甘比亞魚類概論 Fishes in the Gambia River: A synthesis (I)

我們就從甘比亞河和其他西非河川的比較開始吧。Baran 在2000年發表的報告(Baran 2000)比較了數條西非主要的大河川，其中甘比亞河被分類是"正常型"，也就是鹽度從河川入海口開始，沿著往上游而逐漸下降 (和"反向型"剛好相反，如塞內加爾的 Sine-Saloum系統)。甘比亞河和其他西非河川相比，魚類族群有著不同的種類組成和時空分布。

Let's start with the comparison of the Gambia River to other rivers (Senegal River, Sine-Saloum River, Casamance River, Rio Buba and Fatala River) in West Africa (Baran 2000). The Gambia River is a "normal river" that the salinity declines with increasing distance from the sea (, as contrary to "inverse river" in the Sine-Saloum estuary system). Also the fish fauna differed from other river, in the respect of fish species composition and spatial-temporal trend. The Gambia River the marine fish penetrate into upstream region during dry season, and the river fish invade downstream region during rain season.

Baran (2000)所研究的河川

The map showing studied rivers in West Africa. Credit: Baran (2000)

Baran (2000)的圖四: 三條不同西非河川的魚類種類組成

The spatial-temporal trend in a "normal river" as Gambia River. During dry season, the marine fish occupy upstream region, while in the rain season, the river fish invade downstream region

"正常河川"魚類隨著雨季乾季的變化。乾季時海洋起源的魚類會往上游移動，然而雨季時河川起源的魚種會入侵到下游地區。

甘比亞魚類的調查，可以追溯到1950年代。Daget (1960)調查整從幾內亞(Guinea)到大西洋出海口，整條甘比亞河的魚類相，並將甘比亞和根據地理位置特性分成上中下三段。上甘比亞河段指從幾內亞中部Fouta-Djialon高地的發源地，往北延伸至塞內加爾的Kedougou，大約長220公里的河段。因為高低差接近1000公尺，上甘比亞河段多以急流及瀑布為主。中甘比亞河段從圍繞Kedougou山區的河段開始，到Niokolo-koba國家公園最後的急流區，長約500公里的河段。在此段甘比亞河岸被草、森林等植物覆蓋，種類多樣有如森林的展示廊。下甘比亞河從塞內加爾-甘比亞的邊界開始，一直到出海口的區域。下甘比亞河特點是極度平緩，從邊界到首都Banjul只有約0.5m的落差。另外下甘比亞河可感受到海洋漲退潮的影響，然而海水只會侵入大約離出海口180km深的部分。此區域有如綠色走廊，有豐富的森林植被，而且沒有沙洲及礁岩等障礙物，船可以順利逆流而上。

Daget (1960)將整段甘比亞河發現的魚類分成四大類: (1)河口群: 指生活史至少有部分階段會接觸到海洋的物種; (2)蘇丹群: 指主要分布在撒哈拉沙漠以南蘇丹地區的水域，及會在乾季被河水淹沒區域的魚種; (3)幾內亞群:分布中心在幾內亞森林流域的魚種，及(4)其他群: 無法歸類制上三群的魚種。在下甘比亞河區，魚類主要是蘇丹群為主 (50%)，然而河口群也占了可觀的比例(23.75 %)，蘇丹群只占少數(6.25%)。在中甘比亞河，蘇丹群占更大的比例(57.4%)，幾內亞群占的比例也增加(21.3%)，然而河口群僅占極少數(1.6 %)。這主要因為海洋影響只會到下甘比亞河區有關。上甘比亞河區魚類以幾內亞群為主(77.8)，剩下是其他群(22.2 %)。蘇丹群和河口群都沒有發現紀錄。

The study of the fish fauna in the Gambia River can be traced up to the 1950s that Daget (1960) has examined the general fish fauna of the Gambia River. They separate the Gambia River into three sections: High classify fish were classified into upper, middle and lower section.

The upper section, around 220 km, originates from the southern mountains of the Fouta-Djialon highland in the center of Guinea and flows toward the North until Kedougou in Senegal. It is characterized by numerous rapids and waterfalls for high contrast in altitude of approximately 1000 m. The middle section, around 500 km in length, includes a wide loop around the hills in Kedougou and its last rapids locates around the Niokolo-koba national park in Senegal. The river banks of this section are covered by grasses or trees, like a gallery of the forest. The lower section has a very gentle slope that the elevation from Gouloumbo (the border between Gambia and Senegal) to Banjul (river mouth) is only 0.5 m. In the lower section one can sense the tidal influence from the sea, but the "true" brackish water is only for last 180 km. This section is like a green corridor, and there are no sandbanks or hard reef so that the vessels can reach deep in this section safely.

