and geometric mean model( GMM ), and was selected to the apropriate model by use of comparative analysis. Finally, we use this model to forcasting fishing ground for fishermen and fishery managers. The results are as follows:(1) Spatial and temporal distribution of fishing ground of O. bartramii changed significantly during the fishing season of August to October in the Northwestern Pacific Ocean. Inter-annual variations of longitunidal gravity of fishing ground were significant but seasonal variations were not significant. The fishing ground shifted west to east from August to October. In August above 60 percent of fishing efforts and catch came from the waters of 153o-157oE, and during September to October the fishing ground was mainly concentrated in the waters of 155o-159oE. Nevertheless, abundance (CPUE) of O.bartramii didn't show significant variations in longitude. However, inter-annual and seasonal variations of latitudinal grativity of fishing ground were both significant, following the pattern of south-north-south forward during 40o-45oN. The main catch was concentrated in the waters of 42o-44oN, but abundance of O. bartramii ( CPUE) presents remarkable fluctuations.(2) The vertical temperature including SST and deep sea water temperature and latitudinal temperature profile was analyzed in terms of diel vertical migration of neon flying squid. The central fishing ground was mainly distributed in the waters of 150o-160oE, 40o-45oN during August to October, where the monthly vertical temperature structure is different. The optimal range of SST, temperature at 50m, 100m, 200m and 300m and the temperature gradients of 0-50m and 200-300m were 17-21℃, 6-12℃, 4-8℃, 2-6℃, 0.15-0.35℃/m and -0.01-0.02℃/m, respectively, in August, and they were 15-19℃, 6-11℃, 1-6℃, 2-6℃, 0.15-0.35℃/m and -0.01-0.01℃/m respectively in September, and 13-17℃, 8-13℃, 4-9℃, 3-6℃, 0.05-0.2℃/ m and -0.01-0.01℃/m respectively in October. The temperature profile analysis indicated that the fishing ground concentrated in the confluence of the branches of the Kuroshio and Oyashio, warm waters side of front at different depths, where the isothermal line was very dense within 50 meters.(3) The neon flying squid (Ommastrephes bartramii) as a main fishing target for Chinese distant-water squid jigging fleets, the production has maintained a steady level in the recent years. However, it declined sharply in the traditional fishing ground during the high fishing season (from August to October) in 2009, and the daily catch was only half of that in the normal years. Therefore, based on the catch from the Chinese squid jigging vessels and environment data in the Northwestern Pacific Ocean from August to September during 2007 to 2009, the reason on the decline in catch for O. bartramii and variability of fishing ground in 2009 were deduced. The results indicated that there are two points causing a sharp decline in catch: 1) The occurrence of the bigbending of Kuroshio at the spawning ground (130-170°E, 20°-30°N ) caused a sharp decline in squid recruitment; 2) During the main fishing season (August-September), the cold waters (located at 154°-156°E) at 100m depth intruded southward into the traditional fishing ground (150°-165°E,42°-46°N), which split the traditional fishing ground into two parts. And its southerly front (temperature at 100 m layer < 5℃) reached at 42°N, which was distinctly different from that in normal years. Since the above reasons, the fishing ground has been significantly narrowed and not suitable for squid to aggregate.(4) The fishing effort as a relative indicator of suitability index of O. bartramii was created for suitability index (SI) based on SST and vertical temperature (P<0.05). The integrated habitat suitability index model was established based on the arithmetic average method (AMM) and the geometric mean method (GMM). We compared HSI values with the actual fishing effort, catch and CPUE during August to October from 1998 to 2004 in the waters with HSI value greater than 0.6 from August to October, the percentage of catch and fishing effort were 83.4% and 80.9% estimated from AMM, respectively, and CPUEs were all above 2.1t/d. However, the percentage of catch and fishing effort were 73.5% and 69.6% estimated from GMM, respectively, and CPUEs were all above 2.3t/d. AMM was better than the GMM by comparison of two models. The HSI was validated by the catch data from August to October in 2005 and water temperature data. It is found that the main fishing ground of Ommastrephes bartramii was distributed in the areas with HSI greater than 0.6 in the AMM, and fishing effort and catch accounted for 85.6% and 82.5%, respectively, and its corresponding CPUE ranged from 3.2t/d to 4.2t/d, which is stable with small fluctuation. The results shown that the HSI model based on vertical temperature structure can better predict the main fishing ground and potential fishing ground of Ommastrephes bartramii in the Northwestern Pacific Ocean. |
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