6 SURVEYS IN THE ATLANTIC OCEAN OFF NORTHWEST AFRICA

From April 1981 to March 1992 the DR. FRIDTJOF NANSEN was deployed in various large-scale resource surveys off Northwest Africa. The activities formed part of the UNDP/FAO projects GLO/79/011 (Assessment and Development of World Renewable Marine Resources) and GLO/92/013 (Global Investigations of Fishery Resources) to which NORAD gave its support.

History of fisheries and research

Northwest Africa has a considerable history of artisanal and industrial fisheries and of fishery research. The large-scale industrial fisheries for the pelagic stocks off Northwest Africa date from the late 1960s. By the mid 1970s the total catch from FAO statistical area 34, the Eastern Central Atlantic, approached 4 million t, about 60% of which was taken by foreign distant-water fleets. The implementation of EEZs reduced foreign access and the total yield from the region declined for some time, to pick up again in the late 1980s, passing 4 million t again by 1990. Subsequent years showed, however, a trend of decrease in total catch caused mainly by a reduced participation of distant-water fleets in the fisheries for small pelagics.

The history of the fisheries over nearly three decades of industrial exploitation indicates that the pelagic stocks of the Canary Current upwelling system may have been more stable than those found in the three comparable systems, the Benguela Current in the Southeast Atlantic and the California- and Humboldt Currents in the Eastern Pacific. Stock collapses such as those of the Namibian- and Californian sardines and the Peruvian anchoveta do not seem to have occurred in the large stocks of sardine and sardinellas off Northwest Africa. It can be speculated that this difference in stock variability is caused by more stable environmental conditions in the Canary Current system or by a more moderate fishing pressure or both.

Important contributions to knowledge of the region were made through the international Co-operative Investigation of the Northern part of the Eastern Central Atlantic (CINECA). Sponsored by ICES, FAO and IOC, the programme was based on field activities by a large number of research vessels over the period 1970–77. Results were presented and discussed at a symposium in 1978 (Hempel, 1982).

With a mixture of bilateral support and assistance from FAO/UNDP, fishery research centres were established in many coastal countries in the region during the 1960s and 1970s. Other countries, especially France, Spain and the former USSR made important investigations in the region and, in recognition of the potentials of the resources off Northwest Africa, FAO established in 1967 a Committee for the Eastern Central Atlantic Fisheries (CECAF). The work within CECAF has been supported by a series of regional programmes for fisheries development and management.

Hydrography

The investigations with the DR. FRIDTJOF NANSEN in this highly diversified region were as far as possible organized by ecologically defined sub-regions. The surveyed area (Figure 6.1), covers the coastal shelf and immediately adjacent waters and ranges from Safi, in Morocco, to Ghana (from about 32°N to 5°N) a distance of some 2,500 nmi.

Strong seasonal variability and a sharp contrast between the waters in the north and the south are the main characteristics of the Canary Current upwelling system. The upwelling generated by this east boundary current reaches its southernmost extent off Guinea, at about 10°N, in winter when trade winds are strongest, while during summer it is restricted to the region north of Cape Blanc (near Nouadhibou, 21°N), and then a northward flow dominates the surface and sub-surface layers south of Cape Blanc.

Figure 6.1 Map of coast from 32°N in Morocco to Ghana showing the shelf area and names of geographical sites used in this report

The seasonal shift in upwelling results in a coastal surface temperature front in the southern part of the system between cold, upwelled water to the north and warm tropical water to the south. This front is found near Cape Blanc (21°N) in August-September and south of Sherbro Island (6°N) in March, a seasonal shift of some 15 degrees or 900 nmi.

Speth and Detlefsen (1982) studied the seasonality of upwelling off Northwest Africa from an analysis of zonal temperature differences: coastal averages minus mid-ocean temperature. Figure 6.2 shows the mean differences by latitudes for the period 1969–1976. From this study three different upwelling regions can be distinguished:

1. between 20°N and 25°N upwelling exists throughout the year with peaks of activity in spring and autumn;

2. from 25°N and northwards past 30°N upwelling prevails during summer and autumn with a peak at 30°N;

3. from 20°N and southwards to 10°N upwelling takes place during late winter and spring.

Another type of upwelling is found along the coasts of Côte d'Ivoire and Ghana. It is not locally wind-driven, but the sea-surface temperature is seen to drop regularly by several degrees for 14-day periods during the northern summer. This is coupled with reversals of currents on the shelf, periodic lifting of the thermocline and advection of nutrient-rich water towards the coast. At the origin of this event are believed to be offshore depressions and bulges of the thermocline transplanted from the west eastward along the equator (Tomczak and Godfrey, 1994).

These characteristics of the environment must be expected to affect the composition and abundance of the resources. The conditions for the highest sustained biological production should be expected in the area from Cape Juby (28°N) to Cape Blanc (21°N).

Figure 6.2 Seasonality of upwelling off Northwest Africa. Mean temperature difference between coastal areas and mid-ocean (K) for the period 1969–1976. Negative values indicate coastal temperatures colder than mid-ocean.

