SCRS/00/135 REVISED EVOLUTION OF FISHING EFFORT OF GILL NETTING CANOES TARGETING LARGE PELAGIC FISH IN THE GULF OF GUINEA by F. X. Bard 1 and N. Y. N Goran Center for Oceanographic Research,, Côte d Ivoire SUMMARY A canoe fishery using drifting gillnet with large meshes developed off (Côte d Ivoire) since 1984. Fishermen target primarily billfishes, sharks and tuna, setting gillnets at night time at the edge of the continental shelf. This document describes how the fishermen continuously increased the length of theirs nets over the years, funding this by the sale of dried shark fins. The size of the nets has doubled over the past 15 years. The extension of this practice by migrating ian fishermen along the northern shores of Gulf of Guinea might result in a considerable, but still not fully reported, increase in the catches of large pelagic fish. RÉSUMÉ Depuis 1984 une pêcherie piroguière pose des filets maillants dérivants à grandes mailles à proximité du port d. Cette pêcherie opère la nuit à la limite du plateau continental et cible les grands poissons pélagiques, principalement des poissons porte épée des requins et des thons. Le présent document décrit le processus d accroissement constant de la longueur des filets, financé par la vente des ailerons de requin séchés. La taille des filets a ainsi doublé en 15 ans. La diffusion de cette pratique par les pêcheurs migrants ghanéens le long des côtes nord du Golfe de Guinée a probablement permis un important accroissement des captures de grands poissons pélagiques. RESUMEN En 1984 se desarrolló frente a (Côte d Ivoire) una pesquería de piraguas con redes de enmalle a la deriva, de mallas amplias. Los pescadores buscan sobre todo marlines, tiburones y túnidos, calando las redes por la noche en el límite de la plataforma continental. Este documento describe el proceso de continuo aumento de la longitud de las redes a lo largo del tiempo, financiado por la venta de aletas secas de tiburones. El tamaño de las redes se ha doblado en los últimos 15 años. La difusión de esta práctica, debida a la migración de pescadores ghaneanos a lo largo de la costa norte del Golfo de Guinea ha producido probablemente un importante aumento de las capturas de grandes peces pelágicos. KEY WORDS Billfish, Sharks, Fishing effort, gillnets, Gulf of Guinea 1 Fishery Biologist at IRD (ex ORSTOM), E-mail: Xavier.Bard@ird.ci
1 INTRODUCTION A canoe fishery using drifting gill net with large meshes was developed in by 1974, (Mensah and Doyi, 1994). Fishermen target mainly large fish such as billfish, sharks and tunas. Fishing grounds are located at the edge.of the continental shelf. Fishing operations occur during the night. In 1984 the in canoes extended their operations to where fish is highly demanded, (Amon Kothias et al, 1992). The Center for Oceanographic Research in monitors continuously this fishery since 1988. Landings of large fish are recorded along with number of trips. Consequently nominal effort is recorded in number of canoe trips. However a recent frame study showed that length of the net used has increased continuously over the years. The present document evaluates the consequences of this trend for the whole fishery operating in the Gulf of Guinea. 2- MATERIAL, METHODS This fishery is multispecific. By-catches include a wide variety of others fish. (see SCRS/00/63). Among them, sharks have a particular value because, aside of a commercial value, the systematic practice of finning provides a particular income. At he end of every year, dried fins are carried back to where they are sold to specialised traders, who export them. In return fishermen buy additional pieces of net and enlarge their net. A recent enquiry conducted in recorded the history of this evolution for the past 20 years. 3-RESULTS. Table 1 summarises the data collected. Figure 1 displays the evolution of average length of the nets. The average length of the gill nets has more than doubled from 1984 to 2000. Admitting that fishing power is proportional to the length of the net, it is possible to adjust the nominal effort for a more effective effort. The resulting increase of supposed effective effort is noteworthy, as shown in Figure 2. From the fishermen interviews it appeared that the practice of increasing the size of the nets affected as well the canoe fishery in, which developed earlier and reached a considerable volume of catches. Statistics of this fishery in are reported for old years by Mensah (op. cit) and Koranteng, 1994. Similar data for canoes have been recently compiled by Bard and Hervé, 2000. All these elements are displayed for comparison in table 2. 