In this paper he classified the fish into four groups: estuarine group (Groupe estuarien), Sudanese group (Groupe soudanien), Guinean group (Groupe guinéen) and other group (Groupe indifférnt). The estuarine group consists of fish species with at least part of the life history associating with the sea. The Sudanese group includes fish species whose distribution area is centered on the waters of Sudanese savannah or the riverbeds that are flooded during the rainy season. The Guinean group includes the fish species whose range is centered on the Guinean rivers in the forests, and the other group consists of fish species that cannot be classified into above groups. 

The fish of the lower section of Gambia are composed of Sudanese group (50 %), estuary group (23.75 %), and other group (20 %), and the Guinean group contributes little (6.25 %). The middle section is occupied mainly by the Sudanese group (57.4 %), and the Guinean group becomes more important (21.3 %), while the contribution from the estuary group becomes negligible (1.6 %). The fish in the upper section are predominantly contributed by Guinean group (77.8 %), and then by the other group (22.2 %).

Daget (1960)的原始論文

La faune ichtyologique du bassin de la Gambie by Daget (1960)

Three sections of the Gambia River classified by Daget (1960)

Daget定義下的上中下甘比亞河. Credit: Daget (1960).

Composition of fish groups for lower (Basse), middle (Moyenne) and Upper (Haute) section

上中下甘比亞河不同魚類群組成. Credit: Daget (1960)

在Daget (1960)之後，Lesack (1986)繼續研究甘比亞河流域的漁業情況及魚類相。他研究的區域在甘比亞境內的甘比亞河，也就是Daget所定義的甘比亞河下游區。然而與Daget不同的是，他以紅樹林往河川上游分布的最邊緣，再將在甘比亞境內的甘比亞河分成兩段:上游區與下游區。這分界也大致與一年中平均的淡水/半淡鹹水的分界線一致。

After Daget (1960), next available literature examining fish fauna and fisheries for the Gambia River is Lesack (1986), who focused on the section in the nation Gambia (equivalent to lower section of Gambia River in Daget 1960). Different to Daget, he divided the Gambia River in the Gambia into two parts: Upper River Reach and Lowe River Reach, corresponding to the approximate limit of mangrove forest. This line is also roughly the average location of saline from for the whole year. 

Lescak 1986, Journal of Fish Biology 28, 679 - 700

Sampling site in Leack (1986) and his division of Gambia River

Lesck (1986)的採樣點，以及對甘比亞境內甘比亞河的分區圖

他在圖二展示了漁獲量、捕撈努力量以及單位努力漁獲量的時空變化。在下游地區，漁民傾向在乾季中期(二月)以及雨季初期(七月)捕撈。單位努力漁獲量(CPUE)，一個可是為魚類族群豐度的粗淺指標，從乾季中期(一月)到乾季結束時不斷增加。雨季開始時(六月)開始下降，並且在雨季末期(十月)時達到最低點，並且隨著乾季的開始CPUE逐漸回升。在上游地區漁民使用相對固定的捕撈努力量，因此捕撈量應大約與魚類族群豐度有關。CPUE從年初開始一直到雨季開始時(六月)逐漸漸少，然而雨季的第二個月(七月)上升到最高點，但隨即在八月下降到最低點。一直到乾季開始時(九月)，CPUE才逐漸回升。另一方面，雖然資料有些不完整，但圖三顯示: 在每個月份，下游地區的CPUE都大致有高於上游地區的趨勢。另外漁具的不同、漁民間的差異、魚類地理分布及捕獲率的差異，也可能是導致資料高時空變異性的原因。

He also showed seasonal and regional variation in the catch, effort and catch per unit effort (CPUE) in his Figure 2. In the lower river region, the fishermen tended to go fishing in the middle of dry season (February) and early rain season (July). Nominal CPUE, as a rough indicator of fish abundance, increased gradually from January to the end of dry season (April-May) and started to decreased when the rain season started (June). It reached the lowest in the end of rain season (October) and increased again as the rain season started. In the upper river region, fishermen deployed relatively stable fishing effort, and the catch probably reflected the changes in fish abundance reflected by nominal CPUE. CPUE decreased gradually in February to June when the rain season started, and peaked soon in July and decreased again. It bounced back from September when the dry season started again. Meanwhile, although data were fragmentary, the CPUE tended to increase from up-river (Basse) to down-river (Kerewan) within any given month, as shown in his Figure 3. High temporal and spacial variability in catch-effort data probably reflected differences in the equipment, the preference of fishermen, the distribution  of fish and differences in fish catchability.