Source: Speth and Detlefsen (1982) Figure 14

Cape Blanc marks the division of the region into a northern, temperate regime and a southern, sub-tropical regime, thus creating two main sub-regions for assemblages of small pelagics, to the north mainly temperate species: European sardine (Sardina pilchardus), Atlantic horse mackerel (Trachurus trachurus) and chub mackerel (Scomber japonicus), and to the south mainly tropical species: round sardinella (Sardinella aurita), flat sardinella (S. maderensis), Cunene horse mackerel (Trachurus trecae), yellow scad (Decapterus rhonchus), other Carangidae as well as triggerfish (Balistes capriscus).

Survey objectives and operational data

The objectives of the assignments in this region were:

1. to provide information on the state of the main exploited resources, especially the abundant stocks of small pelagic fish;

2. to support the development of ongoing research, in particular to contribute to improved methodologies for acoustic surveys.

The time series generated was, however, discontinuous and with large differences in coverage of the various sub-regions. A review of all survey activities is presented by sub-regions in Table 6.1, while further operational details are given in seperate tables for each sub-region.

Table 6.1 Overview of months of coverage (1–12) of DR. FRIDTJOF NANSEN surveys off Northwest Africa by subregions, 1981–92 and of instruments used

* Incomplete coverage, outside 12 nmi only

** The surveys of the Cape Verde islands have not been reviewed in this report.

Sub-region 1 has been divided into 4 parts:

1a Cape Safi-Cape Ghir (Agadir) (32° -30°30'N)
1b Cape Ghir-Cape Juby (30°30'-28°N)
1c Cape Juby-Cape Bojador (28° -26°N)
1d Cape Bojador-Cape Blanc (26° -21°N)

Division 1d, between Cape Bojador and Cape Blanc could not be covered adequately in the first surveys. For security reasons the vessel only covered the area outside the 12 nmi zone, and only with acoustic instruments.

In later years survey effort has been concentrated on pelagic stocks in the northern part, while sub-areas 3 (Guinea-Sierra Leone) and 4 (Côte d'Ivoire-Ghana) have received less attention. Sub-area 5 (Togo to Congo) and Sub-area 6 the Cape Verde Islands were surveyed only once and therefore have not been discussed in this report.

The main objective of all surveys was to describe the distribution, composition and abundance of the small pelagic fish based on the acoustic integration technique combined with trawl sampling for identification. The acoustic equipment used during each survey has also been indicated in Table 6.1.

The assessment of demersal fish, by bottom trawl surveys, was an additional objective in some areas from Senegal to Guinea Bissau in 1992, off the Gambia in 1986, on the shelf Bissagos Archipelago-Sherbro Island in 1986 and the Cape Verde Islands in 1981.

The surveys were briefly described in cruise reports, e.g. IMR, 1982, 1986g, 1986h, 1987a, 1987b, followed by summary reports:

Cruise reports of surveys made in 1989 and 1992 were more extensive, but real summary reports have not been produced. The relevant reports are:

The present review is limited to a brief description of the main findings by sub-regions and stocks of pelagic fish. These findings will be considered in the light of other information on the stocks and data from the fisheries in the hope that this may give insight into the distribution and fluctuations of the stocks in the region and their state of exploitation in the survey period. Due to the discontinuous nature of the programme, however, the resulting stock histories are far from complete.

6.1 SUB-REGION 1: CAPE SAFI TO CAPE BLANC (32°-21°N)

Survey effort

The operational data for the surveys in this sub-region are given in Table 6.2. The number of days' work gives a rough impression of the effort used. The degree of coverage (d) of the acoustic surveys was gradually increased in the northern area from 13 in 1982 and 14 in 1986, 17 in 1989, to 29 in 1992. Furthermore, in the later surveys the inner shelf, 10–50 m depth, the main area of distribution of sardine and anchovy, was more intensively covered than the outer part of the shelf (50–200 m) (Figure 6.3). Sampling for identification may have been inadequate in the first surveys and therefore the most reliable information is considered to be that based on the 1986, 1989 and 1992 surveys.

Figure 6.3 Course tracks and stations in divisions 1a and 1b, January 1992, showing higher survey density on inner shelf

Table 6.2 Operational data for surveys in sub-region 1

The three first surveys in the southern part, from Cape Bojador to Cape Blanc, were for security reasons only of a reconnaissance nature with a low degree of coverage (d) of about 5 and without sampling, while d was 18 in 1989 and about 15 in 1992, which seems adequate.

Most of the surveys were in the autumn or winter, while the late spring-summer season (May-August) was not covered. There is thus little opportunity for analysing possible seasonal changes in distribution, but there should be a good basis for inter-annual comparisons of the findings. The main pelagic stocks in this sub-region are sardine (Sardina pilchardus), Atlantic horse mackerel (Trachurus trachurus) and chub mackerel (Scomber japonicus).

