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A PLANNING

DOCUMENT

The problem of Hypersa1inity in the Casamance, Sine-Sa1oum and Senegal River Estuaries; its effects on local fisheries and the implications if hypersa1inity becomes a problem in the Gambia River Basin as a result of development.

DRAFT

Paul Andr~ DeGeorges Environmental Advisor O.M.V.G. February 21, 1985

TABLE OF CONTENTS

I.

Hypersalinity in the Casamance and Sine-Saloum Basins

II.

Effects of hypersaline conditions on fish catches

III.

Salinity tolerance of estuarine fish stocks

IV.

Effects of salinity on growth and reproduction of fish

V.

Effects of Hypersalinity on shrimp populations

VI.

Other ecological changes associated with increased year round hypersaline conditions

VII.

The implications of hypersalinity on the fishery stocks of the Gambia River Basin

I.

HYPERSALINITY IN THE CASAMANCE AND SINE-SALOUM BASINS

Normal sea water contains a salinity of 35 parts per thousand (ppt). The Casamance and the Sine-Saloum estuaries have traditionally experienced the natural phenomena of hypersalinities (salinities greater than 35 ppt). For instance in the Sine-Saloum, Bondy (1968) reported salinities as high as 93 ppt at Fatick and 72 ppt at Kaolack, (Figure 1). In the mid-1960's Salinities at Ziguinchor climbed as high as 40 ppt by the end of the dry season, (Le Reste et Odinetz, 1984). These hypersalinities developed as the result of poor flushing by fresh water during the dry season combined with high evaporation rates. Prior to the recent drought which began in the 1970's, yearly rainfall flushed these estuaries resulting in a large dip in salinity during the rainy season. In the mid-1960's the Casamance experienced salinities that fell to as low as 3 ppt at Ziguinchor (Figure 2). During this same period of time the Sine-Saloum, as measured at Foundiougne, experienced salinities which fell as low as 15 ppt, (Figure 3). . In recent years of drought there has been very little flushing of these estuaries by fresh water during the rainy season. The most important flushing that occurs is tidal in nature. In the upper reaches of these estuaries flushing is inadequate and, as mentioned, evaporation rates are high. As a result, there has been a tendency for the yearly salinity curbs to flatten out in the upper reaches of what have become in essence an extension of the ocean, (Figure 2 and 4). Salinities have had a tendency to remain high year round and in some instances to have climbed. II.

EFFECTS OF HYPERSALINE CONDITIONS ON FISH CATCHES

The fish species of these estuarines and the pink shrimp, Penaeus notialus, (formally P duorarum), are physiologically adapted to the normal range of fluctuating-salinities found in an estuary and are classified as euryhaline organisms. However, even they have their limitations. The elevated year round salinities appear to have greatly stressed the euryhaline fish stocks in parts of the Sine-Saloum, and in the Casamance estuaries. Many species have dropped out of the artisanal fish catches while a few species, especially Tilapia spp. and Mugil spp., have adapted to these conditions and flourished, becoming dominant in the artisanal fish catches. This is clearly demonstrated in the yearly artisana1 fi'sh catches from the Department of Kao1ack from 1972 until 1984 (Table I and Figure 1). This area of the Sine-Sa1oum suffers from hypersalinities and is the major area in the Sine-Saloum where declining fish catches, for various fish species, are very obvious. A recent study in the Casamance (A1baret 1984) demonstrates a falloff in fish species from experimental and artisanal catches, moving up river from low to high salinity regimes. Moving from the mouth of the Casamance upstream, salinities ranged from a low of 39 ppt to a high of 83 ppt, (Figure 5). Species abundance decreased with increasing

LEGENDE

NOUAi. 150~'~O":::'

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.p)0' -i;~

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Figure ,'5_

Evolution de la riehesse sp~eifique et de la salinit~ en fonetion de la distance

8 Itembouchure.Rl : Nombre d'especes recens~es pendant la mission, complete par les enque t es de peche du moi s de mars du CRODT (R2).

