Annexe 16. EXTRACTS FROM THE REPORT OF THE JOINT USSR - SIERRA LEONE FISHERY INVESTIGATIONS IN 1976-77

by
K. FERGUSSON
Fisheries Division
Freetown, Sierra Leone

Characteristics of the Area

The coastline of the Republic of Sierra Leone stretches for about 240 miles. The width of the inshore part of the shelf limited by the 50 m isobath approximates 10 mi, in the southern and 60 mi in the northern part of the area. The transition from the shelf to the continental slope lies at depths of 90-100 m. The shelf relief is rather complex due to a number of banks and terraces at the depth of 25 to 90 m. In addition, there are folded erosive valleys on the shelf. The shelf ground is mainly represented by sand with a mixture of gravel and pebbles; small and broken shells also occur. In depressions and river mouth areas significant silt deposits are recorded. In some parts of the external shelf side (below 50 m) there are exposures of hard sedimentary rocks in the shape of inclined slabs, which offer some difficulties for bottom trawling.

The continental slope is narrow, its width does not exceed 15-20 miles. The slope steepness reaches 3-10° at 600-800 m. The slope is broken down by numerous canyons and has a number of ledges with inclination angles of 15° or more. Not much is known about the. slope ground but available information indicates the presence of the original rock exposures within the upper range of the slope down to 500-600 m. There are only a few parts on the continental slope suitable for bottom trawling.

Pecularities of Hydrographic Conditions

In conducting acoustic surveys at 10-15 mile intervals, the temperature and salinity were determined.

The temperature of the surface waters varied between 25.7° and 27.6° C. The peculiarities of the temperature distribution revealed the presence of two upwelling zones around the offshore shelf edge.

1. The northern upwelling zone (8°30-9°00N) the major part of which is evidently situated in the waters of the Republic of Guinea;

2. The southern upwelling zone between 7°20 and 7°50N. The two upwelling, zones, are delimited by the intrusion of warmer oceanic water approaching the shore between 8°40 - 8°50°N.

The salinity of the surf ace waters varied from 28.00 to 33.91 percent. Judging from the peculiarities of its distribution, some streams of less saline water were directed from the coast to the offshore part of the shelf. Two sources of freshwater were found in the port of Freetown and Sherbro Island areas.

The presence of less saline water along the shelf edge between 7°10 and 7°40N was stipulated by its transport from the Freetown area along the frontal zone formed on the left side of the main way of the oceanic water intrusion to the shelf.

The salinity gradients in the southern part of the area affected the distribution pattern of biologically active zones of the Sherbro Island area, which were of interrupted character according to the acoustic survey data.

Results of Experimental Fishing
Catch Distribution by Area and the Stock Size of the Main Pelagic Species

Catches of fish per hauling hour generally ranged from 10-3 000 kg, the average catch made up 580 kg hauling hour. A total of 35 fish species from 20 families were recorded in the catches.

Representative of the families Carangidae (Caranx rhonchus, Caranx senegalus, Chloroscombri chrysurus, Decapterus punctatus), Clupeidae (Sardinella aurita, Sardinella maderensis and Balistidae (Balistes capriscus) were the most numerous.

In the southern shelf zone at depths of 25-40 m the catch of the fishes from the Carangidae family made up about 60 percent or about 4.0 cwt/hauling hour. Larger C. senegalus (2.3 cwt/hauling hour) with the mean length of 29.9 cm were most common (in 75 percent of hauling). Hynnis goreensis also contributed significantly to the catch. The maximum catch of Chloroscombrus chrysurus constituted about 0.7 cwt/hauling hour in the southern zone of the area at depths 20-25 m. In the central zone of the area the representatives of the Carangidae family prevailed among the pelagic fish species. The catch of larger C. rhonchus from the depth of 70-75 m with the mean length of 32.2 cm reached 25 cwt/hauling hour and made up 95 percent of the total catch. Decapterus punctatus and relatively small C. rhonchus (mean fork length 23 cm) occurred in great numbers (about 30 cwt/hauling hour) at depths of 30-50 m. Large aggregations of S. aurita were successfully fished at depths of 20-30 m. The maximum catch per hauling hour reached 27 cwt.

Processing of the acoustic survey data gave the following fish stock estimates:

	- the pelagic fish stock size of the Sherbro Island area:	88 500 t
	- the pelagic fish stock size of the Orontes Bank area:	30 000 t
	- the pelagic fish stock size of the northern area:	40 000 t
	- total pelagic fish stock size:	158 500 t

The commercial fish species make up 76 000 t (48 percent) out of which Sardinella aurita makes up 15 000 t and Horse mackerel, Chloroscombrus chrysurus, V. setapinnis etc. mostly of the Carangidae family comprise 61 000 t.