 Leack (1986) 中的圖二:捕獲量、努力量及單位努力量漁獲的月別變化

Figure 3 in Leack (1986) showing CPUE by months (y-axis) and sampling sites (x-axis)

單位努力捕獲量在不同月份(y軸)及採樣地(x軸)的分布

由圖五來看，在下游區，Bonga魚、馬拔魚(午仔)還有海鯰是最主要出現在漁獲中的物種，這三類就占了總捕獲量的三分之二 (67%)。而這三類魚出現的季節並不一致，似乎有互相交替的現象。Bonga魚主要出現在乾季中期的一月至四月，馬拔多在乾季晚期雨季初期的七月出現，而海鯰則是乾季中期最多，隨著雨季的接近逐漸下降(圖六)。然而捕捉量的種類時間變化，也反應下游區漁業模式的改變: 隨著雨季的結束(十月底十一月初)，漁獲努力量開始增加。此時小型圍網和刺網同樣頻繁地使用，主要捕捉物種為Bonga魚。直到乾季中晚期(四月)，下游區幾乎都只用刺網，一開始捕捉海鯰，然後是馬拔。下游區的漁獲量，在雨季剛開始，第一長大雨之前達到高峰。當雨季開始時，下游區的漁獲量因為水流的增加，以及鹽線(Saline front)的往海邊移動等因素下降。下游區的漁業活動，在雨季中晚期也因為水流增加的關係而在最低點。

在上游區，捕撈量不像下游區般，捕撈量相對比較平均分散在較多的魚種上。慈鯛 (Tilapia spp.)，土虱(Clarias sp.)以及河鯰(Chrysicthys nigrodigitatus)這三類最多的種類，也才占總捕獲量的42%。上游區這三類魚的時間變化大致相同: 雨季結束至乾季的中期較多，雨季除了慈鯛類的之外捕獲量都變少。 然而在上游區，海洋起源的魚種，Bonga魚和烏魚類(Mugil sp.)在乾季晚期(五月至六月)出現最多，此時正好是海水最深入內陸的時候。另外和下游區不同的是，上游區整年皆以刺網為主。

In the lower reach, the bonga (Ethmalosa sp.), threadfins (Polydactylus sp.), and sea catch fish (Arius sp.) constituted 67 % of the total catch (Fig. 5). A succession of catch among dominant fish group was found (Fig. 6): the bonga fish peaked in the middle of dry season (January to April), the threadfins peaked in the end of dry season and starting of rain season (July), and the sea catfish decreased gradually from middle of dry season (April). However, the temporal changes in the catch also reflected the changes in the fishing activity: Fishing effort increased quickly after the rain season (end of October) with surrounding nets and gillnets equally prevalent, and the fishermen target mostly bonga fish. A shift occurred later (April) to almost exclusively gillnet fishing, with Arius sp. and later Polydactylus sp. preferred. The peak in overall catch for the lower reach was slightly before the beginning of the heavy rains (July).  Catches declined thereafter as the rains increased and the salt front was pushed downstream by increasing river discharge. Fishing activity was at a minimum  during  September-October  as  the  rains  ended  and  river discharge peaked.

In the upper reach,the catch was not dominated by a few species as in  the lower  reach. Tilapia (Tilapia spp.), catfish (Clarias sp.), and bagrid catfish (Chrysicthys nigrodigitatus) were the three most abundant sorts,together constituting 42% of the total catch. The temporal distributions of catch for these fishes (Fig. 6) were similar, but the marine fishes showed a different pattern. For example, the bonga fish and mugilids (Mugil sp.) peaked during May and June when the saline front entered the deepest to the upstream. Contrary to the lower reach, gillnet techniques predominated through most of the year in the upper reach.

Figure 5 in Leack (1986), fish catches in lower and upper reach of Gambia River.

Leack (1986)的圖五.上下游區漁獲捕撈量的種類組成。

Figure 6 in Leack (1986), temporal trend in the catch of  important fish species
圖六 Leack (1986).主要魚種捕撈量在上下游區的月別分布

Gillnet is the predominant fishing technique in upper reach of Gambia river.小型刺網是甘比亞河上游區最常使用的漁法

An example for fish catch in the upper reach. 上游區漁獲魚種組成一例. Credit: Allen Huang

Bonga fish is the most important fish species in the lower reach. They are consumed mostly as smoked fish.  Cread: Allen Huang

Bonga魚是下游區最重要的魚種，主要以燻魚乾的形式消費。

Giant African threadfin (Polydactylus quadrifilis ).

四鬚馬拔，同樣也是下游區重要漁獲物種。

North African catfish (Clarias gariepinus) had a good time in the culturing pond.
北非鯰魚(土虱)，吃養殖場的吳郭魚吃得很肥。

Bagrid catfish (Chrysicthys nigrodigitatus).

甘比亞河鯰，上游區常見魚種之一

這篇我們回顧了在1990年代以前，對於甘比亞河魚類的兩大研究。下篇我將針對近代(1990之後)的文章作整理。

Here, we summarized the knowledge base of Gambia River from two critical papers before 1990. In next post I will focus on the recent papers (after 1990).