CHAPTER 1. INTRODUCTION

1.1 Programme background
1.2 Investigational effort
1.3 Methods of investigations

1.1 Programme background

Within the framework of the UNDP/FAO project GLO/001/82 arrangements were made for a programme of fish resource surveys of the continental shelf from Angola to Gabon with the R/V DR FRIDTJOF NANSEN in four seasonal coverages during 1985. The programme was based on an agreement between the governments of Gabon, Congo, Zaire and Angola, the Food and Agriculture Organisation of the United Nations (FAO) and the Norwegian Agency for Development Cooperation (NORAD).

A planning mission was carried out in November 1984, in which background information were collected and the survey plans adjusted to the specific requests of the fishery management and research units of the respective countries. National scientists participated onboard on all surveys.

The objectives were to investigate the marine fish resources of Gabon with emphasis on surveys using acoustic integration technique to study the distribution, composition and abundance of the stocks of small pelagic schooling fish. Additional objectives were to investigate the stocks of demersal fish on the shelf by random trawling, using the swept area method. The plan comprised four seasonal coverages in a full year-cycle.

Cruise reports with a description of the work conducted and outlining some main findings were issued after each of the surveys. A brief summary report was submitted to the Gaboneese authorities upon request in May 1986.

This final report contains an analysis of all main findings concerning Gabon. It is however recognized that the amount of observations and data collected during the four surveys will allow more detailed analyses of the biology and distribution of the commercial species than has been possible in this report. To facilitate such further work, complete data files have been compiled and included.

1.2 Investigational effort

The survey programme covered a period of 1 year from January until December 1985. The details of the four coverages are set out in Table 1.

All surveys covered the shelf area from a depth of about 10 fathoms along the shore out to the shelf edge at about 200 m depth. The density of the course track are indicated in Table 1 as the ratio between nautical miles steamed and units of 100nm covered. The acoustic transects were usually laid out with an interdistance of 10nm, which is generally dense compared with other surveys of similar resources.

Table 1. Details of investigational efforts in the four surveys
SURVEY DAYS	DATES	DISTANCE (nm) TRAELLED IN SURVEY AREA	SURVEY INTENSITY nm/100nm2	NUMBER OF FISHING STATIONS	NUMBER HYDROGR. STATION
I/8	4-12/3	1393	11.7	36	9
II/11	1-12/6	1583	13.2	57	9
III/8	16-23/9	1663	13.9	45	9
IV/8	9-16/12	1414	11.9	32	9

The total survey effort of the programme is demonstrated by the following statistics:

Days at sea:	35
Distance steamed:	6053 nm
Number of fishing stations:	170
Number of hydrographic stations:	36

The layout of the course tracks and the positions of fishing stations are shown in maps in Annex 1.

1.3 Methods of investigations

The main survey effort was spent in investigating the small pelagic fish using acoustic integration technique combined with fishing with bottom and mid water trawls for identification and sampling. Fish near the surface and close to the bottom are not properly observed by this system.

Horisontal ranging sonars were used to observe surface schooling fish, but such observations are not easily quantified in terms of measures of biomass. To include bottom dwelling fish and shrimp a programme of preallocated trawl stations were worked to provide data for swept-area measurements.

The reliability of the acoustic technique in providing estimates of biomass is under continous review by the scientific community. A problem is the limited information available on the acoustic target strength of the species surveyed. Some efforts were made to acquire direct observations of this during the surveys, but the results were not comprehensive, and for the biomass estimates data on target strength of herring in European waters have been made use of throughout. Another limitation is found in the incomplete coverage of inshore waters by the survey, since the parts of the shelf with depths less than about 10 fathoms could not be navigated by the vessel. Lastly, the oil-drilling area from Cape Lopez and southwards is a restricted area for sea navigation. As no permit was received for the vessel to operate in this zone, the investigations do not incorporate this area.

The overall effect of these limitations are assessed to lead to an underestimation of the biomasses.

When allocating the integrator readings on different types of resources use is made of the following categories:

Pelagic fish type 1, clupeids and engraulids;
Pelagic fish type 2, carangids, smaller scombrids, barracudas, hairtails
Demersal fish, seabreams, croakers, snappers etc.

The swept area method involves a special problem of estimation viz. what value to assign to the catchability quotient, q. We have in this case used 1, which implies that all organisms in the path of the trawl are caught and contained in the catch. This is likely to be true for certain types of shrimp, but for fish the q will take values that may be lower than 1, with more fish escaping through meshes or outside the trawl opening than is aggregated by the herding effect of the trawl wires. The assumption q = 1 thus tends to give underestimates of biomass.

All catches were sampled for species and sizes so that a total composition by weight and usually also number could be estimated.

The sea surface temperature was observed continuously and the hydrographic profiles were worked with Nansen bottles, with salinity and oxygen analysed onboard.

A record of the acoustic instruments used and their calibration is presented in Annex 10 together with a description of the fishing gears.