Spatio-temporal variation in the higher trophic level community structure of the western North Pacific pelagic ecosystem

There are increasing demands for evaluation of the current status of marine ecosystems and impacts of environmental change and anthropogenic activities, but for the open ocean available data are quite limited. The Japanese driftnet survey dataset of Hokkaido University was used to analyze the spatio-temporal variations in the higher trophic level (HTL) community structure in the western North Pacific. The analyses were conducted by using subset of the data that was collected along four regularly surveyed transect lines at 155°E, 170°E, 175°E and 180°E spanning from 36 to 48°N and from 1982 to 1996. Non-size-selective multiple mesh driftnets were used in the survey. Total number of individuals, species richness and Simpson index of species diversity indicated a longitudinal gradient with a lower abundance and diversity in the eastern area. Latitudinal gradient was clear for numerical abundance and species richness, but no trend for Simpson diversity index. Temporal patterns in the HTL community structure were not consistent among the three metrics: numerical abundance showed a significantly decrease and increasing fluctuations in later years, species richness was increased along years, and Simpson index did not show any clear temporal trends. The inconsistency in the temporal patterns of community metrics was considered to arise from the effects of the sampling scheme on community measurements and/or complex structural changes such as the emergence of dominant species related to regime shifts. These results suggest the importance of proper consideration of temporal changes in the sampling scheme when analyzing long-term data.