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

In this blog article I am going to summarize the studies about the fish fauna in the Gambia River after 2000. First comes the joint survey between french scientists from IRD (Institut de recherche pour le développement), IFREMER (Institut français de recherche pour l'exploitation de la mer), and Department of Fisheries, the Gambia from November 2000 to April 2002. The survey is composed of two parts: hydro-acoustic and seining, which are summarized in Albaret et al. (2004),  Guillard et al. (2004), Simier et al. (2006) and Guillard et al. (2012), respectively. In addition, I will also briefly introduce the survey done by the IRD team in Dakar, Senegal in the lower estuary in a similar period  (2002). Their results were summarized in Vidy et al. (2004).

在這一篇文章中，我會對在2000年後，針對甘比亞河魚類的調查研究做個整理。首先是法國研究團隊(IRD, IFREMER)以及甘比亞漁業署，在2000年11月至2002年四月於甘比亞河下游區域所完成的一項共同調查研究。這研究分成聲納調查以及圍網採樣兩部分，結果分別發表在Albaret et al. (2004)、Guillard et al. (2004) 、Simier et al. (2006) 及Guillard et al. (2012)這四篇文章中。另外大約在另外一個在塞內加爾IRD的法國團隊，也針對甘比亞河下游做了一次調查，其結果總結在 Vidy et al. (2004)這篇報告。因為調查時間地點接近，就也順便介紹。

Four papers about the join survey by IRD, IFREMER, and the Department of Fisheries of the Gambia:

Albaret et al. (2004).

 Guillard et al. (2004)

Simier et al. (2006)

Guillard et al. (2012)

First let us make a quick review of area covered in their survey and its environmental settings. According to Albaret et al. (2004), the survey examined 44 stations from the river mouth, going upstream until to Deer Island (Figure 1). The surveyed area is shallow, with depths ranging from 2 to 12 m (Figure 2). As summarized in previous studies, the Gambia River has a "normal estuary" with decreasing salinity with increasing distances to the river mouth. Also the saline front moved up- and downstream as the season changed between rain and dry season (Figure 3).

首先我們先針對調查所研究的區域，還有其環境因子作個簡單整理: Albaret 等人的研究，總共在44個站點採樣，涵蓋區域從河口一直到靠近中游區的鹿島(Deer Island, 圖一)。 採樣區域不深，深度在2 ~ 12m 之間(圖二)。就如同先前研究所指出，甘比亞河是"正常型河口"，鹽度隨著離河口距離增加而降低 。同時淡海水分界線，也隨著雨季乾季的交替而在往上游或往出海口移動 (圖三)。

圖一. 2000年 IRD-Gambia聯合調查中44個採樣點的分布圖。

Figure 1. Location map of the 44 sampling stations of 2000 joint survey. Credit: Albaret et al. (2004)

圖二. 甘比亞下游自河口至中游區末端的深度變化 Credit: Darboe (2002)

Figure. 2. Depth profile of surveyed area from river mouth toward upstream.

圖三. 環境因子從河口至上游的變化圖。

Figure 3. Environment parameters from river mouth toward upstream

Their survey consist two parts: acoustic and purse-seine survey. We are going to first introduce the acoustic survey done by Guillard et al. (2004) using the ecosounder (SIMRAD EY500, split-beam, 120 kHz frequency). The acoustic survey is conducted during daytime and two protocols are applied: (1) classical mobile acoustic survey and (2) stationary (moored) sampling (Figure 4). The former means that the echosounder was attached to a boat which was cruising along a zip-zap transect with a speed of around 6 km per hour and broadcasted the sound waves vertically toward the bottom. The second means the boat kept still and the echosounder broadcasted the sound waves. The survey lasted around 30 minuntes for both approaches. The scientists also measured the hydrological parameters when they were conducting the survey.

For the acoustic survey in the Gambia River, two types of fish echoes were encountered, individual targets and schools (Figure 5). The echos of schools occurred mostly in the surface and only occurred in the lower estuary. The mobile and stationary protocol have very similar results in terms of target echo strength with high correlation. Also they observed large seasonal and spatial variation in fish biomass estimate in the sampled area, as indicated by back-scattering coefficient (see this manual for detailed information about scattering coefficient). The fish biomass showed clear contract between winter and summer or dry and rain season that it was high in the lower estuary in winter (Nov-Dec) and higher in the upper estuary in spring and summer (April to June, Figure 6).

他們團隊針對甘比亞河口區域的研究調查分為兩大部分:聲納調查以及圍網調查。首先我們介紹由Guillard et al. (2004) 所做的聲納調查研究，所使用聲納型號為: SIMRAD EY500, 屬於分區聲納(Split beam)、聲波頻率120 千赫茲。他們的聲納調查在白天進行，又分為兩種方式: 動態巡弋以定點調查。動態巡弋指漿聲納裝至在調查船上，在設定好的穿越線上以時速約六公里的速度巡弋，同時聲納往河底發送訊號，並接受河中生物的所反射的回聲訊號。定點調查指船在設定好的調查點固定不同，同時向底部發送聲納訊號並接收回聲。兩種方式(動態巡弋及定點調查)大約都耗時三十分鐘，並且同時量測必須的水文參數資訊。

參照圖五，他們發現有兩種聲納回聲訊號: 個體訊號及(魚)群體訊號，同時魚群回聲主要在表層，並且只有在河口下游出現。定點及動態巡弋所得的目標個體回聲強度之結果類似，並兩者有高度相關。將回聲強度當做魚類豐度指標(細節可參照FAO操作手冊)，可以得知甘比亞河口調查區域的魚類豐度有高度的時空變化(圖六): 在冬天(或乾季初期)，在河口下游魚類豐度較高，另外在春夏(乾季晚期)，則是河口上游有較高的魚類豐度。

圖四. Guillard 等人的聲納調查點分佈圖。Credit: Guillard et al.(2004)

Figure 4. Distribution of transect and stations for the acoustic survey

圖五.聲納顯示出(a)個體(b)(魚)群體回聲訊號。Credit: Guillard et al.(2004)

Figure 5. Echos for (a) individual and (b) (fish) schools.