Sardine

Distribution

Figure 6.4 shows the distribution of the acoustic recordings identified as sardine on the northern part of the shelf in November 1986, September 1989 and January 1992 respectively. The shelf from Cape Safi to Cape Ghir (Agadir) was not covered in 1986. The main features are similar in the three sets of data: the sardine is found on the inner part of the shelf, the main part within 20 nmi from the shore and with the highest densities between Cape Dra (28°44'N) and Cape Juby (28°N). The March-April 1982 survey showed a similar inshore distribution of small pelagic fish.

Figure 6.5 shows the distributions of sardine observed from Cape Juby southwards to Cape Blanc in the 1986, 1989, and 1992 surveys. On the broader shelf south of Cape Bojador the sardine is found further out, 40–60 nmi from the shore. In 1986 and 1992 the shelf south of Cape Blanc was also surveyed and the distribution of sardine was found to extend somewhat south of Cape Blanc, but not beyond 20°N. The Banc d'Arguin seems to form the southern limit for main sardine aggregations although the species may appear in low densities further south in Mauritania and even in Senegal.

With the low position in the food chain of Clupeidae and Engraulidae, their distribution in upwelling regions must be expected to be closely related to centres of active upwelling. Thus Thorne et al. (1977) found that the distribution of sardine between Cape Bojador and Cape Blanc was coincident with maximum abundance of small zooplankton and phytoplankton while that of horse mackerel was associated with a high abundance of large-sized zooplankton.

Figure 6.4 Distribution of sardine in divisions 1a and 1b (see Fig. 6.1) in 1986, 1989 and 1992 from echo integration data. Isotherms are shown for 1986

Although surface temperature observations during the survey period may have been affected by short-term changes in wind, the approximate positions of the prevailing upwelling cells are often disclosed by these observations. A comparison of sardine distribution during the various surveys and that of the simultaneous observations of surface temperatures, demonstrated as an example by the isotherms plotted in Figures 6.4, shows a close correspondence between the centres of cold surface water and fish aggregations. The aggregations south of Cape Dra (28°44'N) may thus be related to the area of local cold water in that region and the wide distribution of sardine on the shelf south of Cape Bojador corresponded with the extensive cold water masses found here in all surveys. The latter area lies within the latitudes 20° and 25°N where the most consistent upwelling of the Canary Current system is found.

Figure 6.5 Distribution of sardine in divisions 1c and 1d (see Fig. 6.1) in 1986, 1989 and 1992 from echo-integration data

Biomass estimates

The details of the acoustic instruments and their operation are described in Chapter 2. The introduction of the EK500 in 1992 may to some extent have affected the comparability of the time series of stock estimates which the surveys provide. The possibility of underestimation by saturation in the previously used EK400 system existed especially where dense schools occurred close to the surface, which is a characteristic behaviour for sardine. In that case high values would be suppressed. This type of bias would be excluded by the wide dynamic range of the EK500 system. A comparison of the frequency distributions of integrator readings from the two systems may indicate whether or not saturation has occurred. Table 6.3 shows the frequencies by levels of integrator values from the 1986, 1989 and 1992 surveys. These represent the main areas of sardine distribution between Cape Bojador and Cape Blanc covering in each case about 90% of the total biomass. There is clearly a greater proportion of observations higher than 500 in the 1992 survey. This could be an effect of a change in the behaviour of the sardine to more densely packed aggregations in 1992 than in previous years, but the difference could also be explained by saturation in the EK400 at high signal levels. This analysis does not allow a direct quantification of a possible bias, but it would seem not to be of very great significance.

Table 6.3 Sub-region 1, Division 1d (Cape Bojador-Cape Blanc) 1986, 1989, 1992. Frequency distributions of integrator readings of the “pelagic I group”, which includes sardine and other pelagics in 1986 and 1989 and sardines only 1992, and the percentage with levels above 500. Unit 0.1x m²/nmi²

The biomass estimates of sardine derived from the three surveys are shown by divisions in Table 6.4. As discussed in Chapter 2 and (IMR, 1992a to d) adjustments were made to the 1986 and 1989 data to correct for different assumptions regarding the condition factor of the fish.

There may be some reservations regarding the estimates from the 1982 surveys. For example, the 750,000 t found between Cape Ghir and Cape Juby represent all pelagic fish, about half of which was assessed to be horse mackerel and mackerel, a much higher proportion of the total biomass than found in subsequent surveys. Also, for security reasons no fishing was done between Cape Bojador and Cape Blanc and furthermore, the coverage there has been incomplete.

The sardine between Cape Safi and Cape Bojador (divisions 1a, b and c) is assumed to constitute the so-called “central stock” which is separated from the “southern stock” found south of Cape Bojador (division 1d) (Shotton, 1984). These two stocks migrate and the situation with regard to their intermixing is apparently not clear.

Sardine found in the Mauritanian part of sub-region 2 (south of Cape Blanc, 21°N) must form part of the “southern stock”. The estimates for this part of the stock were about 500,000 t in 1986 and 20,000 t in 1992. In 1989 sub-region 2 was not surveyed, but observations off the Cape Blanc peninsula in that year indicated a continuation of the distribution of sardine further south.

