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?

Abstract

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.

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

“…A certificate reported from a mandatory investigation (bycatch species and location, type of injury to cetacean, etc.) for each cetacean bycaught by fishermen was issued by the Korean Coast Guard in 2011 [37]. Each cetacean bycaught in fishing gear is individually counted.…”

“…This study was conducted around the widest hotspot of common dolphins, as their main hotspot. In the present study, the BPUE of common dolphins using a set net is used to index the relative density of species in the coastal region [37]. The BPUE was calculated by dividing the bycatch by the gear number of the set nets in each year.…”

“…collecting information in Korean waters was established [37]. Therefore, we initially examined the characteristics of temporal changes in the BPUE from 2011 to 2021 and then investigated the relationships between the annual BPUE and environmental variables using regression models.…”

“…Visual line-transect surveys are commonly used to estimate cetacean abundance [34], but ship and aerial use of such surveys can be prohibitively expensive [35,36]. Recently, our study examined whether bycatch per unit effort (BPUE) can be considered as a possible relative abundance index of cetacean species as part of an inexpensive method for examining changes in abundance and suggested that the annual BPUE of common dolphins by set net could be used to index the relative abundance of species in the coastal regions of the East Sea of Korea [37].…”

“…In this study, we focused on environmental factors that influence annual changes in the BPUE of the common dolphin around their main hotspots in Korean waters. Reliable information on cetacean bycatch has been provided since 2011, when a legal system for collecting information in Korean waters was established [37]. Therefore, we initially examined the characteristics of temporal changes in the BPUE from 2011 to 2021 and then investigated the relationships between the annual BPUE and environmental variables using regression models.…”