圖六.利用聲納估計生物量的時空分佈。Credit: Guillard et al.(2004)

Figure 6. Spatial distribution of biomass estimate from different surveys in different time

A purse-seine survey was conducted simultaneously with the acoustic survey on the second boat. The purse-seine survey has been applied for fish communities in other West African rivers. The purse-seine is 250 m long, 20 m deep with a mesh size of 14 mm. Nine trained fishermen deploy the net without any surface search and the results were summarized in Albaret et al. 2004.

Totally the caught 70 fish species belonging to 30 families. The fish fauna in the Gambia estuary was dominated by the sciaenid Pseudotolithus elongatus, and then followed by the clupeids Ethmalosa fimbriata, Ilisha africana and Sardinella maderensis (Table 1). The estuary in the Gambia River had a distinct fish species composition that some typical estuarine families were unusually scarce. For example, the gerreids Gerres nigri and Eucinostomus melanopterus are usually among the most abundant, widely distributed and frequently occurring species in West Africa. In the Gambia Estuary,  only a few individuals of both species were captured with  occurrences less than 2.5 %. Also only five species of mugilids were recorded and three typically common mugilids, Liza dumerili, Mugil cephalus and Mugil bananensis were represented by only a single individual. Another very common and widespread mullet, Mugil curema, was not recorded in that survey in the Gambia Estuary, either.

They classified the fish species of the Gambia Estuary into the 8 bio-ecological categories. Five categories with affinity to estuary from high to low are: strictly estuarine species (Es), estuarine species from marine origin (Em), the marine-estuarine species (ME), the marine species accessory in estuaries (Ma) and the marine species occasional in estuaries (Mo). The other three categories represented different affinity to freshwater: estuarine species from continental origin (Ec), the continental species with estuarine affinities (Ce), and the continental species, occasional in estuaries (Co). The fish in the Gambia River estuary were composed of mostly Em and ME species, suggesting the influence of marine environment. Moreover, the Em species dominated the fish community in terms of individuals and biomass, which was contributed mainly by Pseudotolithus elongatus, Ethmalosa fimbriata, and Ilisha africana (Figure 7).

The sciaenid Pseudotolithus elongatus was the most common fish species in the fish community with a remarkably wide spatio-temporal distribution in the estuary (> 96 % occurrence). It occurred in large number for all season at all sites, from sea water to totally fresh water (Figure 8). Even during the end of the rain season with maximum flood (September), it was abundant in the upper reaches of the estuary. In addition,P. elongatus was possibly able to breed in the Gambia estuary because all stages of the life cycle were represented in the system (All stages of P. elongatus were also found in Vidy et al. [2004]). 

The clupdeids Ethmalosa fimbriata, Ilisha africana and Sardinella maderensis was the second most important fish species in terms of total fish numbers, biomass and occurrence. Both juveniles and adults were recorded in seine net samples throughout the estuary. However, they showed a clear seasonal-related spatial pattern: They stayed all year round in the lower reaches and their abundances generally decreased as the distance to the sea increased. They also tend to move upstream during the dry season (Figure 8). 

Synodontis gambiensis is a species of continental origin with a strong estuarine affinity. It was more abundant in the upstream sites than in the lower reach stations, but this spatial distribution trend was also influenced by different season. As the dry season progressed, it had started moving back upstream and was confined to the upper reaches during the dry season (Figure 8).

Simier et al. (2006) analyzed the physical environment variables and the fish community based on these eight bio-ecological categories using a combination of factor analysis, principal analysis and multi-table comparison. The physical environment was characterized by a longitudinal gradient (from high to low) of salinity, dissolved oxygen and transparency that explained a majority (77.4 %) of total variance. Different bio-ecological categories of the fish also associated with this east-west environmental gradient, contrasting marine and freshwater affinity assemblages (Figure 9). This longitudinal gradient also exhibited seasonal variations, that it was the most complete in December (early dry season) and was the weakest in June (at the end of the dry season). 

Then Guillard et al. (2012) showed that two simultaneous sampling, acoustic and purse-seine, produced similar spatial trends in the abundance indexes of fish (Figure 10) and two indexes were highly correlated (Figure 11). They also examined possible interferences from other covariates with regression tree, but i think the correlation between two abundance indexes is a bit blurry. It might be clearer if the confounding effects from other covariates can be removed using statistical modelling, like generalized linear model framework.

