Annexe 3. OCCURRENCE OF THE TRIGGER FISH, BALISTES CAPRISCUS (GMEL), ON THE CONTINENTAL SHELF OF GHANA

by
M. Ansa-Emmim
CECAF Project
UNDP, B.P. 154, Dakar, Senegal

1. INTRODUCTION

Before 1969, the trigger fish (Balistes capriscus) was practically absent from the continental shelf of Ghana. A sudden increase in the biomass of this species was observed from 1970. Recorded catches of trigger fish have increased from about 3 000 tons in 1972 to 8 600 tons in 1976. But due to its low market value, it is not possible to estimate the total catches of this species because medium sized trawlers and purse seiners discard some quantities at sea. Increase in the abundance of trigger fish has also been reported in adjacent seas and in the Gulf of Guinea generally. The causes of this sudden increase are not well known. However, this increase coincided with a drastic drop in the catches of Sardinella aurita off Ghana and Ivory Coast. At the Sardinella Working Group held in Abidjan (Ivory Coast) by the Fishery Research Unit of Ghana and the “Centre de recherches océanographiques” of Ivory Coast, to examine the causes of the collapse of S. aurita off Ghana and Ivory Coast, from 28 June to 3 July 1976, a recommendation was made to initiate a study of the biology, ecology and distribution of the trigger fish. Therefore, the objective of this paper is to analyse data collected by the Fishery Research Unit, Tema, Ghana, since 1976.

2. THE FISHERY

B. capriscus is fished mainly by the artisanal as well as the semi-industrial fleet (trawlers and purse seiners) of Ghana, with the bulk of the catches from the small inshore trawlers (8-12 m). The low abundance of S. aurita after 1972 and the consequent processing of this fish for consumption encouraged these small operators, especially those based in Elmina, to land all catches of B. capriscus. Medium-sized trawlers and purse seiners still discard quantities of B. capriscus at sea. Freezer trawlers operating on the continental shelf of Ghana also land some quantities of B. capriscus (212.2 tons and 103.8 tons in 1975 and 1976).

3. CATCHES AND LANDINGS

Tables 1-6 give the monthly landings of B. capriscus by types of vessel for 1972-77. The landings of the purse seiners and medium trawlers do not represent catches of these vessels since most trigger fish are discarded. The landings of the 8-12 m vessels can be said to represent catches of these vessels because there is no discarding at sea.

