In the southwestern equatorial atlantic ocean




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SCRS/2007/085 Collect. Vol. Sci. Pap. ICCAT, 62(5): 1553-1559 (2008)



HISTORICAL CATCH RATES OF BLUE SHARK (PRIONACE GLAUCA)

IN THE SOUTHWESTERN EQUATORIAL ATLANTIC OCEAN

BETWEEN 1958 AND 1962

Felipe Carvalho, Fábio H.V. Hazin, Humberto G. Hazin, Paulo Travassos


SUMMARY
The increasing catches of blue shark (Prionace glauca) in the past years, both as by-catch and as a target species, have enhanced the need of more accurate data in order to assure an adequate assessment of the exploited stocks, which, in turn, constitutes the very basis for a sound management of the fishery. Historical catch rates from the early period of exploitation are of particular importance, since the biomass at that time was close to a virgin state. The objective of the present study, thus, was to recover and analyze the catch rates of blue sharks caught by five tuna longline vessels operating in the southwestern equatorial Atlantic Ocean, between 1958 and 1962, and to compare the results with the catch rates obtained in more recent years. The results show that both catch composition and blue shark CPUE of tuna longliners operating in the southwestern equatorial Atlantic Ocean, from 1958-1962, do not seem to differ much from more recent fishing operations (1986 to 2005).
RÉSUMÉ
Les prises croissantes de requin peau bleue (Prionace glauca) au cours de ces dernières années, à la fois en tant qu’espèces accessoires et en tant qu’espèces cibles, ont renforcé la nécessité de disposer de données plus précises afin de garantir une évaluation adéquate des stocks exploités, ce qui, à son tour, constitue la base même d’une bonne gestion de la pêcherie. Les taux de capture historiques du début de la période d’exploitation sont particulièrement importants, étant donné que la biomasse à cette époque était proche d’un état vierge. L’objectif de la présente étude était donc de récupérer et d’analyser les taux de capture des requins peaux bleues capturés par cinq palangriers thoniers opérant au Sud-Ouest de l’océan Atlantique équatorial entre 1958 et 1962, et de comparer les résultats avec les taux de capture obtenus au cours des années plus récentes. Les résultats indiquent que la composition de la capture et la CPUE du requin peau bleue des palangriers thoniers opérant au Sud-Ouest de l’océan Atlantique équatorial de 1958 à 1962 ne semblent pas différer beaucoup des opérations de pêche plus récentes (1986 à 2005).
RESUMEN
Las crecientes capturas de tintorera (Prionace glauca) en años pasados, como captura fortuita y como especie objetivo, han puesto de relieve la necesidad de disponer de datos más precisos para asegurar una evaluación adecuada de los stocks explotados que, a su vez, constituye la verdadera base de una ordenación sólida de la pesquería. Las tasas de captura histórica del primer periodo de explotación son de especial importancia, ya que la biomasa en aquel momento se encontraba cerca de un estado virgen. El objetivo del presente estudio, por lo tanto, era recuperar y analizar las tasas de captura de la tintorera capturada por cinco palangreros que operaban en el océano Atlántico ecuatorial sudoccidental entre 1958 y 1962, y comparar los resultados con las tasas de captura obtenidas en años más recientes. Los resultados demuestran que tanto la composición de la captura como la CPUE de la tintorera de los palangreros atuneros que operaban en el océano Atlántico ecuatorial sudoccidental entre 1958-1962, no parecen ser muy diferentes de las de las operaciones pesqueras más recientes (1986 a 2005).

