Recovering or Still Depleted? Updated Population Assessment of Tappanaga, a Northern Local Population of Short-Finned Pilot Whales (Globicephala macrorhynchus) off the Coast of Japan

Abstract

The northern form (often called Tappanaga type) population of short-finned pilot whales (T-SPW) is a local population that inhabits the Kuroshio-Oyashio mixed water regions off northern Japan. The population has been severely depleted by past whaling. Rigorous management was first introduced in the 1980s, and whalers have voluntarily refrained from taking T-SPW since 2007. Our primary goal is to investigate whether the T-SPW population has recovered. We estimated recent abundances based on data from the line-transect sighting surveys dedicated to T-SPW. Simple population dynamics models were fitted to these and previously published estimates for 1985–2006. When a detection probability on the track line was assumed to be 1 (g(0) = 1), the estimated abundances in 2007, 2021 and 2022 were 1659 (coefficient of variation, CV = 0.79), 2634 (0.93) and 759 (0.64), respectively. The population dynamics model showed that past whaling substantially reduced abundance, and no clear tendency for population recovery was seen even after the voluntary suspension of whaling. Compared with the estimated carrying capacity, the current population size was still considered to be nearly depleted. To improve the conservation and management measures, we need more efforts to identify the causes of nonrecovery of the T-SPW population.