Table 1

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1972

MONTHS

GEAR

TOTAL

TRAWL
8-12 m

TRAWL
MEDIUM

PURSE SEINE MEDIUM

PURSE SEINE
8-12 m

ARTISANAL

JAN

13.8

7.9

6.7

0.3

25.3

54.0

FEB

125.0

8.4

133.4

MARCH

119.0

21.5

140.5

APRIL

129.7

23.2

2.6

1.0

156.5

MAY

33.3

7.3

0.3

40.9

JUNE

57.6

14.8

2.7

75.1

JULY

36.1

1.8

1.6

39.5

AUG

-

-

-

SEPT

169.7

3.8

1.2

174.7

OCT

640.3

13.9

0.9

655.1

NOV

371.8

9.3

10.7

1.8

393.6

DEC

1 150.3

13.4

4.2

1 167.9

TOTAL

2 846.6

125.3

17.4

7.1

34.8

3 031.2

Table 2

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1973

MONTHS

GEAR

TOTAL

TRAWL
8-12 m

TRAWL
MEDIUM

PURSE SEINE MEDIUM

PURSE SEINE
8-12 m

ARTISANAL

JAN

919.5

12.0

6.9

5.6

944.0

FEB

556.8

7.1

1.8

565.7

MARCH

623.5

7.2

0.9

6.0

637.6

APRIL

348.5

9.1

3.8

2.1

363.5

MAY

292.0

10.0

0.5

6.0

308.5

JUNE

12.1

14.9

0.5

5.4

32.9

JULY

6.7

13.2

2.4

5.3

27.6

AUG

0.9

3.4

7.9

12.2

SEPT

1.8

14.8

16.6

OCT

98.4

49.2

1.5

3.2

152.3

NOV

713.0

30.7

2.0

0.1

745.8

DEC

1 031.7

17.6

1.5

1 050.8

TOTAL

4 604.9

189.2

20.8

11.0

31.6

4 857.5

Table 3

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1974

MONTHS

GEAR

TOTAL

TRAWL
8-12 m

TRAWL
MEDIUM

PURSE SEINE MEDIUM

PURSE SEINE
8-12 m

ARTISANAL

JAN

843.8

32.5

75.1

9.4

960.8

FEB

1 195.7

53.5

0.4

1.4

13.5

1 264.5

MARCH

675.1

61.2

0.1

7.0

743.4

APRIL

505.0

55.6

560.6

MAY

771.6

107.5

879.1

JUNE

24.8

68.3

93.1

JULY

12.8

59.5

0.2

1.7

74.2

AUG

5.7

44.2

0.1

7.6

57.6

SEPT

97.1

73.6

28.8

0.4

5.6

205.5

OCT

421.3

60.5

1.2

50.5

533.5

NOV

596.2

32.6

0.2

34.7

663.7

DEC

1 617.9

15.8

1.7

4.9

1 640.3

TOTAL

6 767.0

664.8

32.7

84.5

127.3

7 676.3

Table 4

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1975

MONTHS

GEAR

TOTAL

TRAWL
8-12 m

TRAWL
MEDIUM

PURSE SEINE MEDIUM

PURSE SEINE
8-12 m

ARTISANAL

JAN

1 633.1

16.9

1.9

3.0

1 654.9

FEB

860.1

15.6

0.1

33.4

909.2

MARCH

459.4

29.9

489.3

APRIL

422.8

42.3

465.1

MAY

245.7

41.7

2.4

289.8

JUNE

76.4

8.1

84.5

JULY

13.6

18.8

0.1

32.5

AUG

10.4

23.2

11.1

17.7

6.0

68.4

SEPT

2.7

5.5

4.7

16.1

29.0

OCT

445.8

35.4

0.2

0.5

481.9

NOV

1 623.8

80.6

0.6

15.3

1 720.3

DEC

2 375.8

20.8

1.0

2 396.6

TOTAL

8 169.5

338.8

19.5

34.5

60.1

8 622.4

Table 5

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1976

MONTHS

GEAR

TOTAL

TRAWL
8-12 m

TRAWL
MEDIUM

PURSE SEINE MEDIUM

PURSE SEINE
8-12 m

ARTISANAL

JAN

1 158.5

25.9

2.8

3.6

70.0

1 260.8

FEB

1 055.8

59.5

0.9

1 116.2

MARCH

861.4

62.1

0.1

15.7

939.3

APRIL

769.1

36.1

1.1

806.3

MAY

27.3

16.2

43.5

JUNE

31.3

10.7

0.6

8.4

51.0

JULY

3.9

2.9

2.8

16.3

25.9

AUG

0.8

4.1

0.6

28.9

34.4

SEPT

60.8

2.5

4.0

6.4

456.7

530.4

OCT

523.3

13.7

2.0

0.6

539.6

NOV

1 208.4

22.5

3.5

0.1

179.5

1 414.0

DEC

2 046.8

13.8

4.4

2 065.0

TOTAL

7 747.4

270.0

22.2

56.5

730.3

8 826.4

Table 6

GHANA: TRIGGER FISH LANDINGS IN TONS BY GEAR IN 1977¹

¹ Excluding catches of artisanal fleet

MONTHS	GEAR					TOTAL
MONTHS	TRAWL 8-12 m	TRAWL MEDIUM	PURSE SEINE MEDIUM	PURSE SEINE 8-12 m	ARTISANAL	TOTAL
JAN	1 226.8	20.8	2.7			1 250.3
FEB	928.1	61.7	3.3			993.1
MARCH	652.3	10.4	14.7			677.4
APRIL	746.5	27.4	15.3	0.1		789.3
MAY	336.9	11.5	1.3			349.7
JUNE	9.8	11.9	10.7	0.4		32.8
JULY	30.0	5.6	14.9	2.4		52.9
AUG	1.4	0.5	2.5	18.6		23.0
SEPT	30.8	1.2	5.4	92.3		129.7
OCT	371.9	11.1		0.3		383.3
NOV	1 276.2	32.7				1 308.9
DEC	1 070.1	20.4	11.5			1 102.0
TOTAL	6 680.8	215.2	82.3	114.1		7 092.4

