CHAPTER 4. OVERVIEW OF DISTRIBUTIONS AND ABUNDANCE OF STOCKS OF SMALL PELAGIC FISH.

4.1 The sardinella stocks.
4.2 Horse - and jack mackerels, scombrids etc.
4.3 The stock of trigger fish
4.4 Morroccan sardine

4.1 The sardinella stocks.

As discussed under section 1.3 above, problems of sampling and identification limit the degree to which the total observed biomass can be allocated to fish types or species. The sardinellas form the most important part of the group identified as clupeids and anchovies from Mauritania southwards. Figure 4.1 shows the distribution of this group during the two survey coverages, in August - September and in November - December. The African ilisha was found in limited amounts in inshore waters off the Bijagos Islands and the Gambia. The dense aggregation shown off Nouadhibou in Survey II consisted mainly of sardine (Sardina pilchardus). Because of incomplete coverages the total distribution of the sardinellas can not be described, but it seems that only limited amounts occurred south of Cape Roxo although it must be noted that the aggregations off Freetown was no doubt substantially underestimated in the survey. From the distribution charts there does not seem to be any clear distributional difference between the two species, either by depths nor latitude, both species were found over the whole area. Table 4.1 shows the catch rates and the incidence of occurrence of the round and flat sardinellas. Because of the clear size related catchability of these fish in trawl gears, too much confidence should not be placed in the interpretation of these data, but the results show that they occur in about the same proportion, in precence in number of total catches. The catch rates for flat sardinella are somewhat higher than those for the round, but this may be a consequence of some difference in depth distribution between the two species which may cause a higher catchability of flat sardinella in trawl hauls in shallow water.

Table 4.1 Mean catch rates and incidence of occurrence in bottom - and mid water trawl of the two sardinella species.

	Flat sardinella		Round sardinella
	Mean catch kg/hr	% Incid.	Mean catch kg/hr	% Incid.
Aug.-Sept.	65	27	31	20
Nov.-Dec.	448	25	121	26

Table 4.2 shows the biomass estimates for the sardinellas by surveys and areas including probable assessments for the smaller uncovered parts of the Senegambian shelf. A rounded likely total estimate for the sardinella biomass is 650 000 tonnes.

The distribution of the sardinellas by these surveys, with the main parts located on the Mauritanian - Senegambian shelf, is very similar to that found in previous surveys of the area e.g. the surveys by DR FRIDTJOF NANSEN between May 1981 and March 1982. The estimates of biomass are, however, now approximately 30% higher than those from 1981/82.

Table 4.2 Biomass estimates of sardinellas by areas and surveys. 1 000 tonnes.

	Aug.-Sept.	Nov.-Dec.
Mauritania	n.s.	300
Senegambia	300	330
C. Roxo - Sherbro Isl.	30	n. s.

4.2 Horse - and jack mackerels, scombrids etc.

Figure 4.2 shows the distribution of this group as a whole. The various species differed, however, considerably with regard to their main areas of location by latitude and by depth. The horse mackerel Trachurus trachurus ranges south to Mauritania where in December it was found overlapping in distribution with T. trecae. This species appeared in concentrations from Cape Timeris to the Gambia, often, and particularly for the larger fish, over the offshore parts of the shelf. The false scad, Decapterus rhonchus again overlapped with this horse mackerel species, but was found in highest concentrations from the Gambia southwards past the Bijagos Islands. The bumper, Chloroscombrus chrysurus was found in high concentrations in many locations from Freetown northwards past Cape Verde especially in the August-September survey and some times mixed with the lookdown, Selene dorsalis. Of the other species in this group, Spanish mackerel Scomberomorus tritor occurred frequently, but in limited amounts inshore from Freetown to Cape Verde. The hairtail Trichiurus lepturus was found in some abundance off Mauritania in December.

The estimated biomass for this group by areas and surveys is shown in Table 4.3.

An attempt can be made to roughly allocate these biomass estimates to species or species groups based on the catch compositions and the geographical distributions observed. The December survey represents a point in time when the seasonal southward shift is well under way and it seems likely that the stock of the horse mackerel T. trecae will then have its main distribution within the Mauritania - Senegambia area. The August - September survey from St. Louis to Sherbro Isl. will probably provide the best coverage of the false scad and the inshore carangids. Table 4.4 shows the results of such a rough allocation.

Table 4.3. Estimated biomass by areas and surveys of horse mackerels, scads, jacks etc 1 000 tonnes.

	Aug.-Sept	Nov.-Dec
Mauritania	-	540
Senegambia	150	170
C. Roxo to Sherbro Isl.	170	-

Table 4.4 Rough allocation of biomass estimates of the carangids etc. on species or species groups. 1 000 tonnes.

		Decapterus rhoncus	Chloroscombrus and others
Aug. - Sept.
	Senegambia	60	90
	Sherbro-C. Roxo	110	60
Total		170	150
		Trachurus trecae	Trichiurus and others
Nov. - Dec.
	Mauritania	380	160
	Senegambia	100	70
Total		480	230

Likely assessments of stock biomass for these fish are thus: close to 500 000 tonnes for horse mackerel; about 170 000 tonnes for scad and 200 000 - 250 000 tonnes for bumper, hairtails, look-down and other jacks, Spanish mackerel, and barracudas, a total of 900 000 tonnes for these types of fish.

