江豚的一生

江豚牙齒被染色後的 橫切面

Cross-section of a finless porpoise tooth after sectioning and staining process

江豚的一生 Life history

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江豚的年齡鑑定工作，主要依靠其牙齒橫切面 的年輪分析(就像計算樹木橫切面的年輪)，而 江豚的牙齒則是從擱淺屍體取下來的。鯨豚專 家以往普遍認為所有鼠海豚壽命較短，大多不 會活超過20歲。但香港的江豚研究發現，本地 最年長的江豚達33歲，亦可能是鼠海豚科的最 老紀錄。其實，在其他地方進行有關鼠海豚的 年齡研究，由於樣本主要來自被漁網誤捕的死 亡個體，所以這些「英年早逝」的鼠海豚可能 未能活到老就已死掉，這可能令研究結果有所 偏差。當然，我們亦不能排除香港江豚較其他 鼠海豚長壽的可能性。

The estimated age of a finless porpoise is based on analysis of dentinal growth layers from a crosssection of the teeth that are retrieved from stranded specimens; this method is similar to reading the rings for growth layers observed in tree trunks. Porpoises are often thought to have short life spans, rarely reaching the age of 20. But to researchers’ surprise, the oldest finless porpoise aged thus far in Hong Kong was 33 years old, which is probably the oldest found in any porpoise species. It is possible that studies of porpoise life spans in other areas are heavily biased by examining by-caught animals only so these animals would not have been able to reach their known maximum age; or maybe finless porpoises in Hong Kong just enjoy a longer life span than other porpoises.

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江豚的成長亦是與眾不同的。剛

出生的幼豚一般身長只有約67-70 厘米，成年最長的亦不會超過1.8 米。幼豚出生時體型十分細小及 纖瘦，就像一尾中型的魚類；牠們大多 在秋末至初冬的月份(即每年的10月至翌 年的1月)誕生，江豚亦正於此時開始移 至香港南面水域。幼豚在出生後首一年 剛出生的江豚只有少於70厘米的體 長，體形纖瘦，身上亦有多條胎褶。

Newborn calves are less than 70cm in length and quite skinny. The foetal folds were clearly visible on this newborn porpoise.

的成長速度非常迅速，一歲過後成長速度才開始放緩。根據一個較小的數據樣 本，研究員估計雄性江豚在4-5歲已達性成熟期，而雌性亦可在5-6歲左右開始交 配繁殖；由於江豚在較年輕時已達性成熟期，約每隔一年就可生產一胎，所以牠 們一生中應能比中華白海豚孕育更多幼豚。

The growth of porpoises is quite interesting too. When they are first born, they are at about 67- 70 cm in length, and can grow up to about 1.8 m in length. The newborn porpoises are very small and thin, pretty similar to a mediumsized fish. Most births of local porpoises occur between late autumn and early winter months (around October to January), and that is when more porpoises start to move inshore to the southern waters of Hong Kong. After birth, 在剛死去不久的江豚頭部可見漂亮 the growth of newborn porpoises is very rapid, 的斑紋及淺色的「唇印」 especially in their first year and then begins to Beautiful body stripes and light patches level-off thereafter. Based on a small sample around lips of the head region can be size, researchers have determined that male found on a freshly stranded porpoise porpoises can reach sexual maturity between the ages of 4-5 years, while females can reach sexual maturity at the ages of 5-6 years. In contrast to the Chinese white dolphins, finless porpoises are likely capable of producing more offspring given their early age of sexual maturity and shorter calving interval.

與中華白海豚相比，在香港生活的江豚較 不會「揀飲擇食」。根據牠們的胃部食物 殘渣分析，本地江豚主要獵食25種魚類、 6種頭足類(包括魷魚、墨魚及八爪魚)及 一種蝦。這些江豚賴以為生的食物主要是 一些近岸底層及中層的品種，所以研究員 估計，江豚主要在近岸不同水深的地方覓 食。牠們所吃的海產種類雖跟中華白海豚 有所重疊(主要是一些河口生活的魚類)， 但江豚更會獵食其他近岸的海洋性魚類。 這兩個品種不同的分佈及棲身地運用中亦 反映了牠們的覓食習慣。

Unlike Chinese white dolphins, finless porpoises in Hong Kong appear to be less selective when it comes to their prey. From examining stomach contents of stranded animals, researchers found that local porpoises preyed upon 25 species of fish, six species of cephalopods (i.e. squids, cuttlefishes, and octopuses) and one species of shrimp. These prey species are primarily inshore, bottomdwelling and mid-water species, suggesting that porpoises probably feed in different levels of the water column in nearshore habitats. Even though there is some overlap in targeted prey species with Chinese white dolphins (mainly some estuarine fishes), porpoises also exploit other coastal, non-estuarine habitats. This is also reflected in the contrasting distribution and habitat requirements of these two resident cetaceans! 00

在這條被漁網誤捕致死的江豚胃 部，發現一些未被完全消化的魚 類，這有助研究員瞭解江豚的覓 食習性。

From the stomach content of this by-caught finless porpoise, some undigested fishes were found, which could aid researchers to study its feeding habits.

這些魷魚及蝦均在一條擱 淺江豚的胃內找到

These squids and a shrimp were found inside the stomach of a stranded porpoise