Seasonal variations of spatial structure of Japanese squid (Loligo japonica) and octopus (Octopus ochellatus) in Haizhou Bay

The cephalopod species, Japanese squid (Loligo japonica) and octopus (Octopus ochellatus) have important ecological and economic values in Haizhou Bay and adjacent waters. Japanese squid and octopus both spawn and nurse in the bay in early spring, and start overwintering migration from autumn. Their spatial structures vary substantially among seasons, driven by the seasonal variations of environmental changes and the monsoon climate. Studies of seasonal variations of their spatial structure contribute to understanding the distributional pattern and life-history of the two species. The spatial autocorrelation of sampling data has long been overlooked in the past. Besides, as most studies are limited by the scale of sampling time and spatial locations, the seasonal variations of spatial structure has been less studied. According to the survey data collected in spring (May), autumn (September) and winter (December) of 2011 using bottom trawls in Haizhou Bay and adjacent waters, we used multiple spatial analyses methods, including the global spatial autocorrelation, local spatial hot spots and semivariogram to examine the seasonal variations in the spatial structure of Japanese squid and octopus distribution. Our results show that the global spatial autocorrelation was weak for the two species, whereas there were strong local spatial aggregation distributions. The semivariogram analysis was consistent with global and local spatial autocorrelation, showing that the scale of spatial autocorrelation had seasonal changes of two species. The spatial structure of the two species was strong in spring and relatively weak in autumn and winter, and spatial autocorrelation in spring is more significant than that in autumn and winter. Both species showed a hot spot in the sea of 35.0°–35.5°N, 120.0°–121.0°E in spring, and there were non-cold spots area in the whole year. The spatial distribution showed non-aggregation or random pattern in the season when there was highest biomass density. The spatial distribution patterns of the two species may be related to seasonal environment changes as well as their feeding habits and the spawning and overwintering migration. This study contributes to a better understanding of the life-history traits and the distribution of suitable habitat for cephalopoda. The study provides research basis for scientific exploitation and management of the two species, which may benefit marine resources conservation and following studies.