另外一艘船上，別組團隊在聲納調查的同時進行圍網調查。圍網調查與此團隊之前在西非其他地區採用同樣的漁法: 使用長250公尺、深20公尺，網目14公厘。網具由九位經過訓練的漁民操作，並且採取"盲目取樣"，不針對魚群下網。數次調查總共採取到30科70種魚類(表一)，石首魚類(Pseudotolithus elongatus)是最常出現、數量最多的種類。其次是三種屬於鯡科的 Ethmalosa fimbriata、 Ilisha africana 及 Sardinella maderensis。另外此次研究並沒有採集到數種在西非河口地區常見的魚種。如兩種鑽嘴魚類 (Gerres nigri 及 Eucinostomus melanopterus)是廣泛在西非分布的常見種，但此次採樣僅採集到幾隻個體。同時也只採集到五種鯔科魚類，同時三種常見種(杜氏鮻 Liza dumerili、烏魚 Mugil cephalus 及Mugil bananensis)採集數也僅有數隻。也沒有採集到庫里鯔 (Mugil curema)。

作者同時將這些河口魚種，依照對於河口及淡水的依賴度分為八大群。以對河口的依賴度由高至低分為嚴格河口群(Es)、河口海洋起源群 (Em)、海洋河口群(ME)、海洋可進入河口群(Ma)以及海洋群偶爾利用河口群(Mo)。從對淡水依賴的程度由高至低分為: 河口淡水起源群(Ec)、淡水親河口群(Ce)以及淡水偶爾利用河口群(Co)。甘比亞河河口主要以河口海洋起源群 (Em)及海洋河口群(ME)類為主，其中又以石首魚及鯡科為主(圖七)。

石首魚(Pseudotolithus elongatus)是此次調查最常見的種類，出現率超過96%，且每個季節都可以從最靠近出海口直到最靠近甘比亞河中游的站點，發現到大量的個體(圖八)。另外此魚即使在雨季的末期，河水水流量最高的九月，仍然可以在最上游的站點中採集到。另外，由於採集到各個發育階段的個體(從稚魚至成魚[Vidy et al. 2004])，因此作者推測此種魚可能可以在甘比亞下游繁殖。

鯡科魚類(Ethmalosa fimbriata, Ilisha africana 及 Sardinella maderensi) 是在豐度、生物量以及出現頻度第二重要的種類，同時同樣可以發現稚魚和成魚。但和石首魚不同的是，鯡科魚類有明顯的區域分布趨勢: 大多分布在靠近海洋的下游河口地區，豐度隨著離海距離增加而降低。然而隨著乾季的尾聲，鹽度往上游推進時也會到河口上游分布。甘比亞倒游鯰(Synodontis gambiensis)是一種淡水魚類，但是喜歡至河口地區生活。其豐度隨著離海距離增加而增加，但是在雨季晚期、水量增加時會往下游地區移動，但在乾季時，分布就主要侷限在上游地區(圖八)。

Simier et al. (2006) 將調查區域之魚類按照先前的方式分為八個生態群，然後利用多變量統計方法中的因素分析、主成分分析以及多表格比較法分析物理環境魚類聚落。甘比亞河下游物理環境可以以東西向的鹽度、溶氧及透明度梯度所解釋(這些變數可以解釋全體變異的77.4%)。而不同的魚類生態群分布在不同的物理梯度的位置，代表這些生態群對於海水或淡水不同的喜好依賴程度(圖九)。然而這樣東西向的梯度同時受到季節的影響，在乾季初期(12月)最為明顯，而在乾季尾期雨季初期(6月)此梯度出現最小的對比。

最後這一魚多吃的調查，竟可在2012又發表了一篇文章(Guillard et al. 2012)，比較魚類豐度指標是否會因為不同採樣方式(圍網或聲納)而有所差異 。兩個採樣方式所得之豐度指標，大體呈現出類似的趨勢(圖十) ，同時不同方式所求得之豐度指標有高度相關(圖十一)。雖然作者用詭異的迴歸樹來嘗試將其他干擾因素排除，但我覺得簡單用泛線性模式家族等統計方法，就可以將這些干擾因素排除，相關不是很好看的圖十一應該也可以變得有說服力些。(基本上我覺得這一篇會被接受也很神奇....可見SCI論文不難寫呀....)

表一.利用圍網調查所得之魚種、生活區域、出現次數、數量及生物量。Credit: Albaret et al. 2004.
Table.1. Summary table for the purse-seine survey.

圖七.八種棲地群的種類豐度、個體數量及生物量。Credit: Albaret et al. 2004.
Figure 7. Species richness, numbers and biomass for the fish from the 8 biological categories.

圖八.一些重要魚種之豐度與離海距離之變化。Credit: Albaret et al. 2004.
Figure 8. The spatial trend of the abundance of some essential fish species.

圖九. 圍網及聲納之魚類豐度指標比較圖。Credit: Guillard et al.(2012)

Figure 9. Fish abund

圖十. 圍網及聲納之魚類豐度指標比較圖。Credit: Guillard et al.(2012)

Figure 10. Fish abundance indexes from purse-seine (left panels) and acoustic survey (right panel)

圖十一. 圍網及聲納之魚類豐度指標相關圖。Credit: Guillard et al.(2012)

Figure 11. Correlation between abundance index from purse-seine and acoustic survey

Now lets move to another study in the same region and period, Vidy et al. (2004). Their object is to examine  the juvenile fish assemblage occupying the lateral mangrove channels of the estuary of Gambia River. In May, September and November, 2002, the team collected fish juveniles by small meshed fyke-nets in six stations from the river mouth to the 130 km upstream (Figure 12). The nets were set at night in channels adjacent to the main channel of the estuary. The sampling was conducted to coincide with the new moon phase and an evening flood tide as close as possible, given the assumption that that juvenile fish enter the mangrove area with the tide.