When considering the time-series, the possibility of an underestimate in the 1986 and 1989 surveys caused by saturation in the EK400 system should be kept in mind. The biomass estimates have been grouped by stock in Table 6.5. The most notable feature is the low estimate of the central stock in 1992. Kifani and Gohin (1992) found that trends in the availability and distribution of this central stock could be related to changes in coastal temperatures and upwelling indices during the period from early 1960s to the late 1980s. Data are not available to determine if unfavourable environmental conditions may have caused the low stock in 1992.

Table 6.4 Sub-region 1: Biomass estimates of sardine by surveys and divisions (1,000 t)

Table 6.5 Biomass estimates of sardine by stock in 1986, 1989 and 1992 (1,000 t)

The estimates for the central stock vary more than those for the southern stock between Cape Bojador coast and Cape Blanc where upwelling occurs on a more stable year-round basis. It seems, however, that the central stock also has been more intensively fished. The combined reported catches by Spain and Morocco from FAO Statistical Area 34 which will mainly represent the yield from this stock increased from about 250,000 t in 1984 to about 450 000 t in the late 1980s, with a mean of 400,000 t in the period 1986–92, a considerable catch compared to a mean estimated biomass of 1.1 million t. For the southern stock, mean catches of 400,000 t in 1986–92 represented a much smaller part of the estimated standing biomass of over 4 million t, and therefore the rate of exploitation must have been relatively low for this stock.

Chub and horse mackerels

Distribution

Contributions to the integrator readings from Atlantic horse mackerel (Trachurus trachurus) and chub mackerel (Scomber japonicus) were estimated from their proportions in the trawl samples and from echo trace characteristics. In the 1989 survey no distinction was made between these two species in the integrator contributions. They were generally found in low densities, less than 10% of that of the sardine. Both species were, in contrast to the sardine, found to be distributed over wider and more offshore parts of the shelf and there were only a few cases of aggregations of high density. One such case was that of Atlantic horse mackerel found aggregated off the peninsula of Cape Blanc in 1989. Dense schools of adult fish near the shelf edge off Cape Blanc seem to be characteristic of the distribution pattern of this species, but the surveys were not designed to specifically cover these aggregations (IMR, 1989e).

According to Shotton (1984) the Atlantic horse mackerel moves outside the shelf in the hot season and may be found in the open sea. The horse mackerel of the Chile-Peru system has a substantial oceanic component, while DR. FRIDTJOF NANSEN surveys in Namibia and South Angola showed that the Cape horse mackerel could at times be found up to 5–10 nmi outside the shelf edge. The observation of the 1989 survey indicates that also the Atlantic horse mackerel may have a tendency to be distributed near the slope or even off the shelf.

The fishing areas of Trachurus spp. in the Eastern Central Atlantic include statistical division 34.2.0, the Northern Oceanic Division (FAO/CECAF, 1994) from which landings close to 100 000 t were reported by the USSR in 1982 and 1983. This indicates that the horse mackerel in the northern part of the CECAF area may have an oceanic component and that therefore the biomass estimates of the Atlantic horse mackerel obtained from surveys limited to the shelf area may be too low.

Biomass estimates

Table 6.6 shows the combined estimates of these two species by surveys. Separate estimates were only attempted in 1986 and 1992 when the proportions chub mackerel/horse mackerel were 17 to 10 and 19 to 50 respectively. The 1992 estimates were very low in all sub-areas.

The reported total catches of horse mackerels in FAO statistical area 34.1.3, Sahara coastal and chub mackerel in FAO statistical area 34 (FAO/CECAF, 1994) in the period of the survey are presented in Table 6.7.

Table 6.6 Biomass estimates of chub and horse mackerel by surveys and sub-areas. (1000 t)

Table 6.7 Reported total catches of horse mackerels in FAO statistical area 34.1.3, and chub mackerel in area 34 from 1982 to 1991 (in 1,000 t)

It is assumed that the main part of the chub mackerel was caught in the survey area. The combined landings of the two species represented 77% and 53% of the biomass estimates in 1986 and 1989 respectively. In general, the biomass estimates seem too low for the level of the landings. This indicates that the surveys have underestimated one or both stocks. For the Atlantic horse mackerel lack of detailed coverage of fish aggregations on the shelf edge and over the slope off Cape Blanc, as reported to have occurred in 1989, and a possible oceanic distribution may have caused an underestimate.

On the other hand, the low landings in 1990 and 1991 could indicate the start of declining stocks as demonstrated by the low 1992 survey estimate (Table 6.6).

Review of biomass estimates and densities

The mean estimates for the surveys 1986, 1989 and 1992 which are thought to have given more reliable and complete data than the 1981/82 surveys, are shown in Table 6.8 by the divisions Cape Safi to Cape Bojador and Cape Bojador to Cape Blanc. The density estimates will also be used as indices of fish productivity of the upwelling system and for this purpose it is thought better to base them on a combination of catches and biomass rather than on biomass alone.