1. Introduction
The blue shark, Prionace glauca, has an oceanic and circumglobal distribution, inhabiting temperate and tropical waters of the world oceans, from about 40°N to 40°S. It exhibits the widest geographic distribution among all large sharks, living predominantly in the open ocean, although it occasionally occurs inshore, in areas with narrow continental shelves, or around islands (Bigelow and Schroeder, 1948; Castro and Mejuto, 1995). Due to its frequent occurrence in gillnet and longline fisheries, it has been intensively studied in both hemispheres. In the southwestern Atlantic, where it is chiefly caught by longliners targeting tunas and swordfish, studies have focused on distribution and relative abundance, reproduction, feeding habits and population dynamics (Hazin, 1991; Amorim, 1992; Hazin et al., 1994).
Driven by a continuous rise in the price of their fins, as well as meat, associated to a declining trend of catch rates of several bony fish traditionally targeted by commercial fisheries, blue shark landings are increasing worldwide. In the Atlantic Ocean, their status in the longline fishery has been gradually changing from by-catch to a target species. The increasing shark catches in ICCAT Convention area, associated to a growing international concern with the conservation of shark stocks caught by commercial fisheries (FAO, 1998), has prompted the Commission to adopt several measures aiming at maintaining shark populations at levels that allow maximum sustainable yield (ICCAT, 2006). In 2004, for the first time in its history, ICCAT carried out a stock assessment of the two shark species most caught by the industrial fishing fleet in the Atlantic Ocean: the blue shark and the shortfin mako (Isurus oxyrinchus). By far, the main problem encountered in the 2004 stock assessment was the lack of accurate data. Therefore, although the general conclusion of the assessment was that both shortfin mako and blue shark stocks in the North and South Atlantic seemed to be in a reasonably good condition, probably at levels close or even above the Maximum Sustainable Yield, particularly in the southern hemisphere, the results of the analyses should be interpreted with considerable caution due to data deficiencies and to the fragility of the assessment methods employed.
During its 2006 meeting, ICCAT has recommended that a new stock assessment for the shortfin mako and blue shark be carried out in 2008, emphasizing, therefore, the acute need for more detailed data, particularly on fishing effort and catches from the main fisheries targeting the species in the Atlantic Ocean. It was noted then that historical catch rates obtained both from commercial and exploratory fishing surveys, conducted in late 1950s and early 1960s, when the tuna longline fishery in the Atlantic Ocean was beginning, would be of particular importance, since the biomass at that time was close to a virgin state. The objective of the present study, thus, was to recover and analyze the catch rates of blue sharks caught by five tuna longline vessels operating in the Southwestern Equatorial Atlantic Ocean, between 1958 and 1962, and to compare the results with the catch rates obtained in more recent years.

2. Material and methods
The catch rates of blue sharks caught from May 1958 through September 1962, during 2,129 longline sets performed along 148 cruises of five tuna longline boats (Albacora, Madre Luna, Galates, Aguja and Delfin) (Table 1) operating in the southwestern equatorial Atlantic were estimated, based on the number of fish caught per 1,000 hooks. The area fished laid between 5°N and 5°S latitude, and 15°W and 38°W longitude (Figure 1). The longline gear employed was similar to the one traditionally used by the Japanese tuna longline fishery in the early fifties, the details of which have been abundantly described (Shapiro, 1950; Shimada, 1951; Ego and Otsu, 1952). The quarterly and yearly mean CPUE was calculated for the 5 boats together. Information on the longline soaking time, available just for three of them (Albacora, Madre Luna, and Galates), was also analyzed (Table 2). For all those 5 vessels, data on the fishing effort, in number of hooks employed, as well as on the number of fish caught, for the main species, were recorded. In the case of Albacora, Madre Luna e Galates, blue shark catches were specifically annotated. For the Aguja and the Delfin, however, only the total shark catch was available. For these 2 boats, therefore, the blue shark catch was estimated, by applying the same proportion found for the 3 other aforementioned boats pooled together. In order to compare the historic catch rates with those obtained in recent years, the yearly mean CPUE of Brazilian vessels operating in the same area, from 1986 to 2005, was calculated, based on a total effort of 21,718,816 hooks, with a total blue shark catch of 74,937.