The DR FRIDTJOF NANSEN survey in May/June 1981 and Feb/March 1982 which covered the same area, gave total biomass estimates of 1 130 000 tonnes and 975 000 tonnes respectively for this group, thus about the same level as observed in 1986. The allocation on species was, however, somewhat different with 670 000 and 610 000 tonnes of horse mackerel, and 330 000 and 150 000 tonnes of scad. The methodological problems of allocating the total biomass estimates on species or subspecies groups are as we have already discussed, considerable, and it seems reasonable to conclude that there is no major difference in the estimates of stock biomass between the two survey programmes.

4.3 The stock of trigger fish

The distribution of the trigger fish from Freetown to Cape Roxo in August - September and off Guinea Bissau in November is shown in Figure 4.3. The species also occurs further north, but during these surveys it was only found in small aggregations here. The distributional area off Guinea and Guinea-Bissau corresponds to that of the western of the two stocks in the CECAF area, the eastern stock being found off Ghana - Ivory Coast. The distributional characteristics for the western stock demonstrated by the present surveys are very similar to those found in previous investigations. The size distribution with predominance of medium sized fish is also similar to that found in the 1981-82 surveys with the DR FRIDTJOF NANSEN.

Assuming that the August - September survey covered the main western stock its estimated biomass is thus some 220 000 tonnes. This should be compared with similar estimates for the same stock of 1 050 000 tonnes in June 1981 and 1 350 000 in February 1982. There is no evidence of large short term fluctuations in trigger fish stocks, but long term changes have been demonstrated over spans of several years. The greatly reduced biomass estimate from 1986 as compared to 1981/82 must probably be interpreted as the effect of a corresponding stock decline over this period. The available estimates of the biomass of this western stock can be summarized as follows: (FAO/CECAF, 1981 and Strømme, 1983), (1 000 tonnes):

USSR survey 1975	80
CAPRICORNE Nov. 1978	440
CAPRICORNE Mar. 1979	440
CORN. DE SAAVEDRA 1980	760
DR FRIDTJOF NANSEN, MAY-JUNE 1981	1 050
DR FRIDTJOF NANSEN, FEB. 1982	1 350
DR FRIDTJOF NANSEN, AUG.-SEPT. 1986	220

These surveys are not all directly comparable as different survey methods have been applied, but even allowing for a considerable possible biases we are left with a picture of a stock increase in the late 1970’s and early 1980’s followed by a sharp decline in recent years. Especially the decline is well documented since the survey method in this period is more or less identical.

Commercial fishing for triggerfish started on a large scale in 1980. Catches from the Eastern Central Atlantic are reported as follows: (FAO, Yearbook of Fishery Statistics vol. 52,56,-60), 1 000 tonnes:

1978	10	1982	92
1979	14	1983	72
1980	70	1984	28
1981	102	1985	26

It is not unconceivable that this fishery may have affected the stocks, and this should be evaluated if access is obtained to more detailed data of the fishery and the biology of the trigger fish.

A decline seems to have taken place also in the Ghana-Ivory Coast stock of trigger fish. In the DR FRIDTJOF NANSEN survey of June 1981 it was estimated at 500 000 tonnes while from an August 1986 survey with the R/V CORNIDE DE SAAVEDRA it is concluded that the biomass of this stock was approximately 140 000 tonnes. (Oliver and Miguel, 1987). This indicates a proportional decline similar to that in the western stock.

4.4 Morroccan sardine

The sardine is the clearly dominating species in Morocco, at least south of Agadir. During the two surveys between Agadir and Cape Juby the distribution were found to be almost identical, with the main part of the species in the southern half of this area. During the second survey, which also covered the region between Cape Juby and Cape Bojador, there were observed considerable amounts in this area as well, also in the shallower waters, within 20 nm from the coast.

The biomass was assessed to about 0.95 million tonnes between Agadir and Cape Juby, and additional 1.1 million tonnes south to Cape Bojador. The stock of sardine in Morocco was composed of two size cohorts of around 14 and 18 cm modal length, and with the largest as the clearly dominant.

The sardine was also observed in Mauritania, during the second survey. The species formed dense registrations in a small area just off Nouadhibou, which also was the southern limit of the species distribution. The sardine in this area was assessed to 540 thousand tonnes. The sardine was composed of one size group only, with a modal length around 24 cm and thus abt. 6 cm longer than the dominating length cohort in the Agadir- Cape Juby area.

The area between Agadir and Cape Juby was surveyed by “Dr. Fridtjof Nansen” also in March 1981. From this survey the total pelagic biomass in the area was estimated to 750 thousand tonnes with abt 350 thousand as sardine and another 350 thousand tonnes as mackerel and horse mackerel grouped together. Compared to the previous survey the 1986 surveys shows that the stock of sardine has increased since the 1982 level. The increase seems to have been partially compensated by a decline in the abundance of the stocks of mackerel and horse mackerel, at least in the area investigated. Some uncertainties are linked to this comparison as the 1982 survey was in spring while the 1986 surveys were in fall/winter period. Some of the differences might thus be ascribed fluctuations due to seasonal migration.