The IUCN/SSC Shark Specialist Group
Shark News 10: January 1998
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Fishery for the shortfin mako Isurus oxyrinchus in southern Brazil
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Fábio E. S. Costa, Francisco M. S. Braga, Alberto F. Amorim and Carlos A. Arfelli
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The shortfin mako Isurus oxyrinchus is one of the most common shark species on the Brazilian coast and in other parts of the world, and among the main species of sharks caught commercially in the North Atlantic, Pacific and Brazilian coast. Nevertheless, there are few studies on the shortfin mako fishery.
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| Shortfin mako Isurus oxyrinchus with parasitic copepods streaming off dorsal fin. |
| Photo: © Jeremy Stafford-Deitsch.
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The main fishing effort on this species is by longliners. Based on the same data sources used in the present paper, Amorim & Arfelli (1992) and Amorim et al. (in press) found that the main species of sharks caught by Santos (SP) longliners were: Prionace glauca (blue shark), Isurus oxyrinchus (shortfin mako), Alopias superciliosus (bigeye thresher), Sphyrna lewini, S. zygaena (hammerhead sharks), and 11 species of Carcharhinus. From 1971 to 1975, sharks represented 13.5% of the total yield of these boats (10 t/year). This percentage reached 51% of the total catch in 1985.
The mako shark fisheries by Santos (SP) longliners (operating in the area 18o -33oS / 35o-51oW) from 1971 to 1990 were studied. The annual catch in dressed weight (without head, gut, gill and fins) fluctuated from 13.3 t in 1975 to 138.3 t in 1990, and average weight of sharks ranged from 41 kg (1984) to 51.2 kg (1977). Fishing effort has increased from 430 thousand hooks (in 1972) to 3 million (in 1990). Catch per unit effort (CPUE, number per thousand hooks) ranged between 0.6 (1982) and 1.8 (1988). The lowest monthly cumulative catch occurred in February (45 t and 869 fishes) and the highest in November (116t and 2,643 fishes). The monthly cumulative fishing effort ranged from 1.3 million (February) to 2.7 million (November). CPUE ranged from 0.4 (January) to 1.1 (September) fish per thousand hooks. From April to November, catches in weight and number were higher, but the average weight of sharks decreased.
The highest annual frequencies of pectoral-caudal length were in the classes between 90 and 150 cm, and mainly from 110 to 130 cm. The length-frequency distribution of pectoral-caudal length in the period 1971-1990 showed high frequencies in the classes from 80 to 150 cm, with the highest in the classes of 110, 120 and 130 cm. The largest amplitude of pectoral-caudal length was seen in the fourth quarter, ranging from 40 to 250 cm. Therefore, the highest frequencies were concentrated in the length classes of 90 to 150 cm in all quarters.
The pectoral-caudal length (Lc)/dressed weight (Dw) was: Dw = 5.69.10-5.Lc2.85 (r2 = 0.847). The relationship total length/pectoral-caudal length was: Lc = 0.277.TL1.15 (r2 = 0.997) (Figure 2).
Pratt and Casey (1983) give the average total length at maturity of female shortfin mako as about 258 cm. According to Compagno (1984) females become mature at 280 cm, while males mature at 195 cm total length. The corresponding pectoral-caudal length (Lc) was estimated to be over 160 cm for females, and about 120 cm for males, based on the total length/pectoral-caudal length relationship. The highest number of shortfin mako caught in the period 1971-1990 was in the length classes from 90 to 150 cm (Lc). So more than half of the total individuals caught were immature.
The increasing trend of catches of shortfin mako from 1971 to 1990, was due to a similar increase in the fishing effort.
The length-frequency distribution showed an occurrence of individuals from newborn stage to adults (near maximum length). Nevertheless, the higher abundance of small sharks indicates that the shortfin makos caught by Santos longliners are mainly young individuals. The fact that this fleet catches mainly immature individuals, but that the average length did not show a decreasing trend, probably indicates that this fishery catches only part of the shortfin mako population.
Figure 1. CPUE (number/thousand hooks) of shortfin mako caught by longliners off Southern Brazil, from 1971 to 1990.
a : annual data.
b : monthly data.
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Figure 2.
a : Dressed weight (Dw) / pectoral-caudal (Lc) relationship.
b : Total length (TL) / pectoral-caudal (Lc) relationship.
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The distribution of the average size of the individuals along the
year, showed that the period from July to September corresponded to
the period of the fishery recruitment of the shortfin mako, for the area
under study.
References
Amorim, A.F., and Arfelli, C.A. 1992. The shark fishery in south and southeastern Brazil. Chondros 3(3): 1-2.
Amorim, A.F., Braga, F.M.S., Fagundes, L., Costa, F.E.S., and Arfelli, C.A. In press. The evolution of tuna fishery in Santos-São Paulo, Southern Brazil (1971-95) In: Collective Volume of Scientific Papers. ICCAT, Madrid.
Compagno, L.J.V. 1984. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO Fish. Synop., Rome, FAO, v.4, n.125 (parts 1 and 2). 665 pp.
Pratt Jr., H.L. and Casey, J.G. 1983. Age and growth of the shortfin mako, Isurus oxyrinchus, using four methods. Canadian Journal of Fisheries and Aquatic Sciences 40(11): 1944-1957.
Summarised from: Costa, F.E.S., Braga, F.M.S., Amorim, A.F., and Arfelli, C.A. 1996. Fishery analysis on shortfin mako, Isurus oxyrinchus, off Southeast and South of Brazil (Elasmobranchii, Lamnidae). Arq. Ciên. Mar. Fortaleza, 30(1-2): 5-12.
Fábio Edir dos Santos Costa
UNESP - Departamento de Zoologia
Av. 24-A, n. 1515 - Bela Vista, 13506-900 - Rio Claro (SP) - Brazil
Email: cbmansur@linkway.com.br
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