3. Results and discussion
The three boats for which operational details were available (Albacora, Madre Luna and Galates) set the longline around 01:00h, finishing close before sunrise. Retrieval begun from 8:20h to 11:20h, ending from about 15:00h to sunset. The mean soaking time was very close for the three boats, ranging from 10:12 to 11:25 h.
The relative distribution of the total catch from the five fishing boats which operated in the late fifties/early sixties (Table 3) shows that sharks and tunas were fairly the most abundant component of the catches, accounting together for nearly 80% of the total. Tunas represented roughly 42% of the total catch and sharks 38%, in number. If catches of sharks and tunas were compared in terms of landed weight, however, sharks would certainly respond to the major part of the catches since they are heavier than tunas. The mean weight of sharks and tunas for the more recent tuna longline fishery in the same region was reported as 39 kg for sharks (Hazin, 1987) and 29 kg for tunas (Hazin, 1986). The blue shark was responsible for about 41% of the shark catches. The relative participation of tunas and sharks, as well as of the blue shark, in longline fishing operations carried out in the southwestern equatorial Atlantic Ocean does not seem to have changed much along time. Hazin (1991), for example, studying the catch composition of Brazilian longiners fishing in the same area, from 1983 to 1988, found that sharks and tunas, together, accounted for about 78% of the total catch, in number, with the blue shark responding for 39% of the total shark catches.
The yearly mean CPUE of the blue shark from 1958 to 1962, remained relatively stable, at around 4.0 (mean= 3.9; s.d.= 0.9) (Figure 2). Hazin and Lessa (2005), analyzing blue shark catch rates of Brazilian longliners based in Natal, which operated roughly in the same area (Fig. 1), from 1986 to 2002, concluded that the several changes of blue shark CPUE along the years were strongly related to the shifts of target species, being relatively stable, though, during the periods of time when the target species had remained the same. According to them, in 1986, sharks, mainly the blue shark, also became a target, resulting in an increase of their CPUE, up to 1988, when it peaked at 6.2. From that year on, up to 1991, the catch of tunas rose again due to the discovery of new fishing grounds around the Archipelago of Saint Peter and Saint Paul, resulting in a decrease of blue shark CPUE to almost a half, remaining close to 4.0. In 1991, the finding of important concentrations of sharks of the genus Carcharhinus, mainly the night shark, C. signatus, over seamounts off northeast Brazil (Hazin et al., 1998), prompted a dislocation of the fishing effort to those areas, resulting in an increase of their catches and a further decline of blue shark CPUE, which stabilized then at around 1.5. In the middle nineties, through the chartering of foreign vessels, new fishing technologies, aimed at higher catches of swordfish, were incorporated by the Brazilian fleet, including the use of monofilament poliamid longlines, together with light sticks and squid as bait. As a result, swordfish CPUE increased by about 6 times, while blue shark CPUE doubled, fluctuating then, from 1995 to 2000, at around 3.0. After 2000, several longliners chartered to China Taipei, begun to target yellowfin tuna, in shallow waters of the equatorial region, resulting in a continuous drop of blue shark CPUE, up to a minimum of 1.4, observed in 2003. Finally, from that year on, after Chinese Taipei vessels ceased their operations in Brazil, blue shark CPUE bounced back to the previous level of 4.0 (4.4, in 2005), similar to the one observed in the late fifties, again probably due to a new shift of the target species to swordfish (Hazin et al., 2008a, 2008b).
The quarterly mean CPUE of blue shark from the five boats fishing in the late fifties, showed significantly higher values during the second and third quarters of the year (Kruskal-Wallis, F= 2.89; p= 0.009) (Figure 3), which is different from the pattern described for the species in the same region, although in a much broader area, for more recent years, with higher CPUEs always happening during the third and fourth quarters (Hazin, 1991). A possible reason for this difference is the relatively narrow fishing ground of the boats operating in older times, associated to their more equatorial location.
Although data on the depth of catch were not available from the fishing cruise reports of all five boats operating in the late fifties, it is interesting to note that all of them pointed out that catches of blue sharks occurred only in depths greater than 1,000 m, a result similar to the one described by Hazin (1991), for the same region, as well as by Gubanov and Grigor`yev (1975), for the western and eastern parts of the equatorial Indian Ocean.
The limited stock assessments carried out to date have shown no evidence of a declining trend in catch rates of blue shark with time in the Atlantic or Indian Ocean (IUCN 2005). However, a 20% decrease was evident in the North Pacific Ocean, between 1971/1982 and 1983/1993 (Nakano et al. 1996), although no consistent decline in catch rates through the fishing season was evident for Japanese longline fishing in Australian waters (Stevens and Wayte, 1999, in IUCN 2005).
The present results show that both catch composition and blue shark CPUE of tuna longliners operating in the southwestern equatorial Atlantic Ocean, from 1958-1962, do not seem to differ much from more recent fishing operations (1986 to 2005). Besides, blue shark CPUE along the years seemed to fluctuate much more due to shifts in targeting strategy than to actual fluctuation in abundance, a fact that highlights the importance of including the targeting strategy as a factor in the standardization of blue shark CPUE. It also emphasizes the great importance of recovering catch and effort data from the early period of the fishery, for the upcoming stock assessment of this important shark species.
4. Acknowledgements
This work was made possible by the Secretariat of Fisheries and Aquaculture (SEAP) of the Brazilian Government and by Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco- FACEPE. We are also grateful to the Tropical Conservation and Development Program (TCD) and the Fisheries and Aquatic Science Department, of University of Florida-USA, for the Fellowship/Assistantship provided.


