CHAPTER 1. INTRODUCTION

1.1 Objectives
1.2 Plan and conduct of the surveys.
1.3 Methods of investigation

1.1 Objectives

The CECAF Cooperative Survey had two principal aims: a) To obtain the best possible description of the composition, distribution and abundance of small pelagic fish along the coastal shelf from Morocco to Benin; and b) To contribute to methodological advancement of acoustic survey techniques in the area through intercalibration exercises and other special experiments. The plans included participation of research vessels from Morocco, Mauritania and Senegal in addition to the two non- region vessels. The Moroccan vessel unfortunately had to be withdrawn from the programme for technical reasons.

1.2 Plan and conduct of the surveys.

The plan agreed for the August - September cruise of the DR FRIDTJOF NANSEN was as follows:

August 8 - 25: Survey Sherbro Isl. - Cape Roxo with possible overlap Senegambia and trawl coverage off the Gambia.

August 26 - September 13: Intercalibration programme and acoustic development work Senegambia - Mauritania in a joint operation with LOUIS SAUGER, N’DIAGO and CORNIDE DE SAAVEDRA.

September 15 - 28: Intercalibration programme and acoustic development work Morocco.

The course tracks with fishing stations for Surveys I and II are shown in Annex 1 and 2 respectively. Table 1 shows the details of the survey efforts spent on the various parts of the coast.

From the CECAF countries the following participated in Survey I:

Sierra Leone: P.A.T. Showers, I.E. Bangura
Guinea: S. Kouyate, Cheik Ahmed Bangura
Guinea-Bissau: A.P.J. Da Silva
The Gambia: J. Ndene, Anna Lloyd Evans
Senegal: B. Samb

Morocco: Mostafa Idrissi Chbani, Mbarek Zouiri

Mauritania: Ibrahim S. Ba, Aliou Dia Mamaoudou
Morocco: Mostafa Idrissi Chbani

Survey I: T. Strømme, G. Sætersdal, A. Abella, O. Alvheim, K. Strømsnes, T. Haugland, E. Molvær,

Survey II: I. Svellingen, A. Abella, O. Alvheim, Ø. Torgersen, T. Mørk.

Survey	Country	Dates	Days	Distance sailed nm	Number of fishing stations
I	S. Leone	Aug. 19-21	3	300	17
I	Guinea	“ 21-25	5	800	32
I	G. Bissau	“ 25-27	3	590	19
		Sep. 7- 9	3	700	22
I	Senegal	Aug. 28-Sp. 5	9	1470	27
I	Gambia	Sep. 10-12	3	180	24
I	Maurit.	Sep. 14-17	3	670	10
I	Morocco	Sp. 23-Oct. 4	12	2000	44
II	Morocco	Nov. 6-13	8	1000	23
II	G. Bissau	Nov. 23-26	4	700	16
II	Gambia	Nov. 27-29	3	300	23
II	Senegal	Dec. 2-7	6	750	18
II	Maurit.	Dec. 7-12	6	1000	19

1.3 Methods of investigation

The main survey effort was spent in investigating the pelagic schooling fish using acoustic integration technique combined with fishing with bottom - and mid water trawls for identification and sampling. Fish near the surface and very close to the bottom are not properly observed with this system. Horizontal ranging sonar was used to observe surface schooling fish, but such observations are not easily quantifiable in terms of measure of biomass. To include bottom dwelling fish a programme of prelocated trawls stations was worked off Sierra Leone, Guinea, Guinea Bissau and the Gambia to provide data for swept area assessments.

The reliability of acoustic techniques in providing estimates of biomass is under continuous review by the scientific community. The intercalibration experiments described in Chapter 2 form a contribution to this process. Opportunities were also taken during the surveys of doing repeated survey coverages of assumed identical biomasses of fish over a time interval of a few days to test the consistency of the resulting estimates. The results which are described in a special report (see Strømme and Sætersdal, 1987) showed that the consistency of the results is high. This does not, however, preclude the possibility of systematic bias in the results deriving for instance from such factors as surface schooling of fish and vessel avoidance. A general problem is also the incomplete information on the target strength of the species surveyed. For the pelagic fish biomass estimates are based on an assumption of a target strength similar to that of the European herring. Another limitation is found in the incomplete coverage of inshore waters by the survey, since the parts of the shelf shallower than about 10 fathoms could not be navigated by the vessel.

The overall effects of these various limitations are thought to lead to an underestimation of the biomass.

The identification of the targets recorded by the acoustic integration system represents an important problem. The basis for this step is the composition of the catches in hauls with bottom - and mid water trawls together with an evaluation of the characteristics of the echo traces. Because the catchability of fish is highly species- and size dependent the catch data must be used with considerable reservations and in areas where many species occur in mixture, identification can only be made by relatively broad groups. Some species occur however in distributional patches or in larger continuous aggregations and this facilitates the identification. The integrator values were allocated to the following groups:

Clupeids and anchovies;
Carangids, scombrids, barracudas etc.;
Triggerfish;
Demersal fish in mid water.

All catches were sampled for species so that a total composition by weight could be estimated. The most common species were sampled for size.

A record of the acoustic instruments and their calibration is presented in Appendix 1 together with a description of the fishing gears.