The catch are dominant by small-sized individuals (< 100 mm fork length), as reflected in the small mean size of most species (Table 2). The juvenile fish assemblage of the Gambia estuary was dominated by the estuary-residing species, while the juveniles of fish species with high affinity to seawater are limited in numbers.Variations in juvenile abundance were also linked to recruitment seasons and the period from May to September-November suggests a breeding season similar to that of

the resident estuary species, as indicated by the join of early juveniles in September to November, for example, Pseudotolithus elongatus. Generally for the species spawning at sea, their juvenile abundance decreased with increasing distance from the sea, with an exception of Polydactylus quadrifilis that its juveniles were found in the upper stations with very low salinity.

接下來，是一篇大約在同時期，採樣區域也重疊的達卡法國研究團隊，針對甘比亞河下游所做的研究，結果整合在Vidy et al. (2004)這篇文章。他們於2002年五月、九月還有十一月，在與甘比亞河主流相接的主要水道設小網目的袋張網採樣，從河口到上游130公里處總共設立 六個採樣點(圖十二)。因為她們假設小體型的稚魚會利用潮汐進入甘比亞河口，因此研究者盡可能在新月的大潮採樣。

樣本以小型個體的稚魚為主，同時大多尾叉長小於100 mm(表二)。魚種多為生活在河口的種類，海洋性魚種數量有限。 稚魚數量的變化也反映了入添的季節，由於稚魚在九至十一月的數量達到高峰，因此可以推測這些魚種在五月至九月產卵，而以石首魚( Pseudotolithus elongatus)為代表。在海洋產卵的魚種，一般稚魚數量隨著離海距離增加而減少，但是馬拔魚(Polydactylus quadrifilis) 是一個例外。研究者在最上游、低鹽度的站點，仍然可以採集到馬拔魚的稚魚。

Vidy et al. (2004)

圖十二. Vidy 等人在甘比亞河下游的採樣點。Credit: Vidy et al.(2004)

Figure 12. Sampling stations in Vidy et al. (2004)

表二. 採集到之物種個數及體型。Credit: Vidy et al.(2004)

Table 2. The number and size of the collected fish species

Then Louca et al. (2009) further examined the fish fauna in the floodplains in the lower reach of the Gambia. The floodplain borders the main channel of Gambia River and the last 200 km was fringed by mangrove forest. The sample sites locate in the North Bank Region east of Farafenni town and are approximately 193 and 209 km upstream, close to the saline water limit in the end of the dry season (Figure 13).  Fish were monthly sampled using cast and hand nets along two transects on the floodplain, and with fyke-nets in two creeks from May to November 2005 and 2006.

Totally 32 fish species belonging to 15 families were collected from the floodplain transects in 2005

and 2006 (Table 3). Greater fish species richness was associated with low conductivity, low pH and deep water. The species richness was greater in creeks than that in floodplain habitats, probably because they acted as conduits for fish passing in and out of the floodplain. The also found that floodplains of the lower reach of the Gambia River is an essential fish habitat as both a habitat for small-sized fish to complete their life history, as well as a nursery habitat for some fish species mainly encountered in the main channel, as indicated by high percentage of immature juveniles.

Louca et al. (2009)則研究甘比亞河下游泛洪區的魚類相。甘比亞河泛洪區將主流給南北包圍，而泛洪區最後200公里則由紅樹林所圍繞。兩個採樣地點分別在北岸省首會Farafenni東方約193及209公里以東，也位於甘比亞河乾季海水入侵的最上游界限(圖十三)。採樣時間在2005至2006年的五月到十一月。在泛洪區，研究者在兩個穿越線上利用手拋網採樣，而在支流則設立袋張網採樣。

總共採集到32種15科的魚類(表三)。低電導度、低PH值以及深水區和高魚類種類數有關，同時在支流有比較多的魚種類數，這可能是因為支流是魚類進入及離開泛洪區的中間通道之緣故。同時他們也發現研究的甘比亞河泛洪區是重要的魚類棲地，讓小型魚類會可以在泛洪區完成完整的生命週期。並且由於在泛洪區發現常在主流發現魚種的稚魚，代表泛洪區同時也會這些魚種作孵育場之用。

Louca et al. (2009).

圖十三. Louca 等人採樣點地圖。河旁的灰色區域為泛洪區域。Credit: Louca et al.(2009)

Figure 13. Sampling sites (BKD and P) in Louca et al. (2009). The grey area indicates the floodplain

表三. 魚種之相對頻度。Credit: Louca et al.(2009)

Table 3. Relative abundance of fish species in Louca et al. (2009).

As far as I know, few studies have examined the fish fauna in the freshwater habitats (salinity zero) of the Gambia River that the Czech team, who has done a couple of surveys in and near the Niokolo Koba National Park in Senegal, provide the best insights of the fish fauna in the freshwater region of Gambia River.

The first comes Reichard (2008) that conducted a visual census, which is also as far the only observation of fish in situ. The survey was done a deep freshwater pool at Badoye in the middle reaches of the River Gambia, where the river flows through Niokolo Koba National Park during daytime in mid March, 2007. A snorkeller viewed each point for the presence of fishes, recorded the species, identified the size (standard length), maturation classes (juvenile, sub-adult and adult), number of individuals and their position in the water column. The snorkeller record their species, size and maturation class (juvenile, sub-dult and adult), number of individuals and their position in the water column. Also the environmental data were recorded.

