stock discrimination of the blue shark, based on irish tagging data - Iccat

2 downloads 46 Views 1MB Size Report
Blue sharks caught in Irish recreational fisheries were tagged since 1970. ... of US and Irish tagged blue sharks recaptured from the Mediterranean Sea led to.
SCRS/2004/124

Col. Vol. Sci. Pap. ICCAT, 58(3): 1171-1178 (2005)

STOCK DISCRIMINATION OF THE BLUE SHARK, BASED ON IRISH TAGGING DATA P. Fitzmaurice, P. Green, G. Keirse, M. Kenny, and M. Clarke1 SUMMARY Blue sharks caught in Irish recreational fisheries were tagged since 1970. The time series of tag and recapture data is the second largest in the northern hemisphere. A total of 16,719 blue sharks were tagged from 1970 and 2001. Most fish tagged in Irish coastal waters were female. There is evidence that this species undertakes a cyclical migration, in a clockwise direction in the north Atlantic. The majority of recaptures off western Europe were females. Most recaptures came from Spanish tuna and swordfish longliners working in the area between Iberia and the Azores. The next most important fleets were the Japanese and Korean longliners off west Africa, including the Canaries and Cape Verde. U.S. and Canadian longliners working off the Grand Banks area were the next biggest source of returned tags, followed by sport fishermen in the NE USA. These results indicate a single north Atlantic stock of blue shark. The small number of US and Irish tagged blue sharks recaptured from the Mediterranean Sea led to the decision to consider the population in that sea as a separate stock. RÉSUMÉ Les requins peau bleue capturés dans les pêcheries sportives irlandaises font l’objet d’un marquage depuis 1970. La série temporelle des données de marquage-recapture est la seconde, par ordre d’importance, dans l’hémisphère Nord. Au total, 16.719 requins peau bleue ont été marqués de 1970 à 2001. La plupart des poissons marqués dans les eaux côtières irlandaises étaient des femelles. Il existe des preuves que cette espèce entreprend une migration cyclique, dans le sens des aiguilles d’une montre, dans l’Atlantique Nord. La majorité des recaptures au large de l’Europe de l’ouest étaient des femelles. La plupart des recaptures provenaient des palangriers espagnols ciblant les thonidés et l’espadon dans une zone comprise entre la Péninsule Ibérique et les Açores. Cette flottille était suivie, par ordre d’importance, par les palangriers japonais et coréens au large de l’Afrique occidentale, incluant les Canaries et le Cap-Vert. Les palangriers américains et canadiens, opérant dans la zone des Grands Bancs, constituaient la plus grande source de retour de marques suivante, eux-mêmes suivis par les pêcheurs sportifs au NE des Etats-Unis. Ces résultats indiquent l’existence d’un stock unique de requins peau bleu dans l’Atlantique Nord. Le faible nombre de requins peau bleue marqués par les Etats-Unis et l’Irlande recapturés en Méditerranée a mené à la décision de considérer la population de cette mer comme un stock distinct. RESUMEN Las tintoreras capturadas por las pesquerías de recreo irlandesas se han marcado desde 1970. La serie temporal de datos de marcado y recuperación de marcas es la segunda más larga en el hemisferio norte. Se marcaron un total de 16.719 tintoreras desde 1970 hasta 2001. La mayoría de los especimenes marcados en las costas irlandesas eran hembras. Hay evidencias de que esta especie realiza una migración cíclica, siguiendo la dirección de las agujas del reloj, en el Atlántico Norte. La mayoría de las recuperaciones de marcas realizadas en las aguas de Europa occidental eran hembras. La mayoría de las recuperaciones se produjeron en palangreros españoles que pescaban túnidos y pez espada en la zona entre la Península ibérica y las islas Azores. Le siguen en importancia. en cuanto a recuperaciones, las flotas de palangreros japoneses y coreanos que pescan en las aguas de África occidental, incluidas las islas Canarias y Cabo Verde. Los palangreros canadienses y estadounidenses que operan en la 1 Central Fisheries Board, Mobhi Boreen, Glasnevin, Dublin 9, Ireland, Marine Institue, GTP Parkmore, Galway, Ireland (author for correspondence), E mail: [email protected]

