A population size estimate of the finless porpoise, Neophocaena phocaenoides, from aerial sighting surveys in Ariake Sound and Tachibana Bay, Japan

Abstract

We estimated the population size of the finless porpoise (Neophocaena phocaenoides) in Ariake Sound and Tachibana Bay of western Kyushu, southwestern Japan, from aerial sighting surveys using line transect methods. All 12 surveys were conducted from May 1993 to May 1994 (8 in Ariake Sound and 4 in Tachibana Bay). In addition to these, 14 surveys were also carried out to obtain information on porpoise occurrence in Tachibana Bay (5 surveys) and in neighboring Sumo Nada (5) and Yatsushiro Sound (4). In Ariake Sound, 225 porpoise groups (369 animals) were detected during all flights totalling 1,694.4 km. In Tachibana Bay, a total of 997.8 km was surveyed and 55 groups (290 animals) were sighted. However, no sightings were recorded in Sumo Nada (distance searched = 148.7 km) and Yatsushiro Sound (208.4 km). In Ariake Sound, few sightings were recorded from waters shallower than 5 m in depth. In Tachibana Bay all animals were detected from waters of less than 50 m depth. The population size was estimated as 1,983 animals in Ariake Sound (95% CI = 1,382‐2,847), 1,110 in Tachibana Bay (95% CI = 642‐1,920), and 3,093 in the 2 waters (1.3 individuals/km2, 95% CI = 2,278‐4,201).

Effects of bycatch on the population viability of the narrow-ridged finless porpoises in Ariake Sound and Tachibana Bay, Japan

“…The sampling bias in the interviewbased survey to estimate the number of by caught porpoises was not incorporated into this study. The most recent available abundance estimate in Ariake Sound and Tachibana Bay was provided from observations taken in 1993 and 1994 (Yoshida et al 1997), and it may be too old to estimate the bycatch mortality rate in 2007 and 2008, or to use as the current population size. Our results suggest that predictions are sensitive to the estimate of M (Table 2; Scenarios I and II).…”

“…For the Ariake Sound and Tachibana Bay population, the most recent abundance estimate is 3093 ind. (coefficient of variation [CV] = 15.7%) in 1993 and 1994 (Yoshida et al 1997); 250 ind. (CV = 22.3%) were estimated to be bycaught annually during gill net fishing .…”

“…The details of the demographic parameters were described in the previous section. The population size in the first year (N 0 ) for all the simulation trials was set to be 3093 ind., derived from the latest abundance estimate for the porpoise population (Yoshida et al 1997). A stable age structure (without any bycatch mortality) was set as the age structure in the first year.…”

“…Tachibana Bay connects Ariake Sound to the open sea. The porpoise population in these waters is one of the most abundant, and has one of the highest population densities in Japanese waters (Yoshida et al 1997, Yoshida 2002, Amano et al 2003, Shirakihara et al 2007. Moreover, the population shows a distinct haplotype cluster (Aizu et al 2013); therefore, it is a valuable resource for discovering the origin of Japanese narrow-ridged finless porpoises.…”

Finless porpoise bycatch in Ariake Sound and Tachibana Bay, Japan

“…1). This population, whose size in 1993/1994 was estimated at 3093 individuals (Yoshida et al 1997), suffers mortalities in bottom-set and drift gillnets (M. Shirakihara et al 1993Shirakihara et al , 2008. To clarify the magnitude of bycatch in gillnet fisheries for this population, we conducted interview-based surveys among fishermen in 2007 and 2008.…”