Albaret,

JJ.

1984

-,--

.--

Mars

1984.

~'--'-"'.~---

..-"-'

_".:.-

Distance I 'embouchure (Kml

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salini ties. Species affected by these changing salinities in the Casamance .' are contained in Tables 2 and ~.. It_sl1ould be noted that the same genera and in many cases the same species have dropp~d off .from apparently high salinities in both the Sine-Saloum (TableLI) an\:!in the Casamance (Tables 2 and 3); the former being docume~ted from temporal observations in one place, the latter from spatial observations over.one period of time. Based upon the Casamance study, preliminary indications are that a critical falloff of species occurs at hypersaline conditions between 56-66 ppt. A critical fall off in biomass could be attained well before these, salinities have been reached. However, information is not currently a~ilable! to substantiate this. J

I

,

Fishery statistics collected from the control posts of the Direction De P~che in the Department of Sedhiou, the Casamance do not demonstrate as drastic a change over time as in the Department of Kaolack, the Sine-Saloum. (Tables 4, 5, 6, and 7). It is believed that this is primarily due to their being very incomplete and to the fact that they coyer a much shorter period of time than the statistics from the Sd.rre+Sa Loum , However certain generalities can be made: At Sedhiou (Table 4), which demonstrated high salinities (82 - 83 ppt in March 1984), mullet and tilapia are dominant in the catches as in the study of Albaret (1984). Ethmalosa stocks appear to be on the decline

in 1984 compared to previous years. Tilapia, mullet and Ethmalosa made up the majority of the 1984 catch at Marsassoum (Table 5). Albaret (1984) found Ethmalosa in very low numbers at this site during his sampling in March 1984. I

At Diattakounda, Tilapia, mullet and ethmalosa make up the majority of the catch in the first half of 1984. Mullet and tilapia catches appear to have declined compared to 1983, while ethmalosa catches show an apparent increase (Table 6). At Goudomp, tilapia, mullet and ethmalosa show increases in the first half of 1984 compared to 1983, (Table 7). Care must be given in assessing these statistics since they are very inco~plete and because the fallout of the Spring 1984 shrimp fishery in the Casamance may have placed more fishing effort on the finfish stocks. wnile this could result in an apparent increase in fish yields, catch per unit of effort could have decreased. Information does not exist to prove or disapprove such a hypo-tb.e sLs , :1 One important factor that should be noted is that fish .stocks have declined from about 10,000 metric tons per year in the early 1970's to 2- 3,000 metric tons per year in the 1980's in the Department of Sedhiou (Personal Communication Malick Sarr, Chef de Secteur, Direction De La Peche. Departement de Sedhiou - Caaainance , ) I.'

" . ~. " ;1

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Tableau 2 - Liste des Esp~ces rencontr~es en Casamance