Table 6.8 Sub-region 1: Cape Safi-Cape Blanc. Mean estimates of standing biomass and corresponding of small pelagic fish from surveys in 1986, 1989 and 1992, mean annual reported (1,000 t). Densities by shelf area and coastline

Division 1d, Cape Bojador to Cape Blanc, where upwelling is reported to be most persistent had by far the highest density with 295 t/nmi², nearly double that of the northern divisions (1a-1c), Cape Safi to Cape Bojador with 160 t/nmi². Measured in terms of coastline the density in the southern region with 12,500 t/nmi was more than three times higher than that between Cape Safi and Cape Bojador. These densities are high compared with those estimated for small pelagic fish by DR. FRIDTJOF NANSEN surveys in other upwelling systems, for example: Somalia 70 t/nmi², Oman 120 t/nmi² and northern Namibia 171 t/nmi² and 5,080 t/nmi coastline.

The final conclusion is that the area between Cape Bojador and Cape Blanc (division 1d) still has a considerable potential for the development of fisheries for small pelagics.

6.2 SUB-REGION 2: CAPE BLANC TO BISSAGOS ARCHIPELAGO (21°N-11°N)

Survey effort

The operational data for the surveys in this sub-region are reviewed in Table 6.9. Three surveys I/81, II/82 and V/92 took place in spring when the coastal surface temperature front was located in the south and upwelling is taking place along most of the coast. The other three surveys in the in the south and upwelling is taking place along most of the coast. The other three surveys in the autumn, September-December, represented the season when the front was in northern Mauritania and warm surface water of 25°–30°C extended along the coast southwards.

Table 6.9 Sub-region 2: Operational data for the surveys (21°N-11°N)

Figure 6.6 shows the survey area with the course tracks of the 1992 survey (VI). This survey had the highest effort with a degree of coverage of 29, but the survey effort was also high in the other surveys as shown by the number of days of work. Degrees of coverage for these varied from 20 to 26. Some swept-area trawl programmes were included in the 1986 and 1992 surveys.

General distribution of pelagic fish

Although the coverage of the surveyed area was good, there are important inshore shallow areas in this sub-region which could not be surveyed. These shallow water areas would add approximately the following proportions to the surveyed parts of shelf: Mauritania 50%, Senegal-The Gambia 27%, and Guinea Bissau 70% (Strømme, 1984e). The validity of the observations of the surveys regarding the shallow water fauna is thus limited. For the small pelagic fish this is thought especially to have affected the juvenile stages of most species and the adult stages of anchovy (Engraulis spp.) and to some extent flat sardinella (S. maderensis) and bumper (Chloroscombrus chrysurus). Adult horse mackerels (Trachurus spp.) and scads (Decapterus spp.) were usually found on the outer shelf, and were therefore less affected.

A characteristic of the distribution of small pelagic species which affects survey and sampling techniques is that where they occur in relatively high abundance they are usually found in school areas, aggregations of higher densities of schools over a smaller part of the shelf. This type of distribution was found to be pronounced on the Mauritania to Guinea-Bissau shelf. The school areas would extend from 10 to 30 nmi in the alongshore direction and from 5 to 10 nmi across the shelf. The composition within the school areas tended to be by species, but a mixture of species also occurred, especially inshore. The 1992 survey indicated that for the sardinellas the size of the individual school areas increases with increasing stock size. It was also noted that the density within the schools was very high for adult sardinella, higher than for most other species. There was thus a higher probability of saturation in the EK400 system when estimating the biomass of sardinellas and therefore a higher possibility of underestimating the biomass of sardinellas than for the other species.

Figure 6.7 shows as an example the distribution of sardinellas found in February-March 1992.

Figure 6.6 Survey tracks and stations in subregion 2, February-March 1992 survey

Figure 6.7 Distribution of sardinellas in subregion 2, February-March 1992 survey

The fauna of small pelagic fish in this region is well known. For the stocks of sardinellas, the round sardinella (Sardinella aurita) and the flat sardinella (Sardinella maderensis), the region is a main distributional area although these species may also be found north of Cape Blanc and southwards on the shelf off Guinea and Sierra Leone.

Cape Timiris (19°23'N) in Mauritania represents the southern boundary for the European horse mackerel (Trachurus trachurus), and the northern boundary for the Cunene horse mackerel (Trachurus trecae). The false scad (Decapterus rhonchus) has a distribution similar to that of the Cunene horse mackerel, while the round scad (Decapterus punctatus) is less abundant and is found further inshore in the southern part of the sub-region. Among the other Carangidae the bumper (Chloroscombrus chrysurus) is common in the central part of the subregion, sometimes mixed with lookdown (Selene dorsalis).

The triggerfish (Balistes capriscus) appeared in the trawl catches as far north as Mauritania in the early surveys. Its main distributional area was, however, much further south from Guinea-Bissau to Sierra Leone.

