8. FINDINGS AND DISCUSSION (contd.)

Species:Pagellus bellottii bellottii
Côte d'Ivoire
Depth: 20–50 m

Species: Pagellus bellottii bellottii
Côte d'Ivoire
Depth: 50–100 m

Species: Pagellus bellottii bellottii
Côte d'Ivoire
Depth: 20–100 m

Species: Pagellus bellottii bellottii
GHANA
Depth: 20–50 m

Species: Pagellus bellottii bellottii
GHANA
Depth: 50–100 m

Species: Pagellus bellottii bellottii
GHANA
Depth: 20–100 m

Fig. 65. P. bellottii. Length distribution for males and females in Western Sierra Leone

Fig. 66. P. bellottii. Length distribution for juveniles in Leone, Stratum A

Fig. 67. P. bellottii. Length distribution for males and females in Liberia, Stratum A

Fig. 68. P. bellottii. Length distribution for males and females in Liberia, Stratum B

Fig. 69. P. bellottii. Length distribution for males and females in Liberia

Fig. 70. P. bellottii. Length distribution for males and females in Côte d'Ivoire, Stratum A

Fig. 71. P. bellottii. Length distribution for males and females in Côte d'Ivoire, Stratum B

Fig. 72. P. bellottii. Length distribution for males and females in Côte d'Ivoire

Fig. 73. P. bellottii. Length distribution for juveniles in Côte d'Ivoire, Stratum A and B

Fig. 74. P. bellottii. Length distribution for males and females in Ghana, Stratum A

Fig. 75. P. bellottii. Length distribution for males and females in Ghana, Stratum B

Fig. 76. P. bellottii. Length distribution for males and females in Ghana

Fig. 77. P. bellottii. Length distribution for juveniles in Ghana, Stratum B

Dentex congoensis

The catch of the sparid Dentex congoensis was good, but three times smaller than that of P. bellottii bellottii. This limited to some extent the number of specimens sampled, an as a result, only total size compositions per country or area were calculated (Figs. 78–80 and Tables 76–78). Although sex sampling was done, it has not yet been possible to prepare histograms for males and females.

Dentex congoensis would appear to occur between stratum B and C in all the countries except Côte d'Ivoire, where it is fished in the 50–100 m depth interval only. The maximum depth at which it was encountered was 155 m, in Ghana.

Size distributions are similar in Western and Eastern Sierra Leone, where lengths range between 11 and 21 cm. Modal lengths are also very similar: 15 and 19 cm in the west and 15 and 18 cm in the east. The larger modal length appears to be more distinct and mean lengths and standard deviations are slightly higher in the east (16.58 ± 2.34) than in the west (16.28 ± 1.8).

Catches of this species were higher in Sierra Leone than in the other countries, and within Sierra Leone itself, almost three times higher in the west than in the east.

The size range of the population taken in Côte d'Ivoire was the same as in Sierra Leone (between 11 and 21 cm) with three modal values; 12, 15 and 19 cm, and the largest number of specimens at the two extremes (12 and 19 cm). Mean size was 15.69 cm and standard deviation 3.33 cm.

The widest size range was in Liberia (5–20 cm). The histogram for this country shows two fairly distinct groups: the first (comprising 60% of the catch) with a clearly defined modal length of 7 cm, and the second showing a number of less well-defined modes and comprising part of the more adult population (Fig. 79).

Thus, the estimated mean size in Liberia is rather low (9.73 cm), while the standard deviation is high (4.25 cm) (see Table 75).

The distribution for Ghana is not very representative, as few specimens were sampled. The larger size classes hardly appear in the histogram, where 98% of the stock falls within three size classes (between 10 and 12 cm (Fig. 80).

Dentex canariensis

This sparid was scarce in the area surveyed. Ghanaian waters provided the most abundant catch (50% of the total). D. canariensis was caught at 20–100 m depth in all the countries except Liberia where specimens were taken only in stratum B. One specimen also appeared at about 300 m depth in Ghana.

In view of the wide variety of sizes among the small number of specimens caught, Ghana was the only country where it was possible to obtain a size frequency distribution representative of the stock surveyed (Table 79 and Fig. 81).

The D. canariensis stock represented in the histogram is characterized by a wide range of sizes, from 8 to 40 cm (one specimen measured 52 cm). Mean length is 23.39 cm, and standard deviation high, 6.68 cm.

Although several size classes may be distinguished, most of the stock is in the 16–30 cm range, with one additional modal size of 14 cm and a group of specimens exceeding 30 cm.