G E N RES ESP E C E S (Abreviation)! !F AMI L L E S Rhinobatos rhinobatos(RRH)! :RHINOBATIDAE !RHINOPTERIDAE Rhinoptera marginata (RMA) !DASYATIDAE ! Dasyatis margarita (DMA) !ELOPIDAE Elops lacerta (EIA) :CLlJ"""PEIDAE Ethmalosa fimbriata (EFI) !CLUPEIDAE Ilisha! africana (IAF) !TETRAODONTIDAE Ephippion guttifer (EGV) :TETRAODONT IDAE Lagocephalus laevigatus (LLA) !BAGRIDAE Chrysichthys walkeri (CWA) !ARIIDAE Arius gambensis (AGA) !ARIIDAE Arius parkii (APA) ! !ARII]}AE Arius heudelotai(mercatoris)(AME)! !OPHI CHTHYIDAE Caecula cephalopel tis ( - ) ! !HENIRHAMPHIDAE ! Hemiramphus brasiliensis (HBR) !SPHYRAENIDAE Sphyraena piscatorum (SPI) !MUGILIDAE Liza! grandisquamis (LGR) !MUGILIDAE Liza falcipinnis (LFA) !MUGILIDAE Mugil cephalus (MCE) Mugil bananensis (MBA) !MUGILIDAE !POLYNEMIDAE Polydactylus quadrifilis (POQ) Galeoides decadactylus(GDE) !POLYNEHIDAE Epinephelus aeneus (EAE) !SERRANIDAE !Po}!ADASYDAE Plectorhynchus macrolepis (PMA) Pomadasys jubelini (PJU) !POHADASYDAE Gerres melanopterus (GME) !GERREIDAE !GE~l;>_EIDAE Gerres nigri (GNI) !SCIAENIDAE !Pseudotolithus(pseudotolithus! senegalensis (PSE) !SCIAENIDAE !Pseudotolithus(pseudotolithus! brachygnatus (PBR) !Pseudotolithus(pseudotolithus! typus (PTY) !SClAENIDAE !SClAENIDAE !Pseudotolithus(hostia) ! moorii ( - ) !Pseudotolithus(fonticulus) elongatus (PEL) !SCIAENIDAE :CAP_tu'l"G IDAE !Chloroscombrus chrysurus (CCH) :CA.l\.ANG IDAE !Caranx senegalus (CAS) !CARANGIDAE :Caranx hippos (CHI) !Lichia glauca ( - ) :CARANGIDAE !CARANG IDAE !Trachinotus falcatus (TFA) !Drepane africana (DAF) !EPHIPPIDAE !EPHIPPIDAE :Chaetodipterus lippei (CL1) !Psettus sebae (PSB) !MONODACTYLIDAE :CICHLIDAE !Hemichromis fasciatus (HFA) !Sarotherodon !melanotheron(heudelottii-SME! !CICHLIDAE !Tilapia guineensis (TGU) !CICHLIDAE !Scomberomorus .! tritor (CTR) !SCOHBRIDAE papilio ( - ) !PERIOPHTHALMIDAE !Periophtha1mus !Psettodes belcheri (PBE) !PSETTODIDAE !Synaptura punctatissima (SPT]) !SOLEIDAE !Synaptura lusitanica (SLU) !SOLEIDAE !Cynoglossus !senegalensis(goreensis)(CSE)! !CYNOGLOSSIDAE !Cynoglossus : tadenati ( - ) ! !CYNOGLOSSIDAE Source: Albaret, J.J. Aofit 1984. Premieres observations sur 1a faune ichtyololique de la Casamance. CRODT. Archive no. l3l.22p.

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Table

4

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- Available Fishery Statistics, Paste de Controle de Sedhiou, The Casamance, Metric Tons.

o.

I

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Espece

1 9 8 4 Jan-Aug.

1 983 Jan-Aout

1 9 8 2 Jan-Dee

1 981 Jan-Aout

198 1 Jan-Dec

1 980 Juin-Dec.

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Mu1et Tilapia

18.0

22.0

37.0

42.0

51.0

20

504.0

254.0

410.0

213.0

354.0

190

Brochet

0.08

1.4

1.5

0.05

Ethmalosa

3.0

16.3

22.0

9.9

14.0

35

Si1ure

0.9

4.2

7.8

8.3

13.0

10

Capitaine

0.0

7.0

7.5

1.8

3.9

2.4

Trachynote

0.0

1.7

1.9

0.0

0

0.8

Otolithe

0.0

2.0

2.2

0.6

0.6

1.0

Albules

0.0

0.0

0.0

0.8

1.2

2.8

0.05

0.3

N

, Sales

0.0

0.0

0.0

0.0

0.0

0.4

Requin

0.0

0.0

0.0

0.0

0.0

a

Total

526.0

309.0

490.0

277.0

Note:

Dept Sedhiou

1975-77 10,000 metric tons per year. 1980's 2-3, 000 metric tons per year. Direction de Peche, Departement de Sedhiou, La Casamance.

