2000년대 초기 제주도에 서식하는 남방큰돌고래(Tursiops aduncus)의 분포특성과 풍도 추정

Abstract

The bottlenose dolphins (Tursiops spp.) commonly inhabit Korean waters, including the coastal waters of the Jeju Island. They are one of the most widely studied species in marine mammalogy field. However, its taxonomic position was unclear because of the validity of this genus. The genus Tursiops has recently been determined to comprise two species: common bottlenose dolphin (Tursiops truncatus) and Indo-Pacific bottlenose dolphin (Tursiops aduncus). Both Tursiops species look very similar. However, Tursiops aduncus is relatively smaller and slender than Tursiops truncatus. Tursiops sp. off the Jeju Island shows several distinct features can be identified as T. aduncus. To confirm the taxonomic position of bottlenose dolphins frequenting the coastal waters of Jeju Island, the external morphology and osteology of the specimen from the Jeju Island were examined. The photographs of free-swimming individuals were also used for determining external morphological characters. The cranial and meristic measurements fell within the ranges of T. aduncus. Osteological ratios were also satisfied with those of T. aduncus. The presence of prominent ventral spot was observed among some individuals. The genetic data agreed with morphological classifications. As results, the dolphins mainly distributed in the coastal waters off the Jeju Island were identified as Indo-Pacific bottlenose dolphins (T. aduncus) in terms of their cranial characters and ventral spotting. I propose its new Korean name, “Nam-bang-keun-dol-go-rae”. Indo-Pacific bottlenose dolphins are mid-sized marine mammals distribute in warm temperate to tropical Indo-Pacific Ocean. The social structure is characterized by a typical fission-fusion type of society, where individuals show flexible grouping patterns. The group size commonly consists between 30~100 individuals. Identification of individuals has become one of standard tools to the study of animal behavior and ecology. Numerous photographic identification works have been successfully conducted for studying various species of marine mammal populations. In this study, the validity of photo-identification method for the Indo-Pacific bottlenose dolphin was attempted from November 2007 to December 2009 sighting surveys. The residency patterns of Indo-Pacific bottlenose dolphins were also investigated using image data. During the 32 sighting surveys, a total of 12 pods were observed. The most frequently sighted size of dolphin pod was 40~50 individuals. The overall sighting rate was 0.01 pod/n.m. Most of Indo-Pacific bottlenose dolphins were sighted within 500 m from the coast line of northern part of the Jeju Island. During the 11 photo-identification surveys, a total of 2,381 dolphin images were taken. Scars and nicks on their dorsal fin as natural markers were constant or gradually changed in shape and location throughout time. These characteristics gave the opportunity to identify individuals within a population for a long survey period. The use of photo-identification has great potential for mark-recapture studies. 89 individuals were photographically identified by natural markers. Ninety four percent of them were re-sighted more than twice during the surveys. Such high percentages of re-sighting rate indicate that the Indo-Pacific bottlenose dolphins are resident stock off the Jeju Island. Population abundance was calculated using mark-recapture methods applied to photo-identification data during a 2-year-study period. Using program CAPTURE, total population of 124 individuals (95% CI = 104-143) in 2008 and 114 individuals (95% CI = 109-133) in 2009 were estimated. Their population size is relatively lower compared with the other dolphin populations reported around the world. A population assessment of the Indo-Pacific bottlenose dolphins off the Jeju Island was conducted using the photo-identification data and several published demographic parameters. In order to assess the anthropogenic threats, bycatch events of the animals in the study area between 2009 and 2010 were investigated. The annual bycatch rate was estimated at 7.9%. Most of bycaught individuals were found in pound nets. The annual growth rate of the population was 3.4%. If present bycatch mortality do not change, the population would be drastically decreased. 85.7% of bycaught individuals in pound nets were found alive. If it is possible to release all of them, gradual population increase is expected. Fisheries bycatch, and where possible other anthropogenic impacts, need to be eliminated to allow population recovery. This is critical if the negative impact on the species is to be minimized and the dolphins are to continue to represent a socioeconomic resource in the region. This study provides the data on distribution patterns, abundance estimates, population projection and status of Indo-Pacific bottlenose dolphins off the Jeju Island, Korea. However, little is known about the demographic parameters for these animals. To conserve resident dolphins off the Jeju Island, long term survey is required to monitor their population fluctuations and changes in age structure.