Species: Dentex congoensis
Western Sierra Leone
Depth: 50–200 m

Species: Dentex congoensis
Eastern Sierra Leone
Depth: 50–200 m

Species: Dentex congoensis
LIBERIA
Depth: 50–200 m

Species: Dentex congoensis
Côte d'Ivoire
Depth: 50–100 m

LENGTH DISTRIBUTIONS

Species: Dentex congoensis
Ghana
Depth: 50– >100 m

Fig. 78. D. congoensis. Length distributions in Western and Eastern Sierra Leone, Stratum B and C

Fig. 79. D. congoensis. Length distributions in Liberia and Côte d'Ivoire

Fig. 80. D. congoensis. Length distributions in Ghana, Stratum B and C

Species: Dentex canariensis
Ghana
Depth: 50– >100 m

Fig. 81. D. congoensis. Length distributions in Ghana

Pagrus coeruleostictus

The main characteristics of the stock of Pagrus caeruleostictus caught during the survey were similar to those of Dentex canariensis small catch (90% obtained in Ghanaian waters; no specimen encountered below 100 m depth; wide range of lengths (between 7 and 38 cm); and only a small number of individuals sampled (given the small size of the catch).

As for Dentex canariensis, representative size distribution was possible only for Ghana, as the species occurred in too small numbers in the other countries.

Figure 82 shows two groups, clearly differentiated in terms of length: one, comprising the greater part of the population and several modal lengths, (with the highest 16 cm), and the other comprising more adult individuals (between 20 and 32 cm), but also different modal sizes, the highest percentage of the sample situated around 27 cm.

The mean size of this species in Ghana is 17.41 cm, with a standard deviation of 6.13 cm.

Epinephelus aeneus

This species was sampled in almost all the hauls in which it was caught, i.e., in the first two strata in all the countries.

Its catch by weight was higher than that of either Dentex canariensis or P. caeruleostictus, but since it comprised only 90 specimens, covering a wide range of lengths, it was not possible to obtain a significant histogram.

Lengths ranged from 26 cm, to one specimen measuring 98 cm. Not all length groups were represented, however.

Côte d'Ivoire provided the largest catch, followed by Liberia. The smallest was obtained in Eastern Sierra Leone.

Using data from biological sampling, a length-weight ratio, which gave a good correlation coefficient, was worked out.

Species: Pagrus caeruleostictus
Country: Ghana
Depth: 50->100 m

Fig. 82 Pagrus caeruleostictus. Length distributions in Ghana

Brachydeuterus auritus

This coastal species, a member of the soft bottom sciaenid community, was fished mainly at depths of down to 50 m, except in Côte d'Ivoire, where it also occurred in the 50–100 m depth range, and in Ghana, where a small catch was obtained at 60 m depth.

Size distributions of Brachydeuterus auritus are given in Tables 82–85 and Figs. 83–86. The length distribution in Côte d'Ivoire differs from that of the other countries. The modal size there is somewhat larger (13 cm in the first two strata) and more distinct, representing more than 40 percent of the stock in both depth ranges. There is a small modal value of 9 cm in the 20–50 m depth range, with the result that mean size in stratum A is slightly lower (13.09 cm) than in stratum B (13.27 cm).

Size ranges, on the other hand, vary fairly widely (from 7 to 22 cm in the first depth interval and 12 to 20 in the second (Fig. 85).

In all other strata modal sizes occur below 10 cm total length.

The main mode is 7 cm in Sierra Leone and Ghana, but larger modal lengths appear in Ghana, raising the mean length to 11.81 cm. Another modal size of 10 cm was encountered in Sierra Leone only, lowering the mean length there to 8.59 cm. Size ranges were nevertheless very similar in these two countries: a minimum of 6 and 7 cm in Sierra Leone and Ghana respectively and a maximum 18 cm in both countries.

In the Western Sierra Leone population, there are two modes, 8 and 11 cm and a size range (comprising only 8 classes) between 6 and 13 cm. Mean length is 9.56 cm ± 1.64 cm (Table 81), similar to that obtained in Liberia (9.89 cm ± 1.61 cm), where, within a length range of 7 to 17 cm the modal value is very distinct at 9 cm.

There are two points worth noting as regards this species: the largest catch was obtained in Côte d'Ivoire (in stratum A particularly), which accounted for almost 90% of the total catch of Brachydeuterus auritus for the entire trip. It was confirmed that this species has a wider bathymetric distribution than the other sciaenids (Domain, 1979), for, while it was most abundant in the 20–50 m depth interval, it also occurred down to 100 m depth.

Merluccius polli

Given its great commercial interest for the Spanish fishing fleet, it was one of the target species for which biological parameters had to be collected, even though it is known to be low yielding in the central part of the Gulf of Guinea (Domain, 1979).

In fact, only a very small quantity was caught (less than 25 kg), which meant that sampling was limited and did not provide a statistically valid size distribution.

Specimens of lengths ranging from 29 to 46 cm were caught in all the countries from 250 to below 700 m depth.

