SCRS/2008/130
Collect. Vol. Sci. Pap. ICCAT, 64(5): 1522-1536 (2009)
DATA ON BLUE SHARK FROM THE IRISH RECREATIONAL FISHERY P. Green 1, D. O’Sullivan1, W. Roche1, P. Fitzmaurice1, D. Stokes 1, S. O’Reilly1, G. Kierse1, M. Kenny1, S. Mariani, 2 and M. Clarke 3
SUMMARY The Irish 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 of sharks and rays. The results of these studies have been reported by Fitzmaurice and Green (2000), and presented to ICCAT (Fitzmaurice et al. 2003; 2004). As part of a partnership with the Irish Marine Institute and University College Dublin, the data collected since 2002 have been updated and are presented in this document. The blue shark is one of Ireland’s largest marine sport fishes and regarded as one of our most valuable angling species (Crummey et al. 1991; Fitzmaurice and Green 2000). They are present in Irish waters during mainly the summer months and are caught by sport fishermen using rod and reel. This document presents updated information on tag and recaptured blue shark and on CPUE data from recreational fisheries logbooks. The tagging data represent a long time series, from 1970 to 2006, whilst the CPUE data span from 1978 to 2006. RÉSUMÉ En 1970, le Comité central irlandais des pêcheries a lancé un Programme de marquage de poissons capturés dans le cadre de la pêche sportive, en conjonction avec les capitaines sélectionnés de navires de pêche à la ligne affrétés. Or, depuis son début, ce programme a fourni un vaste jeu de données sur les déplacements, les migrations et la biologie des requins et des raies. Les résultats de ces études ont été consignés par Fitzmaurice et Green (2000), et présentés à l’ICCAT (Fitzmaurice et al. 2003; 2004). Dans le cadre d’un partenariat avec l’Institut maritime irlandais et l’Université de Dublin, les données recueillies depuis 2002 ont été actualisées et sont présentées dans ce document. Le requin peau bleue est l’un des plus grands poissons capturés par la pêcherie sportive irlandaise et il est considéré comme l’une de nos plus précieuses espèces pour la pêche à la ligne (Crummey et al. 1991; Fitzmaurice et Green 2000). Il est présent dans les eaux irlandaises principalement pendant les mois d’été et il est capturé par les pêcheurs sportifs à la canne et moulinet. Le présent document fournit des informations actualisées sur le requin peau bleue marqué et récupéré et sur les données de CPUE des livres de bord des pêcheries récréatives. Les données de marquage représentent une longue série temporelle, de 1970 à 2006, tandis que les données de CPUE s’étendent de 1978 à 2006. RESUMEN El Programa de marcado de peces marinos deportivos de la Irish Central Fisheries Board se inició en 1970 junto con patrones seleccionados de buques de pesca con caña fletados. Sin embargo, desde su inicio ha facilitado grandes conjuntos de datos sobre los movimientos, migraciones y biología de los tiburones y las rayas. Los resultados de etos estudios han sido comunicados por Fitzmaurice y Green (2000) y presentados a ICCAT (Fitzmaurice et al. 2003; 2004). Como parte de una asociación con el Instituto Marino irlandés y la Universidad de Dublín, los datos recopilados desde 2002 han sido actualizados y se presentan en este documento. La tintorera es uno de los peces deportivos marinos más grandes de Irlanda y se considera una de las especies de pesca con caña más valiosas (Crummey et al. 1991; Fitzmaurice and Green 2000). Está presente en aguas irlandesas principalmente durante los meses de verano y la capturan los pescadores deportivos utilizando la caña y carrete. Este documento presenta información actualizada sobre tintoreras marcadas y recapturadas y sobre 1
Central Fisheries Board, Mobhi Boreen, Glasnevin, Dublin 9, Ireland. School of Biology & Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland. 3 Marine Institute, GTP Parkmore, Galway, Ireland (author for correspondence);
[email protected] 2
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datos de CPUE de los cuadernos de pesca de las pesquerías de recreo. Los datos de marcado representan una larga serie temporal, desde 1970 a 2006, mientras que los datos de CPUE abarcan desde 1978 a 2006. KEYWORDS Blue shark, catch rates, tagging, recreational fisheries
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 of sharks and rays. The results of these studies have been reported by Fitzmaurice and Green (2000), and presented to ICCAT (Fitzmaurice et al. 2003; 2004). As part of a partnership with the Irish Marine Institute and University College Dublin, the data collected since 2002 have been updated and are presented in this document. The blue shark is a large pelagic carcharhinid that is widespread throughout both temperate and tropical oceans. They are viviparous and can reach total lengths in excess of 300cm. Blue sharks commonly occur in large aggregations that can be both size and sex segregated. Blue sharks are caught on a variety of gears in the North Atlantic Ocean, including longlines, gillnets, handlines, rod and reel, trawls, trolls and harpoons, but they are mostly caught incidentally as bycatch in pelagic longline fisheries targeting tuna and swordfish. The blue shark is one of Ireland’s largest marine sport fishes and regarded as one of our most valuable angling species (Crummey et al. 1991; Fitzmaurice and Green 2000). They are present in Irish waters during mainly the summer months and are caught by sport fishermen using rod and reel. This document presents updated information on tag and recaptured blue shark and on CPUE data from recreational fisheries logbooks. The tagging data represent a long time series, from 1970 to 2006, whilst the CPUE data span from 1978 to 2006.