Although B. capriscus is exploited throughout the year, peak landings are observed from October to May, with low landings between June and September. During the period June to September, however, most of the small trawlers (8-12 m) which contribute to the bulk of B. capriscus landings (88-95 percent), change to purse seining for the exploitation of S. aurita.

3.1 Trends in Landings

Landings of B. capriscus have increased on the whole from the 1972 level of 3 031 tons to 8 826 tons in 1976. Total landings for 1977 were 7 092 tons. Of the total landings the small inshore trawlers (8-12 m) contribute between 88 and 95 percent.

3.2 Dominance of B. capriscus in landings of trawlers

The overall demersal catches of inshore trawlers have since 1974 been dominated by B. capriscus. In 1972, 27 percent of the total landings of all inshore trawlers was made up of B. capriscus. In 1973 and 1974, this percentage went up to 43 and 53 respectively. B. capriscus constituted about 89 percent of the catches of the small inshore trawlers (8-12 m) during 1975 and 1976. The dominance of B. capriscus on the shelf has also been confirmed by the Fiolent Survey of 1976 (Table 7). Table 7 shows the percentage representation by weight of selected fish species as they appear in trawls. To the east of Cape Three Points, and between 20 and 50 m depth, B. capriscus accounts for 65.6 percent. The next important species is Brachydeuterus auritus (31 percent). On the Eastern shelf off Accra (including Togo), this species accounts for 46.3 percent of the total weight, followed again by B. auritus.

Table 7

FIOLENT SURVEY: RELATIVE (%) IMPORTANCE (BY WEIGHT) OF SELECTED FISH IN TRAWL CATCHES, GHANA (NOVEMBER-DECEMBER 1976); TOGO (MAY 1976)

Depth range (m)

West of Cape

Three Points

East of Cape

Three Points

Eastern shelf off Accra

20-50

50-100

20-50

50-104

20-50

50-100

20-50

Dentex angolensis

9.1

12.7

2.8

Dentex congoensis

+

26.6

39.6

1.2

21.3

Dentex canariensis

1.2

+

+

2.1

8.5

Pagellus coupei

52.7

15.3

+

+

2.2

57.2

2.4

Pagrus ehrenbergi

3.0

+

+

1.3

4.2

Balistes sp.

8.2

+

65.6

1.1

46.3

67.3

Paracubiceps ledonoisi

+

14.3

33.8

Sardinella eba

+

1.2

+

1.2

Sardinella aurita

+

Brachydeuterus auritus

23.4

3.2

31.0

43.9

Trachurus trecae

+

+

2.7

+

Pomadasys sp.

+

+

1.1

1.7

Saurida parri

+

+

+

+

Trichiurus lepturus

+

+

+

+

Lepidotrigla sp.

+

10.9

+

+

+

12.2

Sarda sarda

+

Caranx sp.

5.6

+

+

+

+

Fistularia sp.

1.3

8.3

+

1.1

+

5.5

Priacanthus arenatus

3.8

3.8

+

4.7

+

1.0

Pseudolithus sp.

+

+

Lutjanus sp.

2.5

2.4

Epinephelus sp.

+

6.6

+

2.3

+

2.8

Weight these fish all trawls

3 715.7

703.6

9 401.5

2 095.3

8 653.8

56.3

10 016.5

Total weight all trawl (raised)

4 372

910

9 428.6

2 200

8 760

120

10 745.0

% selected fish: total weight

84.3

75.8

97.0

93.3

97.3

46.5

86.7

+ = less than 1%

There were occasions when B. capriscus catches exceeded 20 tons per haul.

