Environmental Factors Influencing Annual Changes in Bycatch per Unit Effort of Delphinus delphis around Their Main Hotspot in Korean Waters

In this study, the characteristics of temporal changes in bycatch per unit effort (BPUE) as an index of the relative density of the common dolphin around their widest hotspot in the East Sea/Sea of Japan were examined from 2011 to 2021. BPUE rapidly increased from 2017 to 2019. The annual changes in BPUE were due to changes during March–April, which is the most abundant season for common dolphins. The annual relationship between BPUE and four variables (Pacific herring, common squid catches, chlorophyll-a concentration, and sea surface temperature) around their main hotspot for March–April was investigated using generalized linear models (GLMs) for gamma distribution. A stepwise Bayesian information criterion for the gamma GLM yielded significant retention of two variables, Pacific herring catch and chlorophyll-a concentration, over the study period, indicating that the rapid increase of the relative density of the common dolphin in the early spring during 2017–2019 could be due to the increase of their prey abundance caused by higher primary productivity. Therefore, ecosystem productivity altered by nutrient conditions could be a key biological process that enhances habitat use of cetaceans in highly productive seasons and regions.