Totally 429 individual fishes belonging to 14 species and six families were recorded (Table 4). The most abundant species was Rhabdalestes septentrionalis and R. senegalensis and Hemichromis bimaculatus are the species with the highest appearance (> 30 %). By direct observation, they were able to reveal the species-specific fish-habitat association. For example, for two species with similar size ( ~ 30 mm standard length), Aplocheilichthys normani inhabited shallow marginal areas with no flow and it occurred near the water surface, while Nannocharax ansorgii, preferred deeper region farther from the shore.

就我所知，對於甘比亞河中上游淡水區域魚類種類的文獻不多。而捷克團隊在甘比亞河上游，塞內加爾境內的Niokolo Koba國家公園所作的研究，提供了甘比亞河淡水區域魚類相重要的資訊

首先是他們的一次潛水調查，也是目前針對甘比亞河魚類相唯一的直接在環境觀察的研究。他們在2007年三月中旬，在甘比亞河中游，Niokolo Koba國家公園中Badoye 地區的一個深水淡水池進行研究。研究者潛水觀察魚類存在與否，如果看到魚類，進可能記錄相關生物資訊，例如數量、魚類體型(標準體長)，發育階段(稚魚、 亞成魚或成魚)以及在水中之深度位置。同時也記錄相關的環境水文參數。

他們總共觀察到429隻魚類個體，屬於14種六科(表四)。數量上來說，成群出現的燈魚類Rhabdalestes septentrionalis 數量最多。然而已出現頻度來說，則以塞內加爾燈魚 R. senegalensis 及寶石慈鯛最為常見，出現機率超過30%。因為是潛水員直接在水下觀測，因此他們的研究可以直接反映不同魚種對於環境的不同喜好。例如兩種同樣是約30mm的小型魚，鱂魚Aplocheilichthys normani 喜歡在離岸無水流的表層棲息，而這種Nannocharax ansorgii鯉科魚澤喜好棲息在離岸較遠的水深處。

Reichard (2008)

表四. 在Niokolo Koba 國家公園紀錄之魚種體型及數量。Credit: Reichard (2008)

Table 4. Size and numbers of fish species found in Reichard (2008).

Then it comes their second study, White et al. (2012), which was undertaken also in Niokolo Koba National Park in Senegal. In this 4-year survey (2003-2007, likely) they collected the fish using the seine net and at each site, a single sample consisted of usually three seine hauls (possibly from non-overlapping area swept). Sampling sites were classified into five categories, according to decreasing connectivity to the main channel: (1) main river channels, (2) tributaries, (3) oxbow lakes, (4) temporary water bodies, and (5) spring area (only during dry season). 

They collected totally 49  fish species (Table 5). The habitat use of the fish varied widely among species and seasons. For example, some fish such as Barbus macrops, Barbus pobeguini, and Rhabdalestes septentrionalis occurred generally among all habitats and seasons, whereas other species were observed in certain habitats and/or seasons: Brienomyrus brachyistius occurred only in spring pools, and Ctenopoma petherici was found only in temporary floodplain pools during the wet season (rain season) and in the tributaries and oxbow lakes during the dry season.

 

According to non-metric multidimensional scaling (Figure 15), the fish assemblage depended on both habitats and seasons. During the rain season, the fish assemblage in the main river channel was similar to that in  the tributaries, while the oxbow lakes and temporary water bodies had distinct assemblages (Figure 15). In the dry season, however,  the fish species composition in the tributaries and oxbow lakes altered greatly with high variation along y-axis and x-axis in Figure 15, respectively, which implied joining of different fish species. Also in the dry season, the fish assemblage in the spring pools was distinctly different to main river channel and connecting tributaries.

捷克團隊接下來繼續在Niokolo Koba國家公園進行研究，他們更利用並發表White et al. (2012)這篇文章。在這為期四年的研究中，研究者利用圍網採集魚類，並且在每一個宰樣點，一個樣本通常下網三次(三次捕撈之涵蓋範圍應該不同)。而研 究者根據和主河道的連接程度，將採樣點分成五類: (1)主河道、(2)支流域、(3)牛軛湖、(4)暫時性水體及(5)只有在乾季出現的湧泉區。

他們在這四年研究中採集到49種魚種(表五)，而且也發現魚類棲地的利用，有很大的種間及季節的變異。例如Barbus macrops, Barbus pobeguini, 及 Rhabdalestes septentrionalis 這三種類對棲地選擇性不高，廣泛在每個季節的所有棲地出現。然而有些魚種只會在某些棲地或季節出現，如Brienomyrus brachyistius 只有出現在湧泉區，而 Ctenopoma petherici在雨季出現在支流，而在乾季則分布在牛軛湖。

研究者利用非計量多向度法，分析不同樣本魚類種類組成的差異(圖十五)，結果發現魚類種類組成也受到季節和棲地的影響。在雨季時，甘比亞河河川主流和支流魚類相很接近，然而和牛軛湖及暫時性水體魚類種類組成就差異很大。然而在乾季時，支流和牛軛湖的魚種種類分別在Y軸及X軸出現劇烈的震盪，暗示有不同的魚類種類加入群體。在乾季的湧泉區，也和主流及相連接的支流，有著明顯不同的魚類種類組成。

White et al. (2012)

圖十四. Niokolo Koba 國家公園內之採樣點分布地圖。Credit: White et al. (2012)

Figure 14.Sampling sites in Niokolo Koba National Park

表五. 在Niokolo Koba 國家公園紀錄之魚種豐度。Credit: White et al (2012)

Table 5. Fish abundance index (CPUE) in White et al. (2012)

圖十五. 各樣本魚類種類相成之非計量多向度圖。當樣本點越靠近，表示其魚種類組成越相近。Credit: White et al (2012)

Figure 15. Non-metric multidimensional scaling plot for the samples. Axes are composed of fish species loadings. Close points suggested similar fish species composition.

