The Working Group noted that since the early 1950s, there were several fishery resource surveys aimed at assessing the demersal resource potential on the Ghanaian continental shelf. The list of surveys, as documented by van der Knaap (1985), was updated by the Working Group (Appendix 3).
The Guinean Trawling Survey (GTS) of 1963/64 (Williams, 1968) was the first survey that covered almost the entire shelf area of Ghana, although several ad hoc surveys which were either restricted in area or time, had also been carried out in Ghanaian waters. In 1969, the Fishery Research Unit (FRU) of Tema, Ghana (now renamed Research and Utilization Branch of the Fisheries Department, FRUB) initiated a bottom trawling survey. Since 1979 these surveys have been conducted on a routine basis.
Since reports of many of these surveys have already been published, this section summarizes the design and methodology used in the surveys conducted by FRUB.
Between August 1969 and December 1970, seven seasonal trawling surveys were carried out on the Ghanaian continental shelf at depths of between 18 and 100 m (Rijavec, 1980). The 1969/70 survey programme has been referred to elsewhere as “Research” Survey of 1969/70 or as FRUT1D. The design of the survey is shown in Figure 5. There were 21 sampling stations located on 5 transects which were perpendicular to the coastline. The survey trips were made in all the four distinctive hydrographic seasons that occur off Ghana; namely the major coastal upwelling (July—September), the minor coastal upwelling (January—March) and the two periods of hydrographic stability between them (i.e., April/June and October/December).
Transects and stations were chosen in such that most of the shelf area was covered and the number of stations on each transect depended on the trawlability of the ground and width of the shelf where the transect was located.
Sampling was systematic and the same stations were worked on each cruise. Additionally there were monthly survey cruises on the Tema transect within the period of the survey. All hauls were made during the day and the gear was always towed eastwards along the selected isobath. Each haul lasted for one hour. Hydrographic data were also collected and all fish (and shellfish) caught were sorted into species and weighed.
Two types of trawls were used for the survey: the LARSEN and the ENGEL high-opening bottom trawl nets. Sometimes, a line of bobbins was attached to the latter's footrope. The horizontal opening of both nets was between 14.2 and 16.3 m.
The 1979/80 FRUT survey (referred to elsewhere as KAKADIAMAA Survey of 1979/80 FRUT2D), followed the same design and methodology of FRUT1D (Koranteng, 1981). The same transects and stations were worked. The KAKADIAMAA 1981/82 FRUT survey followed a design as shown in Figure 6 (Koranteng, 1984). The survey covered depths between 15 and 75 m and sampling was done at 37 stations located on a grid. Ten sectors each of 10 nautical miles wide, were demarcated between longitudes 1°10' and 3°W. Each sector carried one transect at a time and the location of the transect was chosen at random within the sector. The 37 stations were contained in 8 strata. The depth zones sampled were 15–23, 24–30, 31–40, 41–50, 51–63 and 64–75 m. The number of stations per transect was between 3 and 6 and depended on the width of the continental shelf in the area where the sector was located.
Four cruises covering the whole continental shelf of Ghana up to the 75m depth contour in all sectors were carried out. These cruises coincided with the four hydrographic regimes as described above. The surveys were in May, August and November 1981 and in January 1982. Additionally monthly surveys were carried out at six stations in the Saltpond sector (see Figure 6).
Subsequent FRUT surveys followed the design described above, but since 1987 the strata have been reduced to 3, corresponding to the shallow belt (i.e., 10–30 m) the middle belt (31–50 m) and the deep belt (51–75 m) respectively.
In the FRUT surveys, certain fish and crustaceans species were selected for length measurement and for detailed biological studies. These normally were species of scientific or commercial interest.
Between 1979 and 1987, the fishing gear used in all FRUT surveys was the ENGEL high-opening net with a horizontal mouth-opening of about 12 m. An ENGEL Balloon trawl with a horizontal mouth-opening of between 17.5 and 19.5m is now used for all FRUT surveys. These differences in survey gear are taken into consideration when data from these surveys are compared or contrasted.
