Can the annual bycatch per unit effort of common dolphins (Delphinus delphis) be used to index the species' relative abundance in the East Sea of Korea?

Visual line-transect surveys are widely used to estimate the abundance of cetacean populations; however, this method can be prohibitively expensive. Thus, a less expensive alternative method was investigated in the present study, i.e., whether temporal changes in the bycatch per unit effort (BPUE) of cetaceans could be used to index the changes in their abundance. Specifically, the relationship between the annual BPUE of two main bycatch fisheries and the relative abundance of common dolphins (Delphinus delphis) was examined in 2012, 2013, 2015, and 2017 when vessel-based line-transect surveys were conducted in the coastal region of the East Sea of Korea. The encounter rate (individuals/km) by study year was significantly correlated with the annual BPUE of set nets. An increase in the density (individuals/km²) of common dolphins, estimated according to a hazard-rate key function by study year, was associated with an increase in the BPUE of set nets. These results suggest that the annual BPUE of common dolphins could be used to index the species' relative abundance. The BPUE of set nets, which are fixed in coastal regions, reflect the densities of common dolphins in the coastal regions better than that of gill nets used in coastal and offshore regions.