台灣的中華白海豚社群結構及生殖參數

Abstract

台灣西岸的中華白海豚(Sousa chienesis)族群因數量不到100隻而被IUCN (the International Union for Conservation of Nature)紅皮書列入”極危”。堅實的保育工作急需瞭解其族群的動態發展，社會結構與生殖動態是其中首要的基礎資訊。本論文以照片辨識追蹤的標記-再捕捉方法以瞭解牠們的行為與生殖動態。2007到2010年中共執行352趟海上調查，藉由超過六萬張的照片辨識出98隻個體，包含71隻非嬰幼兒個體(其中有22隻哺乳中雌性)，以及透過母豚身份鑑定出27隻嬰幼兒。根據分層群聚分析(hierarchical cluster analysis)以及多因次等級分析(multidimensional scaling analysis)結果，台灣西岸中華白海豚族群含有兩個社群(community)，各自有不同年齡層及性別組成，形成不穩定的社交模式，社群之間尚未完全被隔離。同時棲地忠誠性模型指出動物在南北區間移動；雖然部分個體都會利用南北區，但不均質的移動模式指出動物各對北區或南區有所偏好，這個現象可能跟動物的生理狀態有關，間接反映出棲地的特性。兩社群的分隔也可能是人為開發、衝擊造成。動物的社交行為可能與尋覓交配有關。在生殖動態上，全年都有新生兒出生，春到夏季為生殖高峰(calving seasonality)。年自然出生率(crude birth rate)為0.064 ± s.d. 0.037，幼豚生存率(calf survival rate)與存活率(calf survivorship, 至1歲)為0.66 ± s.d. 0.20及0.78 ± s.d. 0.39，年族群補充率(recruitment rate)為0.090 ± s.d. 0.030，生殖周期(calving interval)為3.52 ± s.d. 0.28年，幼豚約在3-4歲左右獨立(weaning age)、不再與母豚形成緊密連結。一系列的族群參數顯示台灣中華白海豚族群增長率低，屬於脆弱的族群、保育狀態極為敏感，牠們需要一個適切恰當的保育政策，以維持其族群延續：兩社群偏好不同的海域，維持與改善其棲地品質對社群的維持極為重要；此外，根據其生殖高峰，在南區實施季節性的禁漁期可確保母豚攝取足夠養分以補育幼豚。

Indo-Pacific humpback dolphin (Sousa chinensis) population that inhabits west coastal waters of Taiwan was estimated to number less than 100 individuals and is listed under the IUCN criteria as “Critically Endangered”. In this study, photo-identification mark-recapture techniques were applied to investigate their behavioral and reproductive dynamics. During a 4-year study period (2007-2010), 352 boat-based surveys were performed, resulting with over 60,000 photographs; 98 individuals were photo-catalogued, of which 22 were reproductively active females and 27 were calves. Hierarchical cluster analysis and multidimensional scaling revealed that the population consists of two communities, each with a moderately fluid pattern of geographic and social fidelity. Although the communities are not entirely discrete, their interactions are limited to 'casual acquaintances” while modeled lagged identification rates suggest that “emigration and reimmigration” best describes their movement and ranging pattern. There seems to be further differences between individuals from both communities, including different percentage of coloration/age-stages and the number of mature females. Although some individuals can be seen in both communities, their heterogeneous movement pattern indicates that over a longer time scale they prefer a certain region which is their respective community range. The degree of separation between the two communities requires further investigation, but it seems to correspond with features of local habitat. It is also possible that the separation is a historically more recent event related to anthropogenic impacts, as there is an area of increased industrial activity located approximately in the middle between the two communities which might act as a human-caused barrier. The social behaviour might be performed as mate-searching behaviour, while the parturition happened throughout the year with a peak in spring and summer, resulted a crude birth rate of 0.064 ± s.d. 0.037. The mean survivorship and survival rate to Age 1 were estimated at 0.66 ± s.d. 0.20 and 0.78 ± s.d. 0.39, respectively; and the recruitment rate (at Age 1) was 0.090 ± s.d. 0.030 with calving interval approximating 3.52 ± s.d. 0.28 years with the weaning age at around 3-4 years old. Estimated suite of population parameters indicating a slow population growth rates highlight further the vulnerability and sensitive conservation status of Indo-Pacific humpback dolphins off Taiwan; indicating that informed conservation management plan is urgently needed.