Surveys in Sierra Leone waters were all undertaken using foreign vessels (Appendix 4) as part of a bilateral fisheries agreement or FAO/CECAF sponsored cruises. Most recent surveys include the ATLANTIDA, MONOCRYSTAL (USSR) and the LAGOAPESCA (Spain/CECAF). The design, methodology and gears used have been described in the reports of the surveys.
During 1985 and 1986 four trawling surveys were carried out on the shelf of Côte d'Ivoire. These are CHALCI 8501 - February 1985, CHALCI 8502 - July 1985, CHALCI 8603 - February 1986, and CHALCI 8603 - July 1986. Data obtained from the CHALCI surveys were used in calculating the mean biomass of all species combined. Table 18 gives the mean yields by survey and bathymetric strata (10–50 and 50–120 m) for the total demersal resources.
Values for the first three surveys might be slightly different from those obtained by Caverivière (1982) because of differences in the surface areas of the strata and also the number of stations worked (not included in the table).
Appendix 5 gives an updated list of all surveys in Ivorian waters.
In Liberia surveys were also undertaken using foreign vessels (Appendix 6). Reports on most of these surveys have been published.
The report of the GUINEA-90 cruise did not include biomass estimation due to the sampling scheme used (systematic transects). This has certain theoretical limitations for its use for fish biomass estimation, as sampling variances cannot be calculated. Despite this limitation and in order to have rough estimates to compare with results of other cruises, the Working Group agreed to calculate the biomasses. The results show estimates for finfishes, crustaceans, cephalopods and total demersal resources; and by depth strata for each of the five sectors (Table 19). Table 20 gives the calculated densities, for total demersal resources.
Biomass estimation was done from the swept area method, using 0.039 km2 as the area swept in 30 min. of trawling. This corresponds to an average horizontal-opening of 14 m and towing at 3 knots. Catchability was assumed to be 1, which leads to an estimation of the minimum biomass.
As the gear used in Côte d'Ivoire was differed from that used in the other areas, the Working Group analyzed the species composition of the two gear types (1 and 2). The comparison was done on the catches of the different fish communities (Table 21) and for five stations where both gears were used. It should be noted that the number of samples was small and therefore the conclusions must be considered with care.
At the level of main groups, important differences were obtained in catchability of the cephalopods. Gear 1 (which had bobbins) was apparently less efficient for these species and could not catch octopus. This could be explained by the fact that gear 2 which did not have bobbins, and the footrope of which therefore touched the bottom, was more efficient for benthic species. There were no other clear differences except in the case of Raja miraletus which was more abundant in gear 1 catches.
The total minimum biomass of demersal species in all five sectors surveyed was about 170 000 t, of which about 90.6% was made up of finfishes and 8.8% of cephalopods. (Table 19) The figures for crustaceans and, to some extent, cephalopods are probably underestimated due partly to the fact that the gears used are not suitable for these species, especially the crustaceans. The largest total biomass was found in the EEZ of Ghana. This was followed by the biomass in Liberia, Sierra Leone and Côte d'Ivoire in decreasing order.
Demersal fish densities by sectors (Table 20, Figure 7) show similar values for Ghana and Côte d'Ivoire (about 36 to 37 kg/ha) and much lower values for Sierra Leone and Liberia. Lowest values were found in the eastern zone of Sierra Leone (about 15 kg/ha).
Table 22 compares the results of the GTS (Williams, 1968), and GUINEA-90. The boundaries used in the GTS for the West Sierra Leone-Guinean area could not permit biomass estimates from the GTS for this area to be compared with those from GUINEA-90; but it was possible to do this for the Sierra Leone East-Liberia area. The estimates are very similar, around 11 000 t for the 15/20 to 50 m zone and 31 000 to 33 000 t for the 50 to 200 m zone.
Concerning densities (Table 23), the results for GTS and GUINEA-90 for the areas in question were in the same order of magnitude. These were 13.6 and 15.8 kg/ha for GTS and GUINEA-90 respectively in the 15/20 to 50 m zone and 20.1 and 22.5 kg/ha for the 50 to 200 m zone. It is worth nothing that in both zones, estimates from GUINEA-90 are slightly higher than those from GTS.