438.0

263.0

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Tableau 3 - Structure des Peup1ements

*****: ****: ***: **: *: Source:

espece espece espece espece espece

1argement dominante tres abondante moyennenient abondante faiblement representee presente (1 ~ quelques individus)

A1baret J.J. AoOt 1984. Premieres observations sur 1a faune ichtyo1ogique de 1a Casamance - CRODT.Archive No. 131-22p.

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Table 5 - Available Fishery Statistics, Poste de Contro1e Marsassoum, The Casamance, Hetrie Tons.

1 9 8 4 Jan-Aout

Species I

1 9 8 3 Juin-Aout

1 983 Jan-Dec

;j

1" !

Mulet Tilapia

.

I

,,,i

I

Ethmalosa SHure

1 9 7 9 Jan-Dee

197 8 Jau-Dee.

18.3

7.2

12.8

13.8

12.2

5.2

105.0

149.0

218.0

66.0

222.0

236.0

2.7

4.2

2.2

7.9

4.3

22.0

13.8

24.4

8.5

47.0

37.0

1.1

4.1

6.5

3.0

8.1

6.7

0.65

Brochet

1 981 Jan-Aout (minus June)

w

I

Capitaine

1.0

3.7

5.4

2.1

7.1

4.9

,

Trachynote

2.3

6.3

8.9

2.0

4.7

7.6

I

Otolythe

4.6

9.9

18.5

5.2

9.0

0.0

i

Crevette

0.0*

92.5

113.0

43.5

40.5

0.0

Sole

0.0

0.0

0.0

1.2

1.5.2

8.1

Albu1es

0.0

0.0

0.0

0.0

5.2

0.5

1

Caranges

0.0

0.0

0.0

0.0

3.8

4.4

I

Drepanes

0.0

0.0

0.0

0.0

0.0

0.05

I

! 1,

j I

I

i

I i

! 1

\

I!

,.

* Too

sma lL not fished

Source:

Direction de Peche, Departement de Sed hi.cu , 1a Casamance Malick Sarr, Chef de Secteur et Sanou Faye, Agent Technique

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Table 6 - Available Fishery Statistics» Paste de Controle de Diattacounda, The Casamance» Metric Tons.

1 9 8 4 Jan-Aug.

1 9 8 3 Jan-Aug

1 983 Jan-Dec

26.0

33.0

51.0

Tilapia

376.0

808.0

1039.0

Brochet

0.0

0.0

0.0

46.0

18.5

36.0 .

4.2

6.8

14.7

10.0

27.1

37.0

Trachynote

9.7

15.1

27.5

Otolythe

7.3

22.3

28.3

Crevette*

0.0

0.0

0.0

Sole

0.0

0.0

0.0

Albules

0.0

0.0

0.0

Caranges ,

0.0

0.0

0.0

Drepanes

0.0

0.0

0.0

Species

Mu1et

!

I

.,

Ethmalosa Si1ure Capitaine

*

Fishing for shrimp forbidden .Source: Direction de Peche» Departement de Sedhiou/La Casamance Malick Sarrt Chef de Secteur et Sanou Faye» Agent Technique.

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T~brn':':-Availtime-YiSllert'-'statTsffcs-,-Posi:e-';cfe' c"(mtrole~--G:6ct~~p;The Casamance, Metric Tons

1 9 8 3 Janvier-Aout

1 9 8 3 Janvier-Decembre

90.0

30.0

54.0

Tilapia

425.0

132.0

176.0

Brochet

0.0

0.0

0.0

Et hna lose

139.0

113.0

139.0

Si lure

11. 3

14.9

20.0

p it

17 .8

11.3

16.9

5.5

2.9

6.0

14.2

40.9

48.2

0.0

0.4

0.4

395.0

,481.0

1 9 8 4 Janvier-Aout

Species

Mulet

Lf)