The drastic environmental changes in this sub-region, with cold, temperate water and upwelling extending southwards past Guinea-Bissau in early spring and tropical water reaching up past Cape Verde (Dakar, 14°40'N) in the autumn, must be expected to cause seasonal changes in fish distribution. Migrations related to the movement of the temperature front are described for the main species by Garcia (1982).

Evidence from the DR. FRIDTJOF NANSEN surveys, which represent a good coverage of the various seasons, indicated that the alongshore seasonal shifts in distribution, although clearly present, had perhaps more characteristics of displacements within a limited range along the coast than of long-distance migrations.

Sardinellas

Distribution

Important parts of the total biomass of sardinellas were still retained between Cape Verde (14°40'N) and Cape Roxo (12°20'N), in September 1981 more than 50% and, in November-December 1986 about 40% (Strømme, 1984e and IMR, 1987a and b). Table 6.10 shows the distribution of the total biomass of sardinellas by countries. There is a clear shift in the distribution towards Guinea-Bissau in spring, but otherwise there were no clear seasonal trends.

The surveys also provided information on the relative abundance of the two sardinella species, round sardinella (S. aurita) and flat sardinella (S. maderensis). For the 1981–82 surveys the occurrence of each species in all trawl stations was weighted with a factor proportional to the size of the catch thus giving a mean abundance index for each survey and sub-area (Table 9 in Strømme, 1984e). The averages of these indices for the whole survey period were very similar for the two species, 49% for the flat sardinella and 51% for the round sardinella, expressed as proportions of the total abundance. The analysis of the 178 fishing stations in the 1986 surveys showed a similar incidence for the two species, 26% for flat and 23% for round sardinella, but with higher mean catch rates for the flat sardinella (about 75%) (Strømme and Sætersdal, 1987a). A higher rate of sampling in the 1992 surveys allowed an estimation of the biomass of the two species separately by sub-areas. Of a total biomass of about 4 million t 58% was estimated to be round sardinella. This species had its highest concentration of biomass in Mauritania, while the flat sardinella was most abundant in Senegal-The Gambia. The accuracy of the method of allocation of biomass between the two species is, however, not thought to be very high and it may be concluded that the survey data indicate that the two species occurred at roughly the same level of abundance in the subregion at the time of the surveys. Since there were considerable variations in the total abundance of both sardinellas over the period of the surveys this indicates that the fluctuations in stock size of the two species must have been largely similar.

Table 6.10 Sub-region 2: Biomass estimates of sardinellas by surveys and divisions, 1981–92

Biomass estimates

Table 6.10 shows the combined biomass estimates of the two sardinella species by surveys and sub-areas. The highly variable total estimates from the 1981–82 surveys were probably caused by inadequate coverage of the shallow inshore parts of the shelf. An interpretation of the survey findings could be that they show stock levels of about 500,000 t in 1981–82, 700,000 t in 1986 and 4 million t in 1992.

These variable stock levels are not in disagreement with the reported landings from the period. From the 1993 meeting of the sardinella Working Group of CECAF (Figs. 20 and 21 in Do Chi, 1994), it appears that the reported landings of both sardinella species were low in the early 1980s, with a total of about 150,000 t, well over 200,000 t in 1986 and exceeding 300,000 t in the early 1990s. Reviewing the findings of various surveys as well as other information, the Working Group found that the biomass had been increasing after 1987. However, the Working Group thought that the 4 million t estimate of the DR. FRIDTJOF NANSEN 1992 survey would need confirmation as it very much exceeded all other estimates of these stocks.

The sharp increase in the acoustic biomass estimate in 1992 may, as discussed above for sardine, to some extent be an effect of the increased dynamic range of the EK500 system that was used in this survey. Sardinella schools are often densely packed and would be susceptible to the effects of saturation in the EKS and EK400 systems. However, there is little doubt that there has been a real increase in the biomass. As shown in Table 6.11, the 1992 estimate of the stocks of Carangidae does not deviate markedly from those of previous surveys, a fact which indicates a consistency in the measurements and therefore a confirmation of the veracity of the sardinella estimate.

Horse mackerels

The Cunene horse mackerel (T. trecae) was mainly found between Cape Blanc (21°N) and Cape Roxo (12°20'N). The distribution of the total biomass of Carangidae along the coast (Table 6.11 indicated a northward shift with the temperature front. In the autumn surveys of 1981 and 1986 most of the stock was located north of Cape Verde (14°40'N). Figure 6.8 shows an example of the spring distribution from February to March 1992.

The Atlantic horse mackerel (T. trachurus) was largely limited to the shelf north of Cape Timeris (19°23'N) and as an average for all surveys only contributed about 20% to the total biomass of horse mackerels in the sub-region.

Figure 6.8 Distribution of Cunene horse mackerel in subregion 2 as shown by the February-March 1992 survey

False scad

The false scad (Decapterus rhonchus) was reported to have about the same distribution as the Cunene horse mackerel in the 1981–82 surveys, but a more southerly one in August-September 1986, when it was found in highest abundance from The Gambia south past the Bissagos Archipelago. In the 1992 survey it was found in highest abundance off Casamance, the southern part of Senegal.