The size-weight ratio gave a high r value (although it was worked out on the basis of a small number of individuals).

Species: Brachydeuterus auritus
Western Sierra Leone
Depth: 20–50 m

Species: Brachydeuterus auritus
Eastern Sierra Leone
Depth: 20–50 m

Species: Brachydeuterus auritus
Liberia
Depth: 20–50 m

Species: Brachydeuterus auritus
Côte d'Ivoire
Depth: 20–50 m

Species: Brachydeuterus auritus
Côte d'Ivoire
Depth: 20–50 m

Species: Brachydeuterus auritus
Côte d'Ivoire
Depth: 50–100 m

Species: Brachydeuterus auritus
Ghana
Depth: 20–50 m

Species: Brachydeuterus auritus
Ghana
Depth: 20–100 m

Fig. 86. B. auritus. Length distributions in Côte d'Ivoire

Fig. 85. B. auritus. Length distributions in Côte d'Ivoire, Stratum A and B Stratum B and C

Fig. 83. B. auritus. Length distributions in Western and Eastern Sierra Loene. Stratum A

Fig. 84. B. auritus. Length distributions in Liberia and Ghana. Stratum A

Sepia officinalis hierredda.

This cephalopod was fished mainly in Côte d'Ivoire and Ghana, but smaller catches were also obtained in Western Sierra Leone, always in the first two depth strata (between 20 and 100 m depth).

Males and females were sampled separately.

Tables 86–92 give the length frequency distributions for the species by depth stratum and country, while Figures 87–98 give the corresponding histograms.

Total length distributions by country (Figs. 87–89) show that the size range of the sampled specimens was identical in Côte d'Ivoire and Ghana (between 9 and 26 cm). The fact that the population structures of these countries are similar would suggest that there is only one stock. The most distinct modal length in the distribution for Ghana is 19 cm, but two smaller modal sizes seem to be emerging. It must be noted that it was possible to sample a much larger number of specimens in Ghana, where Sepia officinalis was very abundant, than in the other countries and this undoubtedly influenced the shape of distribution curve.

The length range in the Sierra Leone samples (Fig. 87) varied between 4 and 22 cm, with a distinct modal size of 7 cm and another less well-marked of 19 cm (as in Côte d'Ivoire and Ghana).

Total length distribution by depth stratum (Figs. 90–91) was identical in the 20–50 m depth range in Côte d'Ivoire and Ghana, with the majority of indivi duals concentrated around the 19 cm mode and a small number at 9 and 13 cm. In Stratum B (50–100 m), distribution was normal in Ghana, a modal length of 17 cm and size range of 10–24 cm, but there is no clear distribution pattern in Côte d'Ivoire, due, no doubt, to the small number of specimens sampled.

Length distribution by sex

There was a majority of females in all hauls, except in Ghana, where males slightly outnumbered females in the 50-100 m depth range (Table 91). Female length distributions are therefore more representative than male.

Given the scant number of samples in Sierra Leone, it was not possible to prepare length histograms by sex for each depth stratum. The histograms only cover the first two strata in which the species was encountered (Fig. 92).

The size range was similar for males and females (4–22 cm), with the females showing a distinct mode of 6 cm.

Few large size individuals were observed.

Male length distributions were based on very few specimens and are therefore not representative.

Distributions in the 20–50 m depth stratum in Côte d'Ivoire and Ghana are identical (Figs. 93 and 96). More than 70% of the females in the sample have a distinct mode of 20 cm, but a small group of individuals range between 9 and 13 cm.

The male distributions do not show distinct modes, but the same two length groups are encountered.

Samples were small in the 50–100 m depth range in Côte d'Ivoire, where the individuals fished did not exceed 14 cm in length, but abundant in Ghana. Two very clear modal sizes may be distinguished in the normal female distribution; one, very distinct, of 19 cm, the other, less distinct, of 14 cm. The male distribution pattern would appear to show only one mode (17 cm), but there is probably another larger size at 22 cm.

It is concluded from the length distributions that the best yields and most marketable size specimens of Sepia officinalis are to be found in Ghana.