Brief Biological Characteristic of the Main Pelagic Species

Caranx senegalus was caught in the southern zone of the area at depths of about 30 m. The species make up 4 to 48 percent of the catch by the pelagic trawl, and accordingly 17 to 234 kg/hauling hour, large individuals, were mainly taken at lengths ranging from 28-34 cm (mean length 29.9 cm) the mean weight being 448 g. All the fish caught were mature, partly spawned, or spawning. The intestines of 50 percent of the fish examined were covered with a thin layer of fat.

Chloroscombrus chrysurus was recorded in trawl catches from the central and southern areas. In the southern area the maximum catch per hauling hour reached 68 kg (18.5 percent of the catch). The pelagic trawl was used at depths of 20-27 m. The fish caught were of 14 to 20 cm length. The bulk of the catch was made up of females (maturity stage VI - IV). In the central area C. Chrysurus was larger (modal length of 19 cm). Maximum catch made up 500 kg per hauling hour.

Sardinella aurita - The species occurred at depths of 20-30 m. The maximum catch was 2.7 t per hauling hour. The size composition varied from 16-22 cm, the average being 18 cm.

Sardinella maderensis - Catches of S. maderensis were insignificant. The maximum catch per hauling hour in the central area was 15 kg (length of 19-20 cm dominated). In the southern region, where the maximum catch of S. maderensis did not exceed 50 kg (12 percent of the catch) per hauling hour, fish of 12-23 cm fork length were recorded (modal length being 17 cm). In both regions specimens at maturity stages VI - IV and VI - II dominated.

Decapturus punctatus - Maximum catches were recorded at the depth of 30 m. Fish of 15-20 cm length were caught (modal length being 18 cm and mean weight 0.65 kg). Males predominated in the catch and the gonads were mostly at maturity stage VI - IV.

Caranx rhonchus occurred in the central zone of the Sierra Leone shelf at depths of 20-40 m and 70-75 m. At depths of 20-40 m the species constitute about 13 percent of the total catch. Relatively small individuals were caught, with length ranging from 21-26 cm and a mean weight of 170 g. In the offshore part, i.e. at depth 70-75m, the abundance of Caranx rhonchus reached 99.2 percent of the total catch. The length of fish in this zone ranged from 27-37 cm (mean length 32.2 cm) and mean weight was 494 g.

Balistes capriscus was caught all over Sierra Leone shelf at depths of 20-75 m. This species was present in 50 percent of the catches. The size composition recorded everywhere was 16 to 25 cm, with 20 cm being the prevalent size.

Description of the Fishery

Sierra Leone's pelagic stocks are mostly exploited by the artisanal fishermen using different types of canoes. The canoes supporting this fishery are about 15 000 scattered along the entire coastline. The canoe fishery involves over 20 000 fishermen. These canoes use ringnets, drift gillnets, gillnets and beach seines.

In addition to the canoes, locally owned trawlers fish for pelagic species, as well as foreign trawlers operating under charter by local companies or as joint ventures with local companies. Scarcity of field personnel had made it difficult to make accurate recordings of catches but, as from 1977, meaningful data have been compiled by the Fisheries Division.

The clupeids have not been allocated by fish species such as Sardinella aurita, Sardinella maderensis etc. but rather have all been added to the Sardinella spp. group. It is hoped that in future, the reporting of the catches will be done by individual fish species.

During the years 1974, 1975 and 1976, because of insufficient field staff, it was not practicable to obtain reliable catch records. In the case of Scomber japonicus the catches reported by the canoe fishery should merely be taken as landings observed and recorded. But in 1977, when more field staff were recruited, the landing figure was definitely significant.

	1974	1975	1976	1977
Sardinella spp.	23 040	23 8001/	24 5001/	25 5001/ tons
Scomber japonicus	6.8	13.48	15.58	119.4 tons
Caranx spp.	-	-	-	239.6 tons

1/ These include estimates for the artisanal fishery which has not been extensively covered.

	1975	1976	1977
Vessel days	360	1 454	4 329
Caranx spp.	13	39	695
Mackerel	-	-	235.7
Sardinella spp.	432	513.9	7 561
Total	445	552.9	8 491.7

Trachurus trachurus and Trachurus trecae are caught by trawlers (Mediterranean type:) of local companies, as well as by some of the USSR fleet but figures are not available for these species.