4. ABUNDANCE OF B. CAPRISCUS

4.1 Catch per Unit Effort

Table 8 shows total catch (landings) effort, catch per unit effort in 8-12 m vessel units (catch per day fishing), and estimates of total effort. It is observed that the cpue has increased since 1972 with the highest during 1976. There was however a slight drop in 1977 due probably to a drop in fishing effort. Except in 1976 and 1977 when there were reductions, the overall effort has remained fairly constant. This situation may suggest increase in the stock of B. capriscus on the Ghanaian continental shelf.

Table 8

GHANA: CATCH AND EFFORT STATISTICS RELATIVE TO B. CAPRISCUS

Year

Total catch
(kg)
all vessels

Total catch
(kg)
8-12 m vessels

Effort of
8-12 m vessels
(days x 10³)

(cpue)
8-12 m vessels
(kg/day)

Estimated
total effort
(x 10³)

1972

3 031 200

2 846 600

14.2

201.2

15.0

1973

4 857 500

4 604 754

14.4

320.8

15.1

1974

7 676 300

6 766 920

13.9

487.0

15.8

1975

8 622 400

8 169 530

13.5

606.9

14.2

1976

8 826 400

7 747 304

10.3

754.4

11.7

1977

7 092 400

6 680 818

9.8

679.8

10.4

4.2 Trawling Surveys

Monthly trawling surveys conducted by the Fishery Research Unit show the abundance of B. capriscus (Appendixes 1-4). But unfortunately, constant breakdown of the research vessel prevented data from being collected throughout the year especially during July to September when landings of B. capriscus are low. However, a look at Table 9 suggests that during those months, the abundance of this species is low. This may also explain the low landings of trigger fish in July-September.

Table 9

TEMA TRAWLING TRANSECTS YIELD IN kg/h BY DEPTH OF ALL FISH SPECIES AND THOSE OF B. CAPRISCUS

Month

Tema 1973

Tema 1974

7

8

10

11

1

2

3

4

6

Depth (m)

18

423

61

16

92

391

56.7

8.0

16.3

36

60

147

267

848

280

40

88

7.7

2.8

181.4

319

279.4

34.2

7.0

45

60

167

14

144

224

124

2.0

0

1.4

20.0

34.6

17.2

54

23

355

967

2.8

1.5

0

Total

166

669

158

290

371

2 587

793

56

564

12.6

4.3

35.5

54.6

210.9

751.5

671.5

42.2

249.6

% B. Capriscus

7.6

0.6

22.5

18.8

56.8

29.0

84.6

75.3

44.2

Month

Tema 1975

Cape Coast 1975

1

2

3

5

9

10

2

5

Depth (m)

18

217

56

4

12

65

945

38

112.8

27.5

1.0

1.0

1.0

799.6

4.0

27

324 0

75

3

5

243

182

363

47

3.0

0

210.4

10.0

36

147

151

451

180

307

394

490

72.8

7.0

442.0

9.0

260

315.6

364.0

45

222

115

213

230

395

108

734

694

52

14.0

17.0

31.5

0.5

78

553.3

610.0

54

250

101

18

148

251

129

59

226

0

1.5

3.0

5

2.5

56.4

16.0

Total

1 160

498

238

846

1 134

726

2 495

1 495

237.6

51.5

19.5

477.5

15.5

340.5

1 935.3

1 005.0

% B. capriscus

20.5

10.3

8.2

56.4

1.4

46.9

77.6

67.2

Source: FRU-ORSTOM (1976)
Rapport du Groupe de travail sur la sardinelle (S. aurita) des côtes Ivoiro-ghanéennes, Abidjan, 28 juin - 3 juillet 1976.

4.3 Fiolent Survey

This survey also showed the abundance of this species on the continental shelf of Ghana. Table 10 shows trawl yields for some selected fish species. These tables also show how important B. capriscus is on the shelf of Ghana. It is also noted that to the West of Cape Three Points (20-50 m depth), the yield of demersal fish species in order of importance during the survey was Brachydeuterus auritus, Pagellus coupei, B. capriscus and Pagrus ehrenbergi. To the East of Cape Three Points (20-50 m) and Eastern shelf of Accra, there is a different situation. B. capriscus is the most important species followed by Brachydeuterus auritus.