Ca

'i'l~ LlC:h

Ot

a Lne

Y ,10 t e

olithe

Bco,:1-tet

C'r eve

*

58.0*

tte

ShrLmp proces sing, stopped.

plant

closed due to a regional

fallout

in this

fishery.

sh r irap

Source:

.. ,

C''' I, Autotrophic 2) rn « I, Heterotrophic 3) Information collected by OMVG Environmnetal 4) Mangrove Estuaries •

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!Hasked by Respiration 1.92

61.44

Np£ative

-2.64

Advisor, Paul Andre DeGeorges, while conducting

Less than 1 -84.4'8

0.42

field work with the University of Michigan.

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its recycling by bacteria, hence high respiratiorr~ates, serves as the basis of the aquatic food chain. Similarly, DeGeorges (1984) found that in the Bas Delta of Mauritania, a part of the Senegal River Estuary, respiration was the driving force in the aquatic food chain (Table 10). Tnis part of the estuary is strongly influenced by the remnant stands of mangrove, Avicennia nitida. A coeficient of determination (R2. 100) of as high as 98 percent was obtained between average community respiration and average fish yield per hectare. It is certain that hypersalinity acts as an indicator parameter in the Casamance, the Sine-Saloum and the Senegal River estuaries. However, it is only one of a complex interrelated set of parameters which have resulted in a . decrease in fish yields and changes in the composition of fish populations, foremost of which is the die-back of mangrove swamps in recent years. VII. THE IMPLICATIONS OF HYPERSALINITY ON THE FISHERIES STOCKS OF THE GN1BIA RIVER BASIN. Currently, hypersalinity·does not appear to be a major problem in the well flushed Gambia River Estuary. With a greater drainage basin, annual flushing by freshwater has kept this condition in a reduced state to date. Certainly one can find bolons such as the Bintang and the Bao bolons where hypersalinities most probably exist. However to what degree and over what period of the year these conditions prevail can not be answered to date due to a lack of field data. A key concern of the current environmental studies is the possibility of hypersaline conditions developing downstream of the Balingho anti-salinity barrage. This barrage, as well as controlled flows from the Kekreti high dam will reduce the amount of flushing of the estuary during the rainy season. Additionally, under intensive irrigation, it is previewed that the Balingho barrage will be closed in December and Will remain closed until sufficient freshwater flow from the next rainy season can maintain salt intrusion below the barrage, approximately 6 - 8 months per year. These man-induced changes may very well result in similar hydrological conditions as those that have naturally developed due to the limited drainage areas and the drought in the Sine-Saloum and the Casamance estuaries. Concern exists that with the decreased freshwater flushing of the rainy season, the closure of the Balingho barrage for a minimum of 6 months over year, and the high evaporation rates of up to 2.5 meters per year as measured at Jenoi, that hypersaline conditions may develop similar to those found in the Casamance and Sine-Saloum estuaries. The hypersalinities will probably develop immediately downstream of the Ba1ingho and extend towards the mouth to a yet to be quantified distance. Likewise, it is unsure as to how high these salinities might rise. The possibility exists that this could and should be modeled. However a model is only man's simplification of a very complex and dynamic situation. Even with a model there would always be doubt as to the outcome. Eventual monitoring and surveillance of the situation, if the· barrage is put in place, is .the only real means of determining what will really happen. At this point in time, it can only be said that there appears to be a great risk for the development of hypersaline conditions, the loss of mangroves and the elimination of fish stocks if the antisalinity barrage is realized. Decision makers from the member states must determine if this risk a~d all that it implies is worth taking.

Table 10

PRIMARY PRODUCTION ESTIMATES USING INSITU DISSOLVED OXYGEN DATA FROM THE LOWER DELTA, SENEGAL RIVER, MAURITANIA - NOVEMBER 14-15, 1984.

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