In conclusion, although locating in the tropics, and generally being considered as homogenous habitats, the Gambia River is in fact heterogeneous in terms of essential physical parameters, for example, salinity and water flow with strong among habitats and between dry-rain seasonal contrasts. Therefore, the fishes in the Gambia River showed large spatial and temporal variation in the species composition, occurrences, abundances, and life-history stages. The Gambia River is by far (2015, end of August) not interrupted by any artificial dam, and therefore it provides an ideal place to study the relationship between fish, marine and freshwater environment in a nearly pristine (or relatively less development pressure) state in terms of connectivity from the river mouth to the upper reaches. However, in most of the studies the research efforts were deposited in the saline part of the lower reach region (corresponding to the West Region and Lower River Region in the Gambia), possibly because of (1) highly productive environment, (2) importance in fisheries as fishing or nursery grounds, and (3) fewer logistic issues. One of the few studies on the freshwater part of the Gambia river located in the Niokolo Koba National Part in Senegal, leaving a vacuum about the fishes in the region starting from the Central River Region and Upper River Region of the Gambia. Studies on these section will have good contribution on the general understanding of the fish in this last few un-distributed large river in the world.

最後總結這篇長文:
雖然熱帶地區過去往往被認為是穩定、缺乏環境條件變異的地區，但是熱帶地區的甘比亞河絕對不符合這陳舊的假設，是個環境因子有著劇烈變化，帶有鮮明棲地間及乾濕季對比的區域。在此環境生活的魚類，其種類組成、出現頻率、族群大小及生活史階段在時空上有相對應的變異。甘比亞河現今(2015年八月底)仍然沒有人為的物體(如水壩)攔阻，因此甘比亞河是一個研究魚類如何在"自然"、"無干擾"、"無障礙"的環境下，與海水及淡水環境互動之理想區域。然而甘比亞河中下游有鹽度之區域可能因為(1)高生產力、(2)作為漁場或孵育場，對於甘比亞漁業有高重要性，及(3)較容易抵達採樣等原因，目前大多數的魚類研究都集中在下游區段。對於甘比亞河淡水區段的研究，所知也僅有捷克團隊在塞內加爾境內的Niokolo Koba國家公園的系列研究，而針對甘比亞河在甘比亞中游省至上游省的區段的研究目前人付之闕如。因此對於分布在這區段的魚類研究，將可在這條少數未被人類阻斷的大河，其魚類分布模式提供更完善的資訊。

沙丁魚和鯷魚 Sardines and Anchovies

The Arabian Gulf is a blessed region with plenty fish resources. The fish market is full of wild-caught fish from various habitats, such as coral reef, rock, seaweed meadow and pelagic waters. In Saudi fishermen mostly target large predatory pelagic fish, the Spanish mackerel, and small tuna and other pelagic fish is regarded almost as "bycatch" and sold with cheaper price. There are not yet fisheries for small-pelagic fish, such as sardines and anchovies. However, in the retail market there are usually "large" sardines available and i guess they may come from other regions, such as Oman or India, for their relatively bad freshness. But one day there are really fresh ones in the supermarket, and thus i gave them a try.

For the first try, i did not clear the stomach, just wash them, and pan-fried the sardines with olive oil. These sardines are really oily, the fish oil kept going out during being fried and they left more oil than i added in the beginning. Next time, i should not need to add too much oil as this time.

Thy tasted wonderful.

As a Taiwanese, fried Pacific saury (Cololabis saira) is a common dish on the table. It is nutritious full of healthy fish oil, large quantity, easy to get for the frozen ones, and cost next to nothing to have one. The fried sardines made me remind the saury, which is a key to the childhood memory. They also had much fish oil, with a white-brown meat being bit hard. When adding some lemon juice, they become more delicious for the sourness from the lemon "neutralized" the greasy from the fish oil. They are really good fish (as long as they are fresh).

沙烏地面對阿拉伯灣的地區，是一塊漁業資源富裕的海洋。漁市場有各種棲地的魚，如珊瑚礁、岩岸、海草床、以及中表層魚類。在中表層魚類中，土魠是沙烏地漁民主要捕撈大型捕食性中表層魚類 ，而其他包括小型鮪魚的中表層魚類，幾乎被漁民認為是混獲，價格無法與土魠相提並論。在這裡，還沒有發展針對像是沙丁魚鯷魚等小型中表層魚類的漁業，但在市場上，有時就可以看到體型相對算大的沙丁魚。因為這些沙丁魚相對來說鮮度較差，因此個人猜測，這些在市場上販售的沙丁魚應該都是從其他地方，像是阿曼或是印度運來的。而在有次去市場購物時，那批沙丁魚鮮度真的不錯，所以我就去嘗試一下。

這次沒有把魚內臟去除，將魚洗乾淨瀝乾後，就把魚用橄欖油煎熟。這沙丁魚還蠻油的，在煎的時候魚油不斷跑出來，最後鍋子還留下比我當初加得更多的油，下次只需要加一點點油就好。

魚好吃。

身為一個台灣人，因為秋刀魚價美物廉又營養豐富，所以煎秋刀魚長出現在餐桌上。而這次的煎沙丁魚，味道讓我強烈連想到秋刀魚，一種童年的味道。和秋刀魚一樣，沙丁魚也是充滿魚油，而且魚肉在煎熟後也是棕色又帶有點硬。加點檸檬汁，用酸味來解來自魚油的油膩感。這次的嘗試非常舒服，這邊市場賣的沙丁魚，只要新鮮，真的是好吃。

Pure-seiners in the Basque country for the small pelagic fish 

巴斯克地區捕撈小型中表層魚類的圍網漁船。

Sardines as the main catch from this Basque purse-seiner

巴斯克漁船所捕撈的沙丁魚。

A large sardine (> 20 cm) from local retail market. Broken stomach probably implies bad freshness. 