The Working Group also calculated fish biomasses for selected commercially important species (Table 24). Dentex angolensis was the most abundant commercial fish. It was especially abundant off Liberia and Côte d'Ivoire at 6 500 t and 4 000 t respectively.
In order to compare the present state of resources in the region as deduced from GUINEA-90, with previous survey results the Working Group followed trends in results of surveys carried out in Ghana and Côte d'Ivoire. Wherever necessary, reference has been made to results of GUINEA-90 for the entire survey area.
From the various surveys carried out on the continental shelf of Ghana, an attempt was made by the Working Group to examine trends in the productivity of the entire demersal resource and also of some important species.
In view of the multiplicity of gears used in the listed surveys, the Working Group decided to compare the results using calculated densities of the total demersal resource and of the selected species.
Table 25.1 gives catch per haul of GTS, FRUT surveys and GUINEA-90. All figures have been adjusted to catch per 30 min. of trawling. For purposes of comparison only data for ENGEL net with bobbins were used for FRUT surveys of 1969–70. Figures from GTS are the means for the two zones (15–50 and 51–200m). Similarly, figures for GUINEA-90 are the means for the two zones 20–50 and 51–100 m.
In Figure 8 and Table 25.2 the evolution of densities (kg/ha) of the total demersal resource on the shelf of Ghana (to 100 m) has been presented. Also presented is the trend of densities when catches of the triggerfish, Balistes capriscus are taken out of the data. It is noted here also that only the FRUT surveys cover a period of one year each.
The following observations were made from Table 25 (1 and 2) and Figure 8:
The proliferation of B. capriscus in the 1970s did not appear to have had any adverse effect on the total productivity of demersal fishes; rather the productivity showed a slight increase during the period that Balistes was most abundant.
The density of the total demersal resource has reduced since about 1981/82. The productivity levels, in the late eighties/early nineties, are similar to the level recorded during GTS.
Since the last Working Group, there have been significant changes in the resources and/or fishery of the triggerfish, globefish and cuttlefish. Some of the recorded changes are discussed below.
Balistes capriscus (= carolinensis)) (Triggerfish)
The proliferation of B. capriscus in Ghanaian waters coincided with the decline of the Sardinella aurita fishery in the area in 1973. During the GTS, B. capriscus was only considered with “others” because it appeared in catches in very small quantities. In FRUT1 (1969/70) a density of only 0.6 kg/ha was calculated for this species. By 1979, B. capriscus had become the most abundant species in Ghanaian waters, representing over 61% of the total estimated biomass (Koranteng, 1981).
Landings of the species were, however, comparatively low and the resource was generally considered to be underexploited (Koranteng, 1984; Koranteng and Quaatey, 1990). Total landings by Ghanaian vessels increased between 1972 and 1986 reaching a peak value of over 18 000 t in 1986. Landings of the species then began to decline when a decline in its occurrence in survey trawls was also observed. The decline was observed during the FRUT surveys in 1981/82 and catches were almost absent from survey hauls by 1988 and 1989.
In the R/V DR FRIDTJOF NANSEN survey of October 1989 (Norway 1989), the biomass of the triggerfish off Ghana and Côte d'Ivoire was virtually put at zero. It may be recalled that in 1981 the same vessel recorded 314 000 t of this species off Ghana (FAO, 1984) whilst R/V CORNIDE DE SAAVEDRA (Oliver, 1986) recorded 115 000 t in the same area in 1986.
During GUINEA-90 only 33 specimens of B. capriscus with a total weight of 17.6 kg were caught and at only 7 of the 120 stations (see Figure 9). These were caught in shallow waters and mainly off Côte d'Ivoire. Of the total weight of Balistes caught during the cruise 80% was from Ivorian waters (see Table 26).
Small amounts were caught in the western zone of Sierra Leone and in Ghana (Figure 9; Table 26). The average weight of the specimens caught ranged between 265 and 680 g.
These data obtained during the warm season are indicative of the decline that had been observed since 1986, especially off Côte d'Ivoire and Ghana.
