Evaluating spatio-temporal dynamics of multiple fisheries-targeted populations simultaneously: A case study of the Bohai Sea ecosystem in China

Development of joint species distribution models allows for improved and simultaneous estimates of various metrics for evaluating spatio-temporal patterns in multi-population dynamics, as essential tools for promoting ecosystem management and conservation. However, such tools are rarely used to simultaneously evaluate the spatio-temporal dynamics of multiple fisheries-targeted populations in an ecosystem. This study aims to fill this gap with a case study of the Bohai Sea in China. A joint dynamic species distribution model was developed to estimate the distributions of 13 major fish and invertebrate populations in the Bohai Sea, using data from bottom trawl surveys conducted in spring during 2014 to 2018. We calculated the spatial ranges (i.e., the distance within which correlation coefficients between locations are always larger than 10%) for spatial variation and spatio-temporal variation in species-specific densities of this fished community. The model-based density estimates for these populations were used to calculate the mean center of density, effective area occupied, and total abundance index for each population by year. Results showed that the spatial ranges were respectively 84 km and 94 km for spatial and spatio-temporal variation in the distributions of targeted populations, suggesting large variability in the community structure at a distance >100 km. Fish populations showed larger fluctuations in the mean center of density and effective area occupied than crustaceans and cephalopods, while most of the 13 populations tended to shift northeastward to deeper waters over the five years. Additionally, 12 of the 13 populations showed a sharp increase in total abundance in the spring of 2017 under the force of increasingly conservational fisheries management since 2017. In 2018, the total abundance indices remained high for most fish populations, but showed an abrupt decrease for all six of the invertebrate populations. These results indicate that major fish and invertebrate populations in a simplified ecosystem could differ in their responses to habitat changes, management or other human activities in a rapid manner, suggesting a demand for adaptive fisheries management at system scale for degraded/declining ecosystems worldwide.