在當地市場買到的大型沙丁魚(全長超過20公分)。然而肚子上的破洞可能代表鮮度欠佳。

Fresh sardines (possibly different species) from the supermarket 

在超市買到的新鮮沙丁魚，和上圖應該是不同種類。

I simply pan-fried the sardine with olive oil. Delicious. 

簡單用橄欖油煎煎，沙丁魚就是一道美食。

In other occasion I literally caught some sardines from the sea. Although they are smaller, I can be certain that they are REALLY fresh. Then I had this second try. Similar to last time, i pan-fried the sardines with little olive oil, and added some garlic slices for more flavor. Then put on the plate and garnished by lemon slice. This time is great, too. The sardines have hard scutes in the belly, making they less favorable in the market. I can easily remove the scutes, as well as the bones. The meat is white-brown, and has less "red meat" (血合 as called in Japanese) that gives it stronger, bloody, and metallic taste compared to the saury.

Again this time further supported that the fresh sardines are good to have. (It would be better if i could have had it with a glass of dry white...).

第二次嘗試，就是正港現撈的沙丁魚。和上次一樣，用少量的橄欖油去煎，但加點蒜片提味，然後和檸檬片一起裝盤。這次同樣很好。這裡人們不喜歡沙丁魚的原因之一，就是沙丁魚腹有一排硬鱗，造成口感不好，所以需要額外挑除。但我覺得這排硬鱗，和魚中骨一樣很容易剔開。煎熟的魚肉介於白色和棕色之間。和秋刀魚相比，沙丁魚魚肉很少血合肉，因此吃起來不會有帶點血味。

這次嘗試一樣是很好的經驗，這邊的沙丁魚還蠻好吃的，如果能有一杯不甜的冰白葡萄酒就更完美了...

Smaller really fresh sardines

新鮮的現撈小沙丁魚

Pan-fried the sardine with olive oil and garlic. Garnished with lemon slice 

一樣用橄欖油煎，這次加點蒜片，起鍋後和檸檬片一起裝盤。

Easy to separate the meat from the bone. Sardines have less smelly "red meat"

很簡單就可以骨肉分離。沙丁魚的肉比較白，同時味道重的血合肉也比較少

When in the Basque country, I had another impressive experience for the small pelagic fish: fried anchovies with garlic. In fact two things made me remember this dish: it is tasty, and the way my friend had it. They can remove the bone and eat only the meat from such small-sized fish. I could not do that delicate work and was only able to eat them whole. However, the anchovies are not consumed fresh here. Mostly they are sold as dried fish or salted fish, and most likely from India or other countries.

In another occasion i had some really fresh anchovies. So I have a chance to try this dish at home. I added some salt on the anchovies and put them in the fridge for a few hours. Then i put the anchovies in a hot pan with a bit more olive oil. Because they had stronger smell after being defrost, i added a few ginger slices (a common practice in Chinese cuisine to remove the bad flavor from the fish) in addition to the garlic slices.

As in Basque, I eat them whole for I cannot remove the heads and bones from the anchovies. They are good, especially for them have been simmered in salts for a couple of hours. Eating with garlic slices and/or ginger slice gives the fish a different flavor and taste. It is in some degree similar to a common Taiwanese dish, fried anchovy dry (炒丁香) that replacing the dried anchovies with fresh ones. It is a nice one as a snack or side-dish.

當我在巴斯克地區時，朋友有吃一道蒜片炸鯷魚。當時對這道菜除了他的美味之外，還有巴斯克友人可以用刀叉把鯷魚骨頭分開來吃的神技。可惜我刀叉沒有用的那麼熟練，只能連著骨頭一起吞下去。在沙的市場並沒有新鮮鯷魚，大多是從印度進口的鯷魚魚乾，就像是台灣的小魚乾一樣。在偶然的機會下，有機會拿到一批新鮮的鯷魚，就來試試看這到在巴斯克吃到的菜。

首先先將解凍好的鯷魚灑點鹽巴，放到冷藏庫醃個幾個小時。將稍微多點的橄欖油放入熱好的平底鍋，將醃好的鯷魚放下去，和蒜片薑片一起半煎半炸，大約不到五分鐘鯷魚表面變得酥脆就可以起鍋了。如果想要硬一點，也可以再炸久點。

就像在巴斯克一樣，我還是懶得去頭去骨頭，就給他這樣直接吃下去。味道很不錯，橄欖油替炸過的魚肉加點特殊的風味。配合炸酥的蒜片和薑片一起吃，又讓味道有些變化，吃起來就像是我們的炒丁香魚，只不過魚不是魚乾，而是新鮮的魚。雖然不知道下一次是什麼時候，但是這次是個很不錯的體驗，當作是配菜或是點心都很不錯。

Another impressive dish in the Basque: fried anchovies with garlic 

另外一道印象深刻的菜:炸蒜頭鯷魚

Fresh anchovies 

這次要嘗試的鯷魚

Frying anchovies in olive oil with garlic and ginger slices 

用較多的橄欖油，和蒜片薑片一起煎鯷魚

Done. I left most oil in the pan.