Biomass estimates

Table 6.11 shows the estimates of the total biomass of the Carangidae for the surveys with good continuous coverages of the shelf. The group totals vary only between 850,000 and 980,000 t, with Cunene horse mackerel being the largest and most stable component as shown in Table 6.12. The total reported landings of horse mackerels from the corresponding statistical area 34.3.1 were 100,000–120,000 t in the survey period with no distinct trends (FAO/CECAF, 1994). This appears not to be inconsistent with the biomass estimates. The group “other pelagics” includes bumper, lookdown, other Carangidae, barracudas, Spanish mackerels and others. This group had a markedly reduced abundance in 1992.

Table 6.11 Sub-region 2: Biomass estimates of Carangidae by surveys and divisions

Table 6.12 Sub-region 2: Biomass of some Carangidae species and of a mixture of other pelagics (1,000 t)

Review of biomass estimates and densities

Table 6.13 shows the simple means of the biomass estimates from each of the surveys, rough averages of the annual catches in the period and estimates of densities by unit shelf area and coastline. As expected the densities were much lower than in the areas of high upwelling further north, between a quarter and a third of the densities found in sub-region 1d between Cape Bojador and Cape Blanc. Measured by unit of coastline the density from Cape Blanc to Bissagos was about the same as that found between Cape Safi and Cape Bojador. By unit shelf area the density in sub-region 2 was only about half of that found on the narrower northern shelf in sub-region 1.

Table 6.13 Sub-region 2, 21°N-11°N. Mean estimates of standing biomass of small pelagic fish from surveys in 1981, 1982, 1986 and 1992 and corresponding mean annual reported landings (1000 t). Densities by shelf area (t/nmi²) and coastline (with and without landings)

6.3 SUB-REGION 3: CAPE VERGA TO CAPE MOUNT (10°N-7°N)

Survey effort

Of the three surveys in this sub-region (Table 6.14) only that in February 1982 covered the southern upwelling, which takes place during late winter and spring.

Table 6.14 Sub-region 3: Operational data for the surveys off Guinea-Sierra Leone (10°N-7°N)

Figure 6.9 shows the course tracks and stations in the 1986 survey.

Figure 6.9 Survey tracks and stations on the shelf of sub-region 3, off Guinea-Sierra Leone, August 1986

All species

Distribution

The triggerfish (Balistes capriscus) was the dominating species in the pelagic regime in this region. The distribution and abundance of this species will be described in Section 6.5. Small pelagic fish were found in more scattered distributions and at lower densities over this shelf than further north. Aggregations of sardinellas were found near Sherbro Island in each of the surveys. The round scad (Decapterus punctatus) was the main species of the Carangidae.

Biomass estimates

The biomass estimates of the small pelagic fish (Table 6.15) were low. The biomass of sardinellas, found mainly near Sherbro Island, was judged to be underestimated due to their occurrence in shallow waters which could not be surveyed. The absence of Cunene horse mackerel (T. trecae) on the outer shelf may be related to the relatively high abundance of triggerfish here. Presumably these two species would be food competitors.

Table 6.15 Sub-region 3: Biomass estimates of small pelagics by surveys and divisions (1,000 t)

6.4 SUB-REGION 4: CAPE PALMAS TO CAPE SAINT PAUL (7°45'W-1°E)

Survey effort

Only two surveys were made in sub-region 4 (Table 6.16), one in June 1981 at the start of the local upwelling season and the other in October 1989 at the end. Figure 6.10 shows the course tracks and stations in the 1981 survey. The shelf off Côte d'Ivoire is narrow (10–20 nmi) while that off Ghana is up to 60 nmi wide at certain points. The degree of coverage was somewhat less in 1989 than in 1981.

Table 6.16 Sub-region 4: Operational data for the surveys off Côte d'Ivoire and Ghana

Figure 6.10 Survey tracks and stations on the shelf off Côte d'Ivoire-Ghana in June 1981

Pelagic fish

In the 1981 survey well over 80% of the biomass as estimated by the acoustic method was allocated to triggerfish (see Section 6.5) and since this species was found together with the small pelagic fish, the precision of the estimates of small pelagics was low.

The small pelagics tended to occur in school areas with the Clupeidae predominantly concentrated on the inner shelf. The estimates of sardinellas (S. aurita and S. maderensis) include some anchovy (Engraulis encrasicholus).

Table 6.17 shows the biomass estimates by species or groups of species of small pelagics and includes also estimates from the August 1986 survey by the Spanish research vessel CORNIDE DE SAAVEDRA as reported by Oliver and Miquel (1987). The acoustic system of this vessel was inter-calibrated with that of DR. FRIDTJOF NANSEN off Senegal in August-September 1986 (Anon., 1986).