Species: Sepia officinalis
Western Sierra Leone
Depth: 20–100 m

Species:Sepia officinalis
Côte d'Ivoire
Depth: 20–50 m

Species: Sepia officinalis
Côte d'Ivoire
Depth: 50–100 m

Species: Sepia officinalis
Côte d'Ivoire
Depth: 20–100 m

Species: Sepia officinalis
Ghana
Depth: 20–50 m

Species: Sepia officinalis
Ghana
Depth: 50–100 m

Species: Sepia officinalis
Western Sierra Leone
Depth: 20–100 m

Fig. 87 Sepia officinalis hierredda. Length distributions in Western Sierra Leone

Fig. 88 Sepia officinalis hierredda. Length distribution in Côte d'Ivoire

Fig. 89. Spia officinalis hierredda. Length distributions in Ghana

Fig. 90. Spia officinalis hierredda. Length distributions in Ghana, Stratum A and B

Fig. 91. Spia officinalis hierredda. Length distributions in Côte d'Ivoire, Stratum A and B

Fig. 92. Spia officinalis hierredda. Length distributions for males and females in Western Sierra Leone, Stratum A and B

Fig. 93. Spia officinalis hierredda. Length distributions for males and females in Côte d'Ivoire, Stratum A

Fig. 94. Spia officinalis hierredda. Length distributions for males and females in Côte d'Ivoire, Stratum B

Fig. 95. Spia officinalis hierredda. Length distributions for males and females in Côte d'Ivoire, Stratum A and B

Fig. 96. Spia officinalis hierredda. Length distributions for males and females in Ghana, Stratum A

Fig. 97. Spia officinalis hierredda. Length distributions for males and females in Ghana, Stratum B

Fig. 98. Spia officinalis hierredda. Length distributions for males and females in Ghana, Stratum A and B

Sepia bertheloti

This species is more widely distributed geographically than Sepia officinalis, being fished in all the countries, always in the 20–100 m depth range (it was caught at greater depths only in a few isolated trawls). Both species occurred together in many tows.

The largest catches were obtained in Liberia and Ghana, and the length distributions by depth range given in this report correspond to these two countries (Tables 93–99 and Figs. 99–108). The sexes were sampled separately.

Overall distribution (males and females combined) (Fig. 102–103) is virtually unimodal in Ghana (5 cm), with lengths ranging from 1 to 15 cm. Overall distribution in Liberia shows a smaller size range, a larger sized population (between 7 and 13 cm) and modal sizes of 7 and 10 cm. There would also appear to be a mode of 13 cm.

A by-stratum analysis of distributions by stratum in Liberia (Figs. 104–105) clearly shows that modes increase with increasing depth (from 3 cm in stratum A to 7 cm in stratum B and 10 cm in stratum C). These modal changes do not appear in frequency distributions for Ghana (Fig. 102), where modal size is the same in the two strata sampled (5 cm), even though the size range between 50 and 100 m depth is much wider.

Size distributions by sex

The striking feature in female distributions in Liberia is the gradual increase of the large mode with increasing depth. In the male distributions, modal size increases between stratum B and C. The few specimens sampled in stratum A were not representative of the population structure.

This does not occur in Ghana, where it has merely been shown that the length range of both males and females (but especially females), widens with increasing depth.

Fig. 99. S. bertheloti. Length distributions in Liberia, Stratum A and B

Fig. 100. S. bertheloti. Length distributions in Liberia, Stratum C

Fig. 101. S. bertheloti. Length distributions in Liberia, Stratum C

Fig. 102. S. bertheloti. Length distributions in Ghana, Stratum A and B

Fig. 103. S. bertheloti. Length distributions in Ghana, Stratum A

Fig. 104. S. bertheloti. Length distributions for females and males in Liberia, Stratum A

Fig. 105. S. bertheloti. Length distributions for females and males in Liberia, Stratum B

Fig. 106. S. bertheloti. Length distributions for females and males in Liberia, Stratum C

Fig. 107. S. bertheloti. Length distributions for females and males in Ghana, Stratum A

Fig. 108. S. bertheloti. Length distributions for females and males in Ghana, Stratum B

Parapenaeus longirostris

Because of its importance for the Spanish shellfish fleet, this was one of the target species.

However, as previous survey reports had indicated, the catch of this species for the entire GUINEA 90 expedition was very small (7 kg in all), with only minute quantities being fished in Sierra Leone and Ghana (see Annex III, Yields).

Only in Liberia and Côte d'Ivoire were slightly more representative quantities caught (2.8 kg in the two countries), not surprisingly in the deeper strata (below 200 m).

A few small-size specimens were caught at shallower depths in Sierra Leone and Liberia.

Since the catches were so small, length frequency distributions are not broken down by country, but males and females are treated separately (Table 100, Fig. 109).

The length range of the female population is wider than that of the male (10–40 mm and 11–35 mm respectively), with females showing a higher main mode than males (Sobrino, 1990), in this case 29 cm. There is, however, another mode at 14 cm, corresponding to a smaller group. The male distribution pattern is not as clear and modal sizes cannot be defined as accurately. They nevertheless seem to occur at 16, 19, 23–24 and 32 mm.

Species: Parapenaeus longirostris
Whole of expedition

Fig. 109. Parapenaeus longirostris. Length distributions for females and males for entire area

Fig. 110. Parapenaeus longirostris. Length distributions for area as a whole