Table 10

FIOLENT SURVEY: TRAWL YIELDS (kg/h) FOR SELECTED SPECIES
(TRAWL 1625-A; GHANA, NOVEMBER-DECEMBER 1976)

Depth range (m)

West of Cape

Three Points

East of Cape

Three Points

Eastern shelf off Accra

20-50

50-100+

20-50

50-104

20-50

50-100

Dentex angolensis

0

13

0

105

0

56

Dentex congoensis

0

37

0

215

100

207

Dentex canariensis

34

0

0.2

5

0

0

Pagellus coupei

300

27

31

8

48

32

Pagrus ehrenbergi

84

0

0.3

14

28

0

Balistes sp.

102

2

3 081

0

1 335

22

Paracubiceps ledonoisi

0

25

0

234

0

6

Sardinella eba

0

0.5

115

0

4

1

Sardinella aurita

0

0

0

0

0.2

0

Brachydeuterus auritus

863

22

2 914

0

1 300

0

Trachurus trecae

2

1

0

0

0

6

Pomadasys sp.

1

3

0

0

31

0

Saurida parri

0

0.1

0.2

0

0.1

0.2

Trichiurus lepturus

0

2

2

1

2

0

Lepidotrigla sp.

0

15

32

2

0

5

Sarda sarda

0

0

0

0

1

0

Caranx sp.

44

0

7

0

10

2

Fistularia sp.

17

12

6

6

21

6

Priacanthus arenatus

38

5

6

23

7

31

Pseudolithus sp.

0

0

70

0

7

0

Lutjanus sp.

5

0

0

0

212

0

Epinephelus sp.

22

17

5

16

9

0

Note: Unusual hauls of more than 20 tons Balistes excluded from calculations

5. DISTRIBUTION

B. capriscus is distributed all over the shelf of Ghana, but seems to be most abundant in the Winneba/Cape Coast area (high catch rates). There does not seem to be any regularity in the distribution by depth. However the area up to 50 m depth (especially 20-50 m) seems to be most important. The Fiolent Survey (November-December 1976) found high yields of 3 081 kg/h and 1 335 kg/h between 20 and 50 m depth to the East of Cape Three Points and Eastern shelf off Accra respectively. (Table 10.)

6. BIOLOGY

Since late 1976, biological data on this species have been collected by the Fishery Research Unit of Ghana. The following only gives preliminary analysis of some of the biological characteristics.

6.1 Sex Ratio
Females dominate the landings of B. capriscus. Data obtained in 1976 snowed the following percentages:

MONTH	FEMALES	MALES
July	72.5	27.5
August	70.5	29.5
September	38.5	61.5
October	100.0	-
November	83.3	16.7

6.2 Maturity, Gonad Weight and Gonadosomatic Indices
The following table gives female percentage representation of the maturity stages in samples during 1976:

MONTHS (1976)	SEXUAL STAGES
MONTHS (1976)	I	II	III	IV	V	VI
July	31.8	40.9	27.3
August		23.1	30.8	28.8	11.5	5.8
September		20.0	80.0
October			56.3	43.7
November	7.7	7.7	15.4	69.2

The following table gives the monthly mean gonad weights during 1976:

MONTH	FEMALE	MALE
July	7.38 g	0.88 g
August	9.2 g	4.6 g
September	9.8 g	2.5 g
October	11.6 g	-
November	31.3 g	4.3 g

It appears that heavier gonads and more advanced stages of maturity occur during the latter part of the year.
6.3 Gonadosomatic indices
Gonadosomatic indices calculated by the following formula

gave the following results:

MONTH	YEAR
	1976		1977		1978
	MALE	FEMALE	MALE	FEMALE	MALE	FEMALE
January			2.48	53.67	-	-
February			5.90	10.97	2.03	7.77
March			0.96	6.83	1.84	6.97
April			1.00	5.64	3.02	3.40
May			0.63	7.51	5.89	6.18
June			1.04	6.51	4.04	5.18
July	1.08	7.42	0.82	6.15	3.42	9.65
August	3.53	18.04
September	2.95	4.43
October	-	9.13
November	1.72	18.83
December

These indices as well as the maturity stages may suggest that peak spawning takes place between October and February, although this is not clear. This is however the period when high catches are obtained.
6.4 Length Frequency Distribution
Table 11 shows annual length frequency distribution obtained from medium-sized commercial trawlers. Samples were not obtained from the small trawlers (8-12 m). The length frequency may not represent the catch since the medium-sized trawlers discard some of the catches. Sizes however range between 13 and 36 cm (fork length). Looking at data on length obtained from trawling surveys, there does not seem to be any definite pattern in the distribution of sizes by depth.
Table 11

7. BIOMASS ESTIMATES

The Fiolent Survey (November-December 1976), as well as the Capricorne Survey (July-August 1977), have attempted to estimate the biomass of B. capriscus on the shelf of Ghana. Results of the Fiolent survey showed a biomass of 68 300 in an area of 1 112 n mi² (an area off Winneba/Cape Coast).

This is the area of high concentration. Although the Capricorne Survey did not sample the fish traces, large concentrations of fishes found in the Tema/Winneba area were believed to be B. capriscus. A biomass of about 70 000 tons of fish believed to be Brachydeuterus auritus and Balistes capriscus was found during the survey in the Tema/Winneba area.

8. DISCUSSIONS AND CONCLUSIONS

B. capriscus has been reported in other areas of the Gulf of Guinea, e.g., off Ivory Coast, Togo, Guinea, Guinea-Bissau, Senegal, Sierra Leone and Nigéria, but the cause of its sudden bloom is not known. The species may occur on the shelf of Liberia, but there is no information yet on this.

This species accounts for high percentages of the catches in the trawl fisheries of the Gulf of Guinea. The dominance of this species on the fishing grounds has also been confirmed by trawling surveys conducted in Ghana and Ivory Coast and the Fiolent Survey (1976).

Hydroacoustic surveys conducted in various parts of the Gulf of Guinea have also confirmed the high incidence of B. capriscus in the area.

The last Working Group on the Ivoro-Ghanaian S. aurita stock made recommendations to have initiated a complete study of the biology, ecology, distribution, migrations and dynamics of this species. In view of the occurrence of this species in the whole of the Gulf of Guinea and also its possible threat to more valuable fish species, it is felt that such a study should be conducted on a cooperative basis with all the institutions in the area taking part.

The following are therefore being suggested:

(i) Estimate the total catches from all commercial trawlers in the Gulf of Guinea (including discards); this should be done at country level.
(ii) Estimate biomass of the species through trawling and acoustic surveys.
(iii) Biological sampling to throw more light on reproduction, feeding, growth, recruitment, etc., to be conducted by the existing laboratories in the Gulf of Guinea. This should also include ecological as well as environmental studies.
(iv) Fish technological institutes to intensify experiments on possible utilization of trigger fish.

9. LITERATURE

CRODT/ISRA, Compte rendu de la Mission ECHOPREG/N/O CAPRICORNE. 16 juin - 4 juillet 1977. (In press)

Mensah, M.A., 1977, Report for the biennium 1975-76 on the Fishery Research Unit. Rep.Bienn.Fish.Res.Unit, Tema, (1975-76): 59 p.

Fishery Research Unit, Tema et Centre de recherches océanographiques, Abidjan, 1976, Rapport du Groupe de travail sur la sardinelle (S. aurita) des côtes ivoiro-ghanéennes. Abidjan, 28 juin - 3 juillet 1976. Abidjan, ORSTOM, 63 p.

Robertson, I.J.B., 1977, Summary report: FIOLENT 1976 Eastern Central Atlantic coastal fishery resource survey. Southern sector. CECAF Tech.Rep.Dakar, (77/2): 115 p.

Appendix 1 to Annexe 3