Table 6.17 Sub-region 4: (Côte d'Ivoire-Ghana). Biomass estimates by species groups and divisions from DR. FRIDTJOF NANSEN surveys in 1981 and 1989 and a survey by the CORNIDE DE SAAVEDRA in 1986 (1,000 t)

These estimates of the standing biomass of sardinellas are low compared with reported landings by the two countries of 47,000 t in 1981 and about 100,000 t in 1986 and 1989. The discrepancy becomes even bigger if Ghana's considerable landings of anchovy are included in the comparison. A possible explanation is that the surveys did not cover inshore aggregations of these species.

The horse mackerel (Trachurus trecae) which with an estimated biomass of 72,000 t in 1989 was the most abundant species of small pelagics in that survey, was not present in the two previous surveys. The mean catch rates of the Carangidae species in the 1981 and 1989 surveys are not comparable because the sampling in 1981 was mainly based on pelagic trawling, while bottom trawl was the main gear used in 1989. The relative proportions of the main Carangidae species (Table 6.18) show, however, the dominance of the Cunene horse mackerel (T. trecae) which occurred in 12 of the 24 hauls in 1989 including pelagic trawls, but in none of the 34 hauls in 1981. Ghana's reported landings of Trachurus spp. increased from a few hundred tonnes per year during 1981–89 to 3,000 t in 1990 and down again to 1,800 t in 1991 (FAO/CECAF, 1994). The increase of the horse mackerel stock took place after the decline of the stock of triggerfish of which none were found in 1989.

Table 6.18 Sub-region 4: Composition (%) of Carangidae in sampling hauls in 1981 and 1989 surveys

The group “other pelagics” included scads, chub mackerel, Spanish mackerels and barracudas. The reported total landings of these fish by Côte d'Ivoire and Ghana in 1989 amounted to 38,000 t and these species were thus severely underestimated in the surveys.

6.5 TRIGGERFISH

The occurrence of the grey triggerfish (Balistes capriscus) in high abundance in the 1970s and 1980s in sub-regions 3 and 4 (Sierra Leone-Guinea Bissau, western stock and Ghana-Côte-d'Ivoire, southern stock) is probably an unusual phenomenon. Most members of the family Balistidae are slow-moving solitary reef-dwellers (Moyle and Cech, 1992) and this seems to be the more normal behaviour and habitat also of the grey triggerfish. However, the stocks in the Eastern Central Atlantic were able to expand their populations to a remarkable and unusual size, probably by utilizing the relatively high productivity of the still tropical regimes of the shelf waters of the Western Gulf of Guinea and the coastal upwelling system south of Senegal. The DR. FRIDTJOF NANSEN surveys in 1981–82 coincided with the culmination of the triggerfish stocks, while those in 1986 and 1989 with their decline and collapse.

The triggerfish was found in layers and schools in mid-water and near the bottom and was a convenient target for acoustic investigations and also easily sampled with mid-water and bottom trawls. Figure 6.11 shows the distribution in June 1981. The two separate areas of distribution seem to have represented sub-stocks with little or no mixing (Strømme, 1984e). In September 1981 the western stock was recorded scattered past Cape Roxo (12°20'N) towards Cape Verde (14°40'N), but in all surveys the highest densities in the west were found between Sherbro Island and the Bissagos Archipelago. A distribution over the outer shelf, with the highest densities in waters with a bottom depth of about 100 m, was a characteristic feature found in all surveys.

Biomass estimates of triggerfish from the relevant DR. FRIDTJOF NANSEN surveys are shown in Table 6.19 together with available data from other similar surveys. The eastern stock was about 0.5 million t in 1981 and had declined to 140,000 t by 1986 and to virtually nil in 1989. For the western stock there seems to have been a rapid growth from about 0.4 million t in 1978/79 (Marchal et al., 1980) to about 1.4 million t in 1982 and a decline to only 0.2 million t in 1986.

The DR. FRIDTJOF NANSEN survey in 1992 was limited to the Guinea-Bissau shelf, which was a main distributional area for the western stock of triggerfish in 1981/82 and 1986. Since no triggerfish was found it seems likely that the western stock had disappeared by that time.

There is less information on the eastern stock in sub-region 4.

Figure 6.11 Distribution of triggerfish from acoustic observations in the June 1981 survey. Source: Strømme, 1984e

Table 6.19 Biomass estimates of triggerfish from acoustic surveys (1,000 t)

According to Ansa-Emmim (1979) triggerfish was practically absent from the continental shelf of Ghana prior to 1969. A sudden increase of the biomass of the species was observed from 1970.

The stocks were fished by Ghana from 1972 on (presumably the eastern stock) and by a distant water fleet from USSR in the west from 1980 to 1987. The mean annual reported landings by four-year periods are given in Table 6.20.

Table 6.20 Landings of triggerfish from the Gulf of Guinea area, 1972–91. Means over four-year periods (t)

The total yield over the period of the fishery was thus well over 0.5 million t, of which the main part was taken from the western area. Since little is known of the population parameters for this type of fish it is difficult to assess which effect the fishery may have had. It does not seem likely, however, that the disappearance of the eastern stock in 1989 can have been caused by the relatively modest catches in this area, and the growth and decline of these stocks is probably a phenomenon of natural ecosystem variability.