Lagocephalus laevigatus (Globefish)
Another significant change in the demersal resource of Ghana since the last Working Group is the apparent increase in the biomass of the globefish, Lagocephalus laevigatus. In 1989, a substantial amount of the species was caught during the FRUT surveys.
In Côte d'Ivoire fishermen have expressed concern over the unusually high abundance of this species which was destroying fishing gears (Bard, pers. comm.).
In view of the possibility of a further increase in the abundance of this species, as had happened with triggerfish in the seventies, the Working Group paid special attention to the information so far obtained on the species, especially the recent data obtained in GUINEA-90.
During the GTS, L. laevigatus was observed in small quantities between 15 and 200 m. During FRUT the species was encountered many times but it hardly constituted even 1% of the total catch at any one station. In FRUT3D (1981/82), a total biomass of about 349 t (or 0.3 kg/ha) was estimated for L. laevigatus. The species was not among the 20 most abundant species caught during the survey.
In FRUT5D (1989/90), an average biomass of 1 063 t (or 0.6 kg/ha) was calculated. It was the twelfth most abundant species caught in the survey. L. laevigatus was found to be as abundant as Sepia officinalis or Dactylopterus volitans and twice as abundant as B. capriscus.
During GUINEA-90, L. laevigatus occurred in 24 of the 120 “valid” hauls and always in waters of 100 m deep or shallower. Table 27 and Figure 10 show the catches by strata in the five sectors surveyed by R/V LAGOPESCA. The data from Côte d'Ivoire have been scaled down by a factor of 3.12 (see Table 6 of the preliminary cruise report (Ramos et al., 1990).
L. laevigatus was most common on the eastern part of Côte d'Ivoire and western Ghana (Figure 11). Off Sierra Leone, it was found mainly between 20 and 50 m deep. Individuals examined had a high average weight which was between 400 and 550 g (see Table 28, Figure 12). The average weight was generally lower in deeper waters. Off Liberia mostly juveniles of this species were encountered. Table 29 gives the estimates of biomass and density of the species off Côte d'Ivoire and Ghana.
The Working Group took note of these developments in the globefish resource; however, there was no evidence to link the increase of its biomass with the decline of the triggerfish.
Cephalopods
The most important cephalopod resource in the Western Gulf of Guinea statistical division of CECAF is the cuttlefish of which Sepia officinalis is the most common.
During the GTS, catch rates of between 0.5 and 6.0 kg per 30 min. were recorded off Ghana for all Sepia species. Surveys since then have recorded corresponding values ranging from 3 to 7 kg per 30 min. In GUINEA-90 an average catch rate of 14.9 kg per 30 min. was recorded off Ghana (to the 100 m depth). This gives a yield value of 3.8 kg/ha.
Appendix 10 includes detailed analysis of the catches made during the GUINEA-90 cruise. The total cephalopods catch during the survey was 1 043 kg (see Table I, Appendix 10). Five of the twelve species of cephalopods caught were cuttlefishes. Sepia officinalis was the most important commercial cephalopod species caught (459 kg or 44%).
The Working Group took note that about 67% of the total catch of S. officinalis was taken off Ghana (Table I, Appendix 10) in waters of between 20 and 100 m deep. The species has a wide bathymetric distribution in Ghanaian waters where it occurred in 90% of the valid hauls (Figure 1, Appendix 10).
The Working Group noted the GUINEA-90 results especially with regard to the apparent discovery of large stocks of S. officinalis off Ghana. It noted also that the Ghana FRUT surveys since 1969 show that the density of S. officinalis in Ghanaian coastal waters (15–75 m) has reduced from 1.2 kg/ha in 1969/70 to 0.6 kg/ha in 1989. Considering, however, that the present FRUT surveys are limited to the 75 m depth contour, the stock of cuttlefish encountered at a depth of 100 m of Ghana during GUINEA-90 could have been there all the time. The Working Group was of the opinion, therefore, that the supposed increase in the stock of cuttlefish off Ghana may only be apparent and far from being real.
Crustaceans
During GUINEA-90 forty-five crustacean species were caught. The highest diversity was found in the families Pandalidae and Penaeidae. Over half the total catch of 86 kg of crustaceans was taken in the Liberian EEZ (Table 30). Penaeus notialis was found between 20 and 50 m deep and mainly off Sierra Leone.