4- DISCUSSION, CONCLUSION. The adaptative character of African small scale fishermen has already noted by various authors. The continuous increase of net size, thanks to an original way of funding reported here is another example. The continuous trend of increasing the length of the net, thus the increase of the fishing power is reality for canoes operating in. But this fishery is relatively minor when compared to the ian one. The question that arises is to know if this trend affected all the canoes in in the same proportion?. If true the increase of effective fishing effort for gillnetting along the coasts of Côte d Ivoire might be considerable. Moreover, Ghanian Fanti fishermen have extended their fishing operations to the whole coast of Gulf of Guinea, (Delaunay, 1988). If among them, the proportion of canoes using gill net is important, and it could be, given the self supporting features of this fishery, as shown here, it is possible that
amount of effort and presumably related catches is considerable. Such possible development of an original fishery, poorly described up to now, might deserve some attention particularly because of its ability to capture species considered as highly exploited such as billfish and sharks. 5- REFERENCES AMON KOTHIAS J. B., F. X. Bard et A. Hervé, 1992. - Description des pêches et des statistiques de poissons porte épée au Port d, 1987-1992. ICCAT, SCRS/92/148. BARD F. X et A. Hervé, 2000- La pêcherie piroguière au filet maillant de grands pélagiques a. Synthèse des connaissances, 1984-2000. DOC.INT.CROA-, 17 P. DELAUNAY K. 1988- L expansion des pêcheurs ghanéens sur les côtes ouest africaines : Le cas de la Côte d Ivoire, Rapport Int ORSTOM, 60p. KORANTENG K.1994 National Fisheries Development Project. Department of Fisheries Doc Int 21p. MENSAH M. A. and B. A. Doyi 1994-The billfish fishery in ICCAT SCRS/92/75, Co.Vol.Sci.Papers. 41 : 265-272. MENSAH M. A., 1994- The catch statistics of the billfish fishery in. ICCAT SCRS/92/146, Co.Vol.Sci.Papers. 41 :287-305
Table 1: Statistics of gill net length evolution, 1978-2000. Year Mean Standard deviation Number Ratio to 79-84 1978 1350 636,4 2 1 1979 944,4 342,3 9 1 1980 866,7 229,2 24 1 1981 862,5 229,5 24 1 1982 863,0 218,2 27 1 1983 866,7 208,6 30 1 1984 904,5 230,6 33 1 1985 1055,3 287,3 38 1,17 1986 1102,3 279,9 44 1,22 1987 1163,0 290,1 46 1,29 1988 1207,8 294,5 51 1,34 1989 1273,6 309,6 53 1,42 1990 1316,1 322,4 56 1,46 1991 1335,7 317,0 56 1,48 1992 1403,6 323,6 56 1,56 1993 1479,3 325,4 58 1,64 1994 1569,5 325,5 59 1,74 1995 1626,2 308,7 61 1,81 1996 1671,8 280,9 62 1,86 1997 1713,7 248,6 62 1,90 1998 1825,4 287,0 65 2,03 1999 1932,0 298,1 64 2,15 2000 2046,1 280,8 64 2,27 Years Table 2. Compared nominal efforts and catches of gill net canoes in Côte d Ivoire and., Sources Mensah, Koranteng, Bard and Hervé Nominal effort in (trips) Billfish Shark Tunas Nominal Effort Billfish Total catch 1984 10,3 28655 1672,8 6830 1985 400 33,4 33964 1521,7 7267 1986 19218 959,8 4834 1987 48162 1520,9 4566 1988 2167 208,1 83,3 28585 1192,8 3533 1989 1717 143,4 31,4 33469 1119,2 4220 1990 1997 153,5 48,5 26076 906,5 3309 1991 3356 161,3 55,8 18887 678,9 2298 1992 3973 161,0 101,4 16530 2943 1993 4689 193,4 90,1 1994 7315 286,0 110,9 1995 7826 261,8 106,5 1996 7794 274,4 103,3 1997 6761 306,6 91,1 237 1998 5092 243,6 55,6 1999 7101 386,7 58,1 2000 5342 271,8 47,4 5720?
Length of the net (m 2300 2100 1900 1700 1500 1300 1100 900 700 500 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 Figure 1: Evolution of the length of gill net, Average +/- 2 standard errors 1999 Year Figure 1: Evolution of the horizontal length of gill nets used by canoes off Trips 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 1984 1986 1988 1990 1992 1994 1996 1998 Figure 2 : Evolution of nominal and adjusted effort of canoes. 2000 Years Nominal effort (trips) Adjusted effort Figure 2: Evolution of nominal and adjusted fishing effort for canoes