1171

zona de los Grandes Bancos fueron la siguiente fuente importante de recuperación de marcas, seguidas por los pescadores de recreo de la parte noreste de Estados Unidos. Los resultados apuntan a la existencia de un único stock de tintorera en el Atlántico norte. El pequeño número de tintoreras marcadas por barcos irlandeses y estadounidenses que se ha recuperado en el Mediterráneo llevó a tomar la decisión de considerar a la población de dicho mar como un stock independiente. 1. Introduction The Central Fisheries Board is Ireland’s statutory agency to conserve, develop, protect and promote inland fisheries and sea angling resources. The Central Fisheries Board’s Marine Sportfish Tagging Programme was initiated in 1970 in conjunction with selected skippers of angling charter vessels. However since its inception it has provided a large data set on the movements, migrations and biology sharks and rays, particularly the blue shark, thornback ray, undulate ray, tope, porbeagle and angel shark. The results of these studies have been reported by Fitzmaurice and Green (2000). As part of a new partnership with the Irish Marine Institute, the CFB collated and analysed the data from the programme. Under the DELASS project (Heessen 2003) data on blue shark Prionace glauca, were analysed to investigate stock structure and basic biology. 2. Materials and methods Preliminary studies of Prionace glauca used a variety of tags, including Petersen Discs, “Spaghetti tags” and Jumbo Rototags. The latter, used for tagging cattle in Ireland, were chosen for ease of supply and use. Skippers were issued with logbooks to record the following information: tag number, species, location, length, date, and other observations. Some skippers recorded the weight of the species. Over 70 charter skippers were involved in collecting the information. A total of 16,719 blue sharks were tagged from 1970 and 2001. Skippers were issued with logbooks to record the following information: tag number, species, location, length, date, and other observations. Some skippers recorded the weight of the species while others recorded the wingspan. The wingspan of the fish should be less than its length so the minimum value of the two was taken when both were reported. Initially a monetary award was given, but more recently, T-shirts are awarded. A list of reliable skippers was compiled. Their information, along with that of the CFB biologists was used for the development allometric conversion factors. Allometric conversion factors based on these data are presented in Table 1. 3. Results and discussion The numbers tagged and recaptured are presented in Figure 1. Length frequencies for blue sharks tagged by the Central Fisheries Board are presented in Figure 2, by 10 cm group. There is some error in these data, because not all tagging vessels had access to accurate measuring equipment. However the size range of blue sharks tagged in the area is 60 cm to 240 cm. The main modes appear between 120 and 130 cm, between 140 and 160 cm and between 170 and 190 cm. In a study of blue sharks in offshore waters SW of Ireland beyond 11ºW (McNaughton et al. (1998) found a similar length range, but with a modal length between 120 and 140 cm. The sex of most tagged fish was not always recorded, but it can be seen from Figure 3 that the majority of the sharks that were sexed were female. The dominance of females in Irish coastal waters was previously reported by Crummey et al. (1991) and in the coastal waters of southwest England (Stevens, 1974). This is in contrast to the situation reported by McNaughton et al. (1998) for offshore waters in Division VIIk and VIIIe (west of 11oW), where the sex ratio was approximately 1:1. Thus it appears that larger female sharks move further inshore than males. The distribution of recaptures by sex is presented in Figure 4. The majority of recaptures off western Europe were females. Kohler et al. (1998) also reported a dominance of females in this area. However these authors reported a predominance of males off west Africa and around Cape Verde. There is no similar trend in the recaptured Irish-tagged sharks. However this may reflect the dominance of females in the tagging area, around Ireland. In contrast, blue sharks on the Grand Banks and off Newfoundland were predominantly (86%) male (McKenzie and Tibbo 1964). 1172