煎好的鯷魚。

Previous try. It was fried a bit longer but is still tasty. 

同場加映: 之前的一次嘗試，炸的有點久但是一樣好吃。

Crustaceans (II): Lobsrers 甲殼類 (II): 海戰車和龍蝦

In this blog, I am going to introduce the "star" of the crustaceans, the lobster. It is interesting to state here that in Arabic here, the lobster is called "Um al Rubiyan", meaning the mother of the prawn. The coast of the western Arabian Gulf is mostly sandy and muddy, and therefore, it is rare to catch "rock lobster" in the wild. Presently some concrete breakwaters have been deployed to create "corniche" place for local residents. The crevices between breakwaters might be suitable habitats for the rock lobsters, as they did in Taiwan, but so far I have never seen or heard that rock lobsters are caught there. Although the rock lobsters are available in some local retail markets, they are (as per the fish salesperson) imported from rocky area, such as Oman, or the Red Sea.

在這一篇文章，來介紹甲殼類的明星: 龍蝦。有趣的是，在這邊阿拉伯文龍蝦稱為"Um al Rubiyan"，蝦子的媽媽，也顯示阿拉伯人的幽默。通常龍蝦喜歡棲息在礁岩的縫隙中，但阿拉伯灣西側基本上都是沙泥灘，所以幾乎都不會看到龍蝦出現。近幾年沙烏地政府改造沙灘，在海邊放置許多人造石頭並建成海岸公園供居民遊憩。因此這些人造礁岩的縫隙，可能增加一些龍蝦類的潛在棲地，就如同台灣有釣客會在消波塊的縫隙中找到龍蝦。但是至少目前為止，我都沒有在這邊聽到在人造礁岩中發現龍蝦的案例，可能還少了什麼因素吧。雖然在當地魚市場偶爾可看到長鬚的龍蝦，但是幾乎都可以確定是來自國外的進口貨，可能是南邊的歐曼，也可能是來自於紅海那側吧。

Rock lobster from local retail market, possibly imported from Oman.

這邊市場上買得到的"真"龍蝦，應該是從阿曼進口的舶來品。

The most common lobster in the Western Arbaian Gulf, Sand lobster Thenus orientalis

阿拉伯灣西岸最常見的龍蝦，沙龍蝦(海戰車, Thenus orientalis)

Take two sand lobsters for a try

拿兩隻來嘗試看看

Simply wrapped in the alum foil and roasted in the oven. They are more being steamed.

用鋁箔紙包好後直接放進烤箱烤。因為包起來烤，所以反而比較像是用蒸的。

More lobsters. This time add they are roasted with some salt.

更多的龍蝦。這次加點鹽烤。

It seemed that they were in the oven for too long. The meat was better but also drier. It is OK, just like a bigger bite of a prawn.

加鹽烤有比較好。可惜好像烤太久了，肉有點乾。吃起來...就像是比較大塊的蝦肉，是不錯啦，但如果便宜一點就更好了。

Trying another way. I made the stock from the head and the carapace.

換另外一種做法，先將蝦頭和殼用水煮出蝦高湯。

They I made a lobster pasta with the stock. A more luxury version, but without one key ingredient...

取出來的蝦肉，就和蝦湯一起做成鮮蝦義大利麵。可惜就是少了一味..

Another try. This time I cut them in half.

更多的龍蝦。這次對切

Adding some flour on the meat side, and frying them in the pan. This time was better.

在肉的切面撒上麵粉，在平底鍋用油煎熟。這次就不錯，看起來就很奢華。

More lobsters. 

更多的龍蝦。

A feast on lobsters. 

龍蝦大餐

A bigger one in dinner. 

某天的晚餐，一隻大龍蝦

Another lobster species, very rare... 

另外一種龍蝦種類，在珊瑚礁棲息，在魚市場非常罕見!

The gonad of this lobster, quite large in quantity. 

那隻龍蝦的生殖腺，還蠻大量的

The meat was taken, along with the gonad. 

蝦肉生殖腺取出冷凍保存。

I decided to make them a shrimp pasta. I fried chopped garlic with the olive oil, and then put the lobster roll in. 

後來決定做龍蝦義大利麵，將蝦肉和切碎的大蒜和橄欖油一起煎炒。

Done! I should have not added the gonad.... 

做好了! 味道嘛...我不該加生殖腺進去的...

In conclusion, the sand lobsters are an interesting species to try, if the budget is not too tight. It don't appear all year round in the fish retail shops, mostly during the trawling season and caught by a special technique. In some fishing market, at times they are very fresh that the sand lobsters are moving. I think they need better culinary skill to make them out-standing, especially considering their high prices. I will not recommend to have rock lobster here, because they are imported somewhere and usually with lower quality.

沙龍蝦，算是種有趣的物種，有錢有閒可以嘗試一次。在特定的季節，通常是下半年的拖網季節，才會在沙烏地東岸的魚市場偶爾出現。有時候，鮮度相當好，龍蝦甚至還會動。然而，個人廚藝技術太差，目前覺得龍蝦沒有特別好吃，特別考量到它尊貴的價格...我更不會推薦在沙烏地東岸吃岩龍蝦，畢竟都是進口，經過冷凍解凍幾次後，狀況只能說慘不忍睹。要吃那種龍蝦，不如在產地吃新鮮的，比較對得起荷包。