5. References
AMORIM, Alberto F. 1992. Estudo da biologia pesca e reprodução do cação azul, Prionace glauca L. 1758, capturado no sudeste e sul do Brasil. Ph. D. Thesis, Universidade Estadual Paulista, Rio Claro, São Paulo. Brazil, 176 p.
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CASTRO, J.A. and Mejuto, J. 1995. Reproductive parameters of blue shark, and others sharks in the Gulf of guinea. Mar. Fresh. Res. 46: 967-973.
CASEY, J.G. 1976. Migrations and abundance of sharks along the Atlantic coast. In:Seaman, W. Jr. (ed.), Sharks and Man – a perspective. Fla. Sea Grant Program, rep. 10. p. 13 – 14.
EGO, k. and Otsu, T. 1952. Japanese tuna-mothership expeditions in the western equatorial Pacific Ocean. (June 1950 to June 1951). Commercial Fisheries Review, 14(6): 1-19.
FAO (Food and Agriculture Organization of the United Nations). 1998. International Plan of Action for the Conservation and Management of Sharks. Document FI:CSS/98/3, Rome.
GUBANOV, Ye., Grigor’yev, V.N. 1975. Observations on the distribution and biology of the blue shark Prionace glauca of the Indian Ocean. J. Ichthyol. 15:37-43.
HAZIN, F.H.V. 1986. Pesca de atuns e afins com embarcação de pequeno porte no nordeste brasileiro. Graduation thesis, Universidade Federal Rural de Pernambuco. Recife, 107p.
HAZIN, F.H.V. 1987. Capturas de tubarões por atuneiros em operação no nordeste brasileiro. III Reunião do grupo de trabalho sobre pesca e pesquisa de tubarões e raias no Brasil. July 1987, Fortaleza, 25pp.
HAZIN, Fábio H.V. 1991. Ecology of the blue shark, Prionace glauca, in the southwestern equatorial Atlantic. M.Sc. Dissert. Tokyo University of Fisheries, 123 pp.
HAZIN, Fábio H.V., C.E. Boeckmann, E.C. Leal, K. Otsuka, K. Kihara. 1994. Reproduction of the blue shark, Prionace glauca, in the southwestern equatorial Atlantic Ocean. Fisheries Science, v.60, n.05, p.487-491.
HAZIN, Fábio H.V.; Zagaglia, Juliana R.; Broadhurst, M.K.; Travassos, P.E.P.; Bezerra, T.R. Q. 1998. Review of a small-scale pelagic longline fishery off northeastern Brazil. Marine Fisheries Review, 60(3):1-8.
HAZIN, Fábio H.V., P.B. Pinheiro, M.K. Broadhurst. 2000. Further notes on reproduction of the blue shark, Prionace glauca, and a postulated migratory pattern in the South Atlantic Ocean. Ciência e Cultura, v.52, n.2, p.114-120.
HAZIN, Fábio H. V., and Lessa, R. 2005. Synopsis of biological information available on blue shark, Prionace glauca, from the southwestern Atlantic Ocean. Collect. Vol. Sci. Pap. ICCAT, 58(3): 1179-1187.
HAZIN, H. G., Hazin, F., Travassos, P., Carvalho, F.C. and Erzini, K., 2008a. Standardization of swordfish CPUE series caught by Brazilian longliners in the Atlantic Ocean, by GLM, using the targeting strategy inferred by cluster analysis. Collect. Vol. Sci. Pap. ICCAT 62, in press.
HAZIN, Fábio H. V., Hazin, H. G. and Travassos, P., 2008b. CPUE and catch trends of shark species caught by Brazilian longliners in the Southwestern Atlantic Ocean. Collect. Vol. Sci. Pap. ICCAT 62, in press.