Most recaptured Irish-tagged blue sharks came from Spanish tuna and swordfish longliners working in the area between Iberia and the Azores. The next most important fleets were the Japanese and Korean longliners off west Africa, including the Canaries and Cape Verde. U.S. and Canadian longliners working off the Grand Banks area were the next biggest source of returned tags, followed by sport fishermen in the NE USA. Recaptures of sharks tagged within one month were from off Ireland, along the edge of the continental slopes of the Bay of Biscay and in the area between Iberia and the Azores. There is a clear trend for sharks that are recaptured further away from Ireland to have been at liberty for longer periods (Figure 5). All sharks recaptured south and west of Cape Verde, off South America, the Caribbean and off US and Canada had been at liberty for more than 6 months. The mean nominal distance travelled by sharks increased up to 4 years at liberty. However there was evidence of a decrease in distance from tagging site for sharks at liberty for more than 4 years (Table 2). No sharks at liberty for more than 4 years were found off Ireland, in the Bay of Biscay or between Europe and the Azores (Figure 6). Sharks recaptured south of 25oN tended to be at liberty for more than a year. Specimens recaptured around Ireland, the Bay of Biscay and west of Iberia were mainly sharks at liberty for up to 2 years. The season (quarter) of recapture is shown in Figure 7. Sharks recaptured off Ireland and in the Bay of Biscay were mainly in the third quarter (July to September). Sharks recaptured west of Iberia and around the Azores were mainly taken in the second quarter of the year (April to June). Recaptured sharks off NE America were mainly taken in the third quarter. There is no clear trend in the season of recapture off west Africa or around Cape Verde. These results may reflect the movements of fishing fleets as much as they reflect blue shark movements. Based on the percentage recapture rate there is evidence that this species undertakes a cyclical migration, in a clockwise direction in the north Atlantic. Sharks tagged off Ireland move in a generally south-westwards direction. (Fitzmaurice and Green 2000). Those tagged off northeast USA moved either eastwards or southeastwards (Kohler et al. 1998). These results indicate a migration eastwards, probably along the Gulf Stream, from America to Europe. Migration from Europe south-westwards seems to be along the Atlantic North Equatorial current. An interesting result becomes apparent when both Irish and U.S. recapture data sets are compared. There is a large area roughly bounded by 17ºN and 30ºN and 30ºW and 50ºW for which there are no recapture records. This tends to support the view that there is a cyclical migration. However further analysis of both recapture datasets is required, including a study of effort patterns of high seas fishing fleets, before firm conclusions can be made on the movements of this species. A very small number of blue sharks tagged off the United States were recaptured south of the equator (Kohler et al. 1998). One shark tagged off southwest England was recaptured in the South Atlantic too (Stevens 1990). Whilst a number of recaptured sharks, tagged off Ireland, were taken within 150 miles of the equator, none were taken in the southern hemisphere (Fitzmaurice and Green 2001). 4. Conclusions Based on these results, it was considered by DELASS that a single stock exists in the north Atlantic, and this was adopted for stock assessment purposes in Task 4. The small number of U.S. and Irish tagged blue sharks recaptured from the Mediterranean Sea led to the decision to consider the population in that sea as a separate stock. For the purposes of stock assessment, separate analyses should be carried out for the North Atlantic and the Mediterranean. There is no information in this study on the structure of stocks of blue shark, south of the equator, but they are most probably a separate stock from that in the North Atlantic. References CRUMMEY, C., M. Ronan and E. Fahy. 1991. Blue shark Prionace glauca (L.) in Irish waters. Irish Naturalists Journal, 23 (11): 454–456. FITZMAURICE, P. and P. Green. 2000. Results from tagging of blue shark in Irish waters. Irish Scientist Yearbook, 3: 51. 1173

HEESSEN, H.J.L. (ed) 2003. Report of the Delass Project. KOHLER, N.E., J.G. Casey and P.A. Turner. 1998. NMFS cooperative shark tagging program, 1962-1993: An atlas of shark tag and recapture data. Marine Fisheries Review, no 60, National Marine Fisheries Service, 86 pp. MCKENZIE, R.A. and S.N. Tibbo. 1964b). A morphometric description of blue shark (Prionace glauca) from Canadian Atlantic waters. Journal of the Fisheries Research Board of Canada, 21(4): 865–866. MACNAUGHTON, R., E. Rogan, V. Hernandez-Garcia and C. Lordan. 1998. The importance of cephalopods in the diet of blue shark (Prionace glauca) south and west of Ireland. ICES-CM-1998/M:7, 12 pp. STEVENS, J.D. 1976a). First results of shark tagging in the North-East Atlantic. 1972–1975. Journal of the Marine Biological Association of the United Kingdom, 56: 929–937. Table 1. Allometric conversion factors for blue shark, based on sportfish data. Slope

SE Slope

r2

N

Length to round weight -4.977

1.542

0.3654

0.6

14

Prionace glauca (f)

Length to round weight -7.899

2.171

0.0821

0.794

183

Prionace glauca (both)

Length to round weight -5.775

1.739

0.5156

0.672

558

Prionace glauca (both)

Fork length to TL

1.191

0.911

234

Species

Type

Prionace glauca (m)

Intercept

2.605

Table 2. Nominal distance travelled by tagged blue sharks and time at liberty in Irish waters. Years free

< 1 year

1-2 years

2-3 years

3-4 years

> 4 years

Mean of Distance (miles) 941

1521

1651

1908

1810

1 S.D of Distance

762

817

849

546

559

1174

2000 1800 1600 1400 1200 1000 800 600 400 200 0 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Number

Numbers Tagged

120 110 100 90 80 70 60 50 40 30 20 10 0

Numbers recaptured

1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Number

year

year

Figure 1. Number of blue sharks tagged and recaptured by year.

1400 1200

Number

1000 800 600 400 200 0 0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Length (cm)

Figure 2. Length frequency of tagged blue sharks, marked by the Irish Central Fisheries Board, length increments by 10 cm groups, N = 8,804. 1175

Figure 3. Locations around Ireland where blue sharks were tagged by the Central Fisheries Board Marine Sportfish Tagging Programme. Sex ratio and numbers tagged are indicated.

1176

Figure 4. Location of recaptures of blue sharks tagged by Irish Central Fisheries Board, by sex.

Figure 5. Variation in weeks at liberty after tagging of recaptured blue sharks tagged by Irish Central Fisheries Board. 1177

Figure 6. Variation in days at liberty after tagging of recaptured blue sharks tagged by Irish Central Fisheries Board.

Figure 7. Location of recaptures of blue sharks tagged by Irish Central Fisheries Board, by month of recapture, 0-2 = January to March. 1178