台灣中華白海豚的傷痕研究

Abstract

台灣西海岸的中華白海豚族群被列為極度瀕危，亟需保育。海豚身上的傷疤可以反映海豚在過去所面臨到的環境或掠食者壓力。本研究利用海上調查照片與擱淺中華白海豚解剖資料來檢視中華白海豚的體表傷疤，並探討其所面臨的人為與自然環境壓力。本研究將傷疤分為兩大類，包括六種人為肇因傷疤以及一種自然的鯊魚咬痕。研究發現，在71已辨識的海豚中有42%的海豚身上帶有人為相關傷疤，年增加率為2±0.707(傷/年)。帶傷比例隨年齡增長而增加，尤以青年期進入成年期階段增加幅度為劇。雖然台灣白海豚的擱淺率相當低，但與香港和南非的白海豚族群比較，台灣帶傷之比例較高，值得關注。人為肇因最相關的兩種傷痕：纏繞傷與船槳傷，身上有此類傷疤的海豚個體多出沒於苗栗—台中以及台中—北彰化一帶，與漁業以及船隻作業繁忙地區重疊，可能受衝擊機率大。 自然肇因的傷疤皆來自鯊魚咬傷，檢視的71隻海豚中發現25.4%的海豚身上帶有鯊魚咬痕，年增加率為0.8±0.98(傷/年)。此比例與南非Natal地區白海豚族群結果相仿，推測與海豚和鯊魚棲息環境的鹽度相同有關。比較四個年齡群中各群內帶傷個體比例發現，帶傷比例增加幅度同樣以青年期進入成年期階段最劇。另外自性別比例來看，在受傷的海豚中有43.75% 為雌性，推測與海豚懷孕以及保護幼體相關。58%的鯊魚咬痕集中於海豚身體後半處(包含尾幹)，與前人推論的海豚迴聲定位盲點假說一致。致傷的鯊魚種類則根據相關生物資料推論，可能為污斑白眼鮫(Carcharhinuslongimanus)或鼬鯊(Galeocerdocuvieri)。 綜合結果發現，不論是來自人為還是自然，台灣的中華白海豚族群都面臨到相當大的環境壓力。雖然我們難以改善白海豚所面臨的自然環境威脅，但仍應盡速採取相關行動降低牠們所面臨人為環境的壓力，以達保護這個脆弱族群的目的。

The humpback dolphins (Sousa chinensis) off the west coast of Taiwan are listed as a critically endangered species in the IUCN list that call for instant protection. Scars on the dolphins can be studied as records of the environmental or predatory threats these cetaceans encountered. In this study, I used photos obtained from boat surveys and necropsy reports of stranded/ by-caught humpback dolphins to examine the scars on them and to elucidate the anthropogenic and natural threats they may have encountered. There scars are classified into six types of anthropogenic scars and one type of natural predatory scars caused by shark bites. Of the 71examined Sousa, 42% carried anthropogenic scars, and the annual increase rate of anthropogenic scars was 2±0.707 (scars/year) in this population. The scarring ratio increased with age, and it increased sharply from the speckled stage to spotted adults. Although the stranding rate was very low in this population, the scarring rate on the other hand was relatively higher than those of the Sousa populations in Hong Kong and South Africa. The dolphins bearing impression marks and propeller wounds, the two types of scars most likely related to anthropogenic activities, were most frequently sighted in coastal waters from Miaoli to Taichung and between Taichung and north Changhua, overlapping with the areas characteristic of heavy vessel traffic and fishery activities. With regard to natural predatory scars, all the scars we examined were determined to be shark bites. Of the 71 dolphins 25.4% displayed shark bites. The annual increase rate of shark bites was 0.8±0.98, which was similar to the result found in Natal, South Africa. The consistency might be related to the similar salinity of the dolphins’ and sharks’ habitats. The predatory scarring ratio also increased fast during the period bwtween the speckled-stage and spotted adults. Among the sexually mature, scarred dolphins, 43.75% were females. This result may be associated with the adult dolphins’ protecting behaviors of the infants and calves. Based on the fish fauna records in Taiwan, the predatory scars are probably inflicted by the oceanic whitetip sharks (Carcharhinuslongimanus) and tiger sharks (Galeocerdocuvieri). This study reveals that humpback dolphins in Taiwan face both specific anthropogenic and environmental threats. Although it is difficult to reduce the natural predatory pressure on the dolphins of this vulnerable population, reducing the anthropogenic threats must be a priority task for their conservation.