1. A total of 17,242 zooplankton samples collected from the Tohoku Sea Area between 1951 and 1990 was examined in this study. The samples were taken by vertical hauling of a conventional net, so called Marutoku net (net opening 45 cm, mesh aperture 0.33 mm) from 150 m depth to the surface. Using the collections, wet weight measurements and identifications to species were conducted. Long-term variations of zooplankton biomass and distributions of the dominant species were investigated. Relatively small variations of water temperature are observed at a depth of 100m. The Oyashio Front and the Kuroshio Front are commonly represented by the contour lines of 5 and 15°C at that depth, respectively. The present study also divided the Tohoku Sea Area into these three water masses based on this water temperature at a depth of 100m, that is, the Oyashio Water (<5°C), the Transitional Water (5-15°C), and the Kuroshio Water (>15°C).
2. Monthly mean biomass of zooplankton within grids of every one degree of latitude and longitude was calculated for all the data collected. The biomass of zooplankton between January and March was less than 1 g/net although the sampling area covered only the western part from 150°E off the Joban Coast. High biomass was observed from May to October, during which observations occupied the whole of the Tohoku Sea Area. In particular, biomass of more than 10 g/net was commonly observed off the Sanriku Coast, the eastern Hokkaido, and the southern Kurile Islands. From November to December the biomass became small again to level as observed from January to March. The extensive survey conducted off the Joban and the Sanriku Coasts to the meridian of 160°E in May of 1973 revealed that high biomass occurred in the Kuroshio Water (>15°C). Observation along the meridian of 145°E conducted in September of 1986 also showed that zooplankton was abundant in Oyashio Water and less so in the Transitional and the Kuroshio Waters. These results indicated that distributions of zooplankton are largely affected by the oceanographic conditions.
3. The range of water temperature at the sampling sites was among 0.1 and 24°C. The highest biomass of zooplankton (34.1 g/m2) occurred in the lowest water temperature range (0-1°C), zooplankton biomass decreased with increasing water temperature. Mean abundance in water temperature ranges of 5-10, 10-15, and >15°C were 11.6, 7.4 and 6-5 g/m2, respectively. Ninety-seven percent of samples were less than 20 g/m2 in the Kuroshio Water, while only the 51% was less than 20 g/m2 in the Oyashio Water.
4. Seasonal changes in the zooplankton biomass were also investigated in the Tohoku Sea Area. The annual maximum biomass usually occurred in May in the Oyashio Water. Eighty percent of observations in the forty years had an annual maximum in May or June. The mean biomass in May reached 40 g/m2. From May to August the biomass was high (20-30 g/m2) and decreased in autumn. In the Transitional and the Kuroshio Water the biomass was also high in May and decreased in summer, but a small increase of biomass was observed in November.
5. Mean biomass of zooplankton was 17.3 g/m2 in the Oyashio Water, although a long-term variation was noted for the yearly mean biomass. That is, relatively high biomass was observed from 1956 to 1962 (mean biomass, 16.5 g/m2), from 1965 to 1978 (21.0 g/m2), and from 1987 to 1989 (16.3 g/m2). The mean biomass in the Transitional Water was 7.8 g/m2. Year to year variation was also observed in this water mass, but the trend was not as consistent as that observed for the Oyashio Water. In the Kuroshio Water the mean biomass was 5.8 g/m2 with a small year to year variation.
6. Zooplankton biomasses in the three water masses of the Tohoku Sea Area were estimated based on the mean biomass and the area of the respective water masses. The estimates were conducted using data collected in the western sea area form 146°E from 1964 to 1990. During this period the total biomass fro the whole Tohoku Sea Area varied between 1.934×106 and 5.358×106 ton. The mean biomass in the Oyashio, the Transitional, and the Kuroshio districts were 1.731×106, 1.857×106 and 0.299×106 ton, respectively. In the Oyashio district, year to year variations of the biomass were corresponded to the long-term variations of biomass and not to the changes of the area of the water mass. This trend was also noted in the Transitional district.
7. A total of 259 species was identified in the present study. Species composition represented 33 species occurred in the Oyashio Water off the Joban and the Sanriku Coasts, were indicator species of cold water masses (e.g., Calanus cristatus, Calanus plumchrus), while ten of 50 species that occurred in the Kuroshio Water were indicator species of warm water masses. In the Transitional Water both cold and warm water species were observed. Fifty-six species were regarded as dominant species based on the rank order of the individual number. Seven species were determined as the most important species in terms of the body weight (Calanus cristatus, Calanus finmarchicus, Calanus plumchrus, Eucalanus bungii, Metridia pacifica, Calanus helgolandicus and Themisto japonica). All of the seven species were indicator species of cold water. Although Paracalanus parvus was numerically dominant (5,886 inds/net), the total abundance of them was equivalent to only 24 individuals of Calanus cristatus. Calanus cristatus and Calanus plumchrus are widely distributed off the Sanriku Coast, the southeastern coast of Hokkaido, and the southern Kurile Islands, which are the main fishing grounds of the Pacific saury, from early summer to autumn. The increase of total zooplankton biomass observed in the Oyashio Water was due to Calanus cristatus and Calanus plumchrus.
8. Pacific saury (Cololabis saira) hatches out in the Kuroshio Water, and stay off the Joban Coast during their juvenile stages. Stomach content analysis showed that they preyed upon small body size copepods of the warm water species in the Joban Sea Area. Pacific saury starts to migrate northward with growth, and reach the Oyashio Water, where zooplankters are abundant. In this water mass they prey on larger body size zooplankters than those eaten in the Joban Sea Area. The mean amounts of stomach contents of Pacific saury (body length, 19-31cm) were 3-5g. The amount in small sized Pacific saury was less (2-5g) and that of larger ones was more (max. 8g). Although the weight of stomach contents increased with body size, the ratio to body weight was less in large size Pacific saury (4%) than in small size one (7%).
9. Total amount of zooplankton consumed by Pacific saury was estimated based on the growth from the juvenile stage in the northward migration to the adult stage in the southward migration assuming a transport efficiency of 10%. The fisheries of Pacific saury are usually conducted in the latter stage. From 1979 to 1985 the annual mean consumption of zooplankton by Pacific saury was 1.55×106 ton, which was 7-fold higher than the annual mean yield of Pacific saury during the same period.
10. Long-term variation observed in the yield of Pacific saury corresponded to that observed in zooplankton biomass in the Tohoku Sea Area. The present study revealed that zooplankton, especially the most important seven species in the cold water, played an essential role in maintaining resources of Pacific saury in the Tohoku Sea Area which is the main feeding ground of them.
The present study shows variations in zooplankton biomass and species compositions in relation to a planktivorous pelagic fish species. The results provide valuable information to investigate fishery resources, especially to consider trophodynamics of zooplankton and plankton feeder in the Tohoku Sea Area.