It is essential to note that the fishing gears used in the survey were not appropriate for these crustacean species and also any part of the resource in waters shallower than 20 m would have been missed.
Appendixes 11 and 12 synthesize information obtained during GUINEA-90 for Parapenaeus longirostris and Parapenaeopsis atlantica respectively.
The CHALCI surveys undertaken in Cote d'Ivoire between 1978 and 1986 were based on a stratified random sampling design having two depth strata: 10–15 and 50–100 m. Seven surveys were undertaken during the warm season, between January and May. These were code-named CHALCI 78.02, 79.03, 83.01, 83.03, 84.01, 85.02 and 86.02. Four other surveys took place during the cold seasons, July-September and these were CHALCI 80.08, 84.07, 85.07 and 86.07. It must be noted that there were no surveys in 1981 and 1982 and also between 1986 and 1990 when the GUINEA-90 survey was conducted.
All the CHALCI surveys used the same vessel, sampling design and gear.
In the following paragraphs, comparisons between the CHALCI and GUINEA-90 surveys have been made. In the accompanying tables and figures, the labels “CHALCI methodology” and “GUINEA methodology” have been used to enable the reader to know how the results were obtained.
CHALCI methodology means that the depth (zone) stratification used during the CHALCI surveys have been considered for the calculations. GUINEA methodology on the other hand refers to calculations based on the GUINEA-90 stratifications.
The data from GUINEA-90 were also re-analysed using CHALCI methodology. To do this the following approach was taken.
For each depth stratum or community of fish conversion factors were obtained from analyzing the same set of data with both methodologies. It may be recalled that off Côte d'Ivoire the depth strata used for GUINEA-90 were 20–50, 50–100 and over 100 m. These factors were then used to correct the GUINEA-90 results. The same approach was taken to convert CHALCI to GUINEA-90 as if the latter had covered the same area as CHALCI. The results are shown in Tables 31–33.
Total biomass
From the study of densities (in kg/ha) of the demersal resource biomass for the warm and cold seasons (Tables 34 and 35 and Figures 13.1 and 13.2) it may be deduced that the density of the total demersal resource increased between 1978 and 1983. This reached a maximum in 1983 and started to decline in later years until 1986. The increase in biomass was due essentially to the proliferation of triggerfish (Balistes capriscus).
Considering the density excluding triggerfish, it seems that no clear trend can be observed in the above period; figures having fluctuated between 26 and 46 kg/ha. This would imply, as is the case in Ghana, that the said proliferation of triggerfish could not have been detrimental to the total demersal resource.
Balistes capriscus (Triggerfish)
It has not been possible to follow trends in abundance of triggerfish from commercial catches in Côte d'Ivoire. This is because this fish is not commercially exploited in the country.
The evolution of the abundance of triggerfish could only be followed through fish surveys, bearing in mind that the catchability of this species is dependent on the season (Caverivière et al., 1981). During the cold season, the fish is less abundant at the bottom, but is most abundant at the beginning of the warm season (i.e., November to January).
From the results of pelagic surveys undertaken since 1981 in Ghanaian and Ivorian waters (Appendixes 3 and 5), great variation in the recorded biomass of triggerfish is noticeable. Such variations may be caused inter alia, by:
a real variability in abundance
differences in the efficiencies of the acoustic instruments used in the surveys and
seasonal migrations (e.g., between Ghana and Côte d'Ivoire) or inshoreoffshore where differences will come by way of areas covered in the respective surveys.
Taking the above into consideration, the Working Group concluded that the biomass of triggerfish in Ivorian waters had decreased between 1981 and 1990.
Concerning demersal surveys held during the warm seasons, the analysis shows that at the end of the seventies, triggerfish accounted for 10–15% of the total biomass in February-March. In January 1983, this fish accounted for 72% of the total biomass which was the largest estimated in the period under consideration.
As can be observed, in a survey carried out only two months later, there was a considerable difference in the estimates of triggerfish biomass.