ICCAT. 2006. Report of the Standing Committee on Research and Statistics (SCRS). In: ICCATs (Ed.), Col. Vol. Sci. Pap., Madrid, pp. 1-194.


IUCN/SSC Shark Specialist Group., 2005. Sharks, Rays and Chimaeras: The Status of the Chondrichthyan Fishes. Gland, Switzerland and Cambridge, UK, 461 p.
LOPEZ, A.M., McLellan, D.B., Bertolino, A. R. and Lange, M.D. 1979. The Japanese longline fishery in the Gulf of Mexico, 1978. Mar. Fish. Rev., 41(10): 23 – 28.
NAKANO, H. 1990. Ecological study on age, breeding, and migration of blue shark from the North Pacific Ocean (In Japanese).
NAKANO, Hideki, and Kazuya Nagasawa. 1996. Distribution of Pelagic Elasmobranches Caught by Salmon Research Gillnets in the North Pacific. Fish. Sci., 62(5): 860-865.
SHAPIRO. 1950. The Japanese longline fishery for tunas. Commercial Fisheries Review, 12 (4):1-26.
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Table 1. Basic characteristics of the five tuna longline boats operating in thesouthwestern Equatorial Atlantic Ocean, from May 1958 until September 1962.

Boat

Length

Gross Ton

Baskets/ set

Fishing Period

Number of sets

Blue shark catch (n)

Albacora

16 m

34 G.T

92

May 58/ Oct. 60

282

711

Madre Luna

18 m

52 G.T

135

May 58/ Dec. 60

409

1,234

Galates

23 m

136 G.T

157

Sep. 59/ Sep. 62

527

1,636

Aguja

17 m

-

101

Jul. 60/ Sep. 62

367

844

Delfin

-

-

91

Nov. 59/ Sep. 62

544

1,384

Total













2,129

5,809

Table 2. Average setting, retrieving and soaking time, with standard deviations (SD), for the longline operations of three boats in the southwestern Equatorial Atlantic Ocean from May 1958 until September 1962.






Albacora

Madre Luna

Galates







Time / S.D

Time / S.D

Time / S.D

Setting

Beginning

1:42 / 00:16

01:09 / 00:11

00:36 / 00:45

Time

End

05:33 / 00:38

04:13 / 00:25

04:39 / 01:24

Retrieving

Beginning

11:20 / 00:10

09:17 / 00:21

08:23 / 01:11

Time

End

18:08 / 00:21

15 :01 / 01:03

19 :11 / 01:24

Soaking time




10:12 / 00:12

11:16 / 00:25

11:25 / 01:34


Table 3. Total catch (n), with its relative distribution, of the five tuna longline boats operating in the southwestern Equatorial Atlantic Ocean, from May 1958 until September 1962.




Sharks

Tunas

Billfishes

Others

Catch (N)

5,809

6,404

2,044

991

%

38.1

42.0

13.4

6.5


Fishing Ground




Figure 1. Location of the fishing ground area of the five tuna longliners in the southwestern Equatorial Atlantic Ocean, from 1958 to 1962.




Figure 2. Yearly catch per unit of effort (number of fish per 1,000 hooks), for the blue shark, from Brazilian longliners operating from Natal (1986 to 2005) and from five tuna longline boats operating in thesouthwestern Equatorial Atlantic Ocean from May 1958 until September 1962 .



Figure 3. Quarterly mean CPUE of blue sharks caught by the five tuna longline boats operating in the southwestern Equatorial Atlantic Ocean from May 1958 until September 1962. Values are means  1 SE.






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