The Changes in Ocean Climate Affecting the Abundance of Pacific Saury


The variations in ocean and climatic conditions have facilitated the abundant fluctuation of pelagic fishes. Why?

The ocean-atmosphere system of the North Pacific and the abundance of pelagic fishes have been changing with the decadal climate changes. Research from 1950 to 2010 have been conducted on the basis of variances in sea surface temperature (SST), climate indices, body size and catch information obtained from the region of Japan Sea and East China Sea having the Tsushima Warm Currents (TWC) to find out how changes in ocean climate interfere with the different population structure of Pacific saury. The winter oceanic changes resulted in the fluctuations of body size and abundance of Pacific saury within the area of TWC. It is evidenced that the rate of catch and the large size saury abundance was highly below average in the years of extreme cool winters when the TWC region experienced northward migration. Nevertheless, the years with warm winters resulted in figures that were above average based on the abundance of large-sized saury and catch rates. The climatic changes presented a pattern that shows fluctuations of decadal-scale along with enormous inter-annual variances of Pacific saury abundance and structure about climatic changes in the ocean. Such results together with the interchange of dominant species of pelagic fish suggested that the TWC region has a saury population that is in a good state, same as that of the 1980s in the region of Kuroshio-Onyashio Current (KOC) which experienced warm regime after climatic changes.

Pacific saury (Cololabis Saira) within the North Pacific Ocean has been making large migrations from the regions of subtropical to subarctic as the breeding season progress from the autumn and spring. The Pacific saury is settled in high and warm salinity waters which makes North part of the Yellow Sea and the East western parts of China Sea to lack adult saury. In the regions of KOC and TWC experience a gradual change in the catches of saury with an average of 310,000 tons and 28,000 tons every year respectively for the last five decades. The Korean waters experienced shifts in climatic regime in 1976 and 1988 which impacted the fisheries resources. The composition of abundance and size of Pacific saury within the regions of KOC and TWC have huge inter-annual differences. Despite fishing having big effects on the inclination in abundance, environmental factors have been crucial in facilitating vast variations of inter-annual abundance and composition of sizes. The human factors have influenced sea warming in those regions which have affected the ecosystem in marines and resulting in a lack of dominant fish species in Korean seas. Sea surface temperature (SST) has affected the abundance of Pacific saury through the interactions of large volumes of oceans and the atmosphere. The conditions of oceanographic will measure the performance recruitment of Pacific saury and fish can denote the bio-indicators of regime variations within the northwest Pacific. Various oceanic systems thus influence the saury which has two size groups (large and medium) in the northwestern Pacific.

Various climatic indices were selected to study how ocean climate changes influence the Pacific saury abundance. This comprised of the Monsoon Index (MOI), Arctic Oscillation Index (AOI), Southern Oscillation Index (SOI) and the Pacific Decadal Oscillation (PDO). The climatic indices were connected to the variability of decadal and inter-annual in oceanic and atmospheric conditions. To explore the oceanic variations, the sea surface temperature anomalies, the springtime series from April to June, annual time series and winter series from January to March was used to denote the variation in climate regime in TWC region from 1950 to 2010. The sequence of yearly catch together with the catch per unit fishing effort (CPUE) was analyzed and compared. As the data of CPUE in the TWC region is restricted to the period of the 1950s to 1970s which is the fishing period, the gathered info was also used to explore the trend of abundance for long-term. Synthetic fibre gillnets did fishing of Pacific saury in TWC region while the KOC region utilized the dip nets with stick-handles. The Pacific saury data on body size composition in the spring and summer represented their northward migration. This data was collected in the areas of TWC which included off Hokkaido, Japan in northeastern, northwestern (off Russia), of Korea in Southwestern and off Honshu in southeastern.

The sequence of SST anomalies implied that the TWC region experienced seasonal differences when changes in oceanic regimes occurred. The gill net and set net were responsible in fishing about 80% of the Pacific saury in the TWC region in the spring-summer northward migration and autumn-winter southern migration. It was evidenced from Figure 1 that the PDO in winter indicated decadal scale differences with discrete changes in 1989 and 1977. There was a positive shift to negative anomalies with the SOI pattern in 1976 with abrupt changes in cumulative sums in 1976 and 1998. There was also considerable variations in the AOI during 1971, 1977 and 1998. The MOI in winter illustrated gradual changes from positive to negative during anomalies during the late 1980s implying a weakening Asian Monsoon. The gradual changes in 1976 and 1977 were evident for AOI, SOI and PDO. During the late 1990s, there were slight variations in SOI and PDO. It was evident that a shift in climatic regimes was showed by four indices which happened in late or mid-1990s, at the end of 1980s and the mid and early 1970s. The western TWC region indicated decadal and inter-annual scale variances based on the series of catch and CPUE according to figure 3. During the early 1970s, the catches were high with the decreased CPUE due to increased fishing grounds and some gill nets. The Pacific saury of size group L (large) with the mode of 28-32 cm was abundant in the 1990s and 1960s during the summer and spring seasons. The Medium, M size (25-27.8cm) were abundant from 1980 to 1990 similar to the small, S size (21-24 cm). By using the body size composition, it became easier to estimate the condition of Pacific saury abundance. During the spring and summer season in the TWC region, the saury population was poor due to lack of the large size group. Such a pattern indicated that the enormous inter-annual and decadal-scale changes in the abundance and structure of Pacific saury was attributed to the oceanic and climatic changes.

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