From the foregoing discussions and also from Table 36 and Figures 14.1 and 14.2, it may be concluded that triggerfish biomass:
increased between the end of the seventies and the beginning of the eighties,
decreased from 1983 to 1986 and was almost non-existent at the end of the eighties. During GUINEA-90, very few triggerfish were caught.
Given that no other survey took place in April 1990 and as it is known from the CHALCI surveys that the demersal component of the biomass of Balistes, a semi-pelagic fish, normally decreases from January to May, can be taken in concluding that the stock of triggerfish in Ivorian waters has totally collapsed. Furthermore, the oceanographic conditions that prevailed during the first week of April 1990 (when the M/V LAGOAPESCA surveyed Ivorian waters) were exceptionally unusual for that time of the year. It was quite cold and an upwelling could even be observed in the western part of the country (see Table 37 and Figure 15). With such conditions having some similarities with those obtainable during cold seasons in the year, it is not surprising that the species compositions observed in the catches during GUINEA-90 resembled those normally observed during cold seasons in the CHALCI surveys.
In conclusion, there are indications which suggest that the stock of Balistes capriscus in Ivorian waters had almost disappeared at the end of the 1980s. This was also the case in Ghana where possibly the same stock had occurred.
Lagocephalus laevigatus (Globefish)
GUINEA-90 results appear to have revealed an increase in the biomass of globefish (Lagocephalus laevigatus) in Ivorian waters. This confirms recent observations by Ivorian artisanal fishermen who have been complaining about the excessive damage to their fishing lines caused by the globefish (Bard, pers. comm.). The fish bite the lines with their incisorlike teeth.
During the CHALCI surveys of the eighties, L. laevigatus never accounted for more than 0.1% of the estimated biomass at any time. From GUINEA-90, the species represented about 1% of the estimated biomass in Ivorian waters. This represents a tenfold increase between the CHALCI and GUINEA-90 results. This proportion would probably have been higher if the 10–20 m depth zone had been surveyed during GUINEA-90.
Figures 16 and 17 show that there were relatively large amounts of L. laevigatus in Ivorian waters during the mid-seventies but the biomass decreased between 1978 and 1984 only to be encountered in large amounts again during the GUINEA-90 survey of April 1990. One striking feature in the evolution of densities is that whereas the densities were much lower in the deeper stratum than in the inshore stratum in the period 1978 to 1986, the recent increase in biomass has been observed in the deeper stratum, (i.e., 50–100 m deep).
Cephalopods
Comparing the results of the CHALCI and GUINEA-90 surveys, it is clear that the size of the cephalopod resources in Ivorian waters increased.
Representing not more than 1% of estimated biomass in any CHALCI survey, the share of this group rose to about 4% in the GUINEA-90 survey.
Contrary to the observation in Ghana, where Sepia officinalis accounted for over 84% of the cephalopods, the proportion in Côte d'Ivoire was only 26%. Octopus vulgaris accounted for 21%.
Table 38 shows also that the relative importance of S. officinalis among the cephalopods was rather lower in 1990 than in the past. One possible explanation for this apparent anomaly is that in the CHALCI surveys, other Sepiidae may have been considered as S. officinalis. The importance of Octopus vulgaris, on the other hand, has not really changed and it appears that the observed increase in cephalopod biomass may be attributed mainly to S. officinalis.
It may be recalled that trawl 2 which was used exclusively in Côte d'Ivoire during GUINEA-90, not only fished three times better than trawl 1, on the average, but it was also more efficient for cephalopods (see Table 21). This group was 3 to 4 times better represented in trawl 2 than in trawl 1 catches.
If one takes into consideration the correction made on the Ivorian data because of the gear effect (see Section 6.2) then the importance of cephalopods in Ivorian waters would be reduced. Consequently, the Working Group concluded that probably no major change had occurred in the cephalopod resource in Ivorian waters.
Crustaceans
One of the main aims of the GUINEA-90 survey was to explore the resources of the deeper part of the continental shelf and the slope area. Crustacean resources were included in the list of priority species. Unfortunately, the results, with regard to crustaceans, were disappointing for Côte d'Ivoire. Large areas of the slope were not trawlable and the average catch rate obtained for the most abundant crustacean species Parapenaeus longirostris, 0.5 kg/30 min., was 20–100 times lower than estimates obtained in similar surveys off Senegal or Gabon-Angola (Sobrino, pers. comm.). As the CHALCI surveys did not go beyond 120 m deep, it was not possible to compare the GUINEA-90 results with the CHALCI results.
Other fish communities
As mentioned above, the CHALCI surveys showed fluctuations in the biomass of sparids, sciaenids and other species but with limited amplitude and without any clear trends. It is clear from Tables 31 and Figure 14, however, that there is a progressive reduction of the importance of sciaenids and an increase in that of sparids. This could be a consequence of the concentration of commercial fishing effort in inshore waters to the detriment of the sciaenid community.
GUINEA-90 results also appear to confirm the above observation on changes in the sciaenid and sparid biomasses. The Working Group agreed, however, to view this with caution considering the range of operation of the GUINEA-90 survey (there was no sampling in waters shallower than 20 m) and the near upwelling conditions that prevailed at the time of the survey. The cold environment favours the sparid community as the CHALCI survey results show.
Appendix 9 summarizes data on sparids obtained from the GUINEA-90 survey.
The state of demersal stocks on the Ivorian continental shelf has been studied by the Centre de Recherches Océanographiques (CRO, Abidjan) through the use of global production models.
The proliferation of Balistes in the Gulf of Guinea permitted three periods to be distinguished. These were: 1959–1970, 1971–1980 and 1981–1989. Global production model was applied to catches and standardized fishing effort for each period (Figure 18). For the period 1959–1970, the maximum sustainable yield (MSY) was about 10 000 t/year.
During the period 1971–1977 when Balistes was very abundant, the MSY of demersal stocks on the Ivorian continental shelf was reduced to not more than 6 000 t/year.
After the period 1978–1980, trawlers operating in Ivorian waters increased their catches which stabilized at the level of about 5 000 t/year. With an MSY of more than 7 000 t/year, this may probably be interpreted as a recovery for the stocks.
The time-series data, especially on fishing effort, required to update the global model produced by the last Working Group in 1988, were not available to the present Group.
Some results of work done to estimate the maximum sustainable yield (MSY) of the Sierra Leonean shrimp resource, using global production models, was presented to the Working Group by the Sierra Leonean participant.
Of the two commercially-important penaeid species found in Sierra Leonean waters, the pink shrimp (Penaeus notialis) is the most predominant, making up over 80% of total landings. From data available to the Working Group for the years 1981–1988, the annual production of the Sierra Fishing Company fluctuated between 237.4 and 611.8 t. The highest catch occurred in 1985. Catch rates also fluctuated between 150 and 217 kg/day, the highest value occurring in 1982.
The MSY was estimated to be 1 428 t annually by Schaefer's method and 1 893 t by Fox's method. The corresponding derived efforts imply optimum catch rates of 101.6 kg/day and 74.7 kg/day respectively. These results show that the shrimp fishery in Sierra Leone is operating above the levels required to achieve the MSY.
The Working Group deplored the recent increases in the fleet of shrimp vessels (in numbers, sizes and horsepowers) and recommended that further expansion be discouraged. It noted, however, that because only landings made by the Sierra Fishing Company were used in the analysis, the results must be interpreted with caution.
The Working Group also recommended that this work be continued under the general framework of a detailed study of the shrimp fishery in Sierra Leone. It also recommended that similar work be done on the demersal finfishes of Sierra Leone.
This study was undertaken by Caverivière and Bard (1989, in press) for the seven main species that are landed in Abidjan. A yield per recruit model was applied to each to enable a total production for all the species to be calculated. The results have been discussed for three characteristic years: 1976, 1980 and 1989 (see Figure 19). It is suspected that a mesh size of 30 mm was used in 1989 instead of the 40 mm used previously.
The low level of efforts observed for these three years show that there is nearly no loss of productivity with the supposed decrease of mesh size (from 5 000 t in 1976 to 4 500 t in 1989). If the effort increases in the future it will, however, be beneficial to use a minimum mesh size of 40 mm which will also ensure that the juveniles, especially those in inshore waters, are not overexploited.