2. Materials and methods 2.1 Tagging studies 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 16 803 blue sharks were tagged from 1970 and 2006 using Jumbo Rototags. Blue sharks tagged using other types of tags were not included in this analysis because those tags would have different retention, visibility and application properties. Also tagging data from 2007 is not reported on, as all information from that time period was not available. 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 upon communication of re-capture data, 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 of allometric conversion factors. Allometric conversion factors based on these data are presented in Table 1. The tagging data were collated in the standardised ICCAT format for exchange of tagging data. They were submitted to ICCAT in August 2008, for use in stock assessment of blue shark. The data will be published separately in the scientific literature and then further studies to combine with other tagging data, from the US and UK will be investigated. 1523
2.2 Nominal CPUE data The Central Fisheries Board has compiled sea angling statistics from skippers’ logbooks in the Irish sportfish fishery since 1978. This information includes the number of days an individual skipper spent shark fishing in a given year, the number of angling boats operating under a given skipper / owner and the number of anglers who fished on the skippers vessels in a given year. From these data it has been possible to determine the catch per unit effort of blue shark over a time period where sufficient data were available both for individual skippers and for the fishery as a whole. Data from 2007 has not been included in the CPUE analysis because all information from this period has not been collected yet. The number of blue shark caught per boat per day, and the number of blue shark caught per angler, per boat and per day (directed shark angling) was also recorded. The number of anglers is taken from CFB angling logbook data given by the owners of the boats and includes data from the ground fish fishery (not directed at sharks). The best index of effort is the number of boats in the fishery. This is because each boat has an associated odour plume. Ten reliable skippers were selected. These have been fishing continuously for a period of at least seven years from 1989-present. The information given by these skippers is considered very reliable because the same skippers have been involved in the work for many years and are responsible for collecting and returning logbooks each year. CPUE for ten reliable skippers is presented up until 2005 only; data from 2006 has not been included because an unstandardised method of data entry was used for this year.
3. Results and discussion 3.1 Tagging studies 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, and separated into 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 30 cm to 300 cm. The main modes appear between 120 and 130 cm, between 140 and 160 cm and between 170 and 190 cm. Please insert length frequency graph below and number appropriately. In a study of blue sharks in offshore waters SW of Ireland beyond 110 W, McNaughton et al. (1998) found a similar length range, but with a modal length between 120 and 140 cm. The locations of where blue sharks were tagged by the Central Fisheries Board are illustrated in Figure 3. Most blue sharks were tagged off the western coast of Ireland, while some were tagged off south and southeastern Irish shores. This observation is similar to that of Fitzmaurice and Green (2000) who reported a similar pattern. The sex of most tagged fish was not always recorded, but it can be seen from Table 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 110 W), where the sex ratio was approximately 1:1. This shows that females move further inshore than males. The distribution of recaptures by sex is presented in Figure 4. The majority of recaptures off western Europe that were sexed 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). 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. US 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 1524
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 5). Sharks recaptured south of 250N 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 6. 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 4th and 1st quarter of the year. 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 the northeast United States moved either eastwards or south-eastwards (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 US recapture data sets are compared. There is a large area in the centre of the Atlantic where 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 USA 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, 2000). 3.2 Nominal CPUE data CPUE for the entire Irish recreational fishery ranged from 4.18 to 0.73 and fluctuated considerably by year (Figure 7). The decline in the nominal CPUE for all skippers (Table 4), to the lowest in the series in 2004 highlights the current decreasing trend in this recreational fishery. The catch and effort data for the ten reliable skippers are presented in Figures 8 and 9. These also show decreasing trends with less blue sharks being caught and a fewer number of days spent shark fishing. Table 5 shows the numbers of blue sharks caught, days of shark fishing and the CPUE. A similar trend is seen for the 10 reliable skippers (Figure 10), which further refines the current pattern of lower numbers of blue shark. Fishing practices have not changed over the time series. Therefore these data provide a reasonably good estimate of stock abundance in Irish waters. The catch data in this series are considered reliable, as there is no incentive to misreport. Thus the series can be considered as a very reliable CPUE series. Findings from this study need to be given further consideration because of the seriousness of the declining patterns observed. These nominal CPUE data were presented to ICCAT (Fitzmaurice et al. 2004a). However they were not used in further analyses. It was planned to conduct further investigations of these data, and to update the series. However the raw data are not readily available at present. This is because individual records from logbooks remained the property of the individual fishermen and were not archived electronically. A GLM standardisation of these CPUE data would be very useful. Potential categorical variables identified are; SST, salinity, location, year and sea surface roughness. However, at present it is not possible to conduct such work, unless all original records are collected from the participating fishermen, for the entire time series.
4. Conclusions This study suggests that a single stock exists in the north Atlantic, north of the equator. However, further work on genetic stock identification of blue shark is being conducted by Queens University Belfast. In the meantime, based on these result and those of Kohler et al. (19xx) it seems appropriate to consider that a separate stock exists in the North Atlantic. This is the approach taken by ICES and ICCAT. 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. 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 1525
shark, south of the equator, but they are most probably a separate stock from that in the north Atlantic. The next step is to combine these data with other tagging data on North Atlantic blue shark. This work may allow estimates of mortality and population size to be derived. Nominal CPUE data show a decline in most recent years, with minor fluctuations suggesting a continued downward trend is likely. The decline in abundance of blue sharks in the Irish recreational fishery since the late 1990’s is a cause for concern.
5. References Crummey, C., Ronan, M. and Fahy, E., 1991. Blue shark Prionace glauca (L.) in Irish waters. Irish Naturalists Journal, 23(11): 454–456. Fitzmaurice, P. and Green, P., 2000. Results from tagging of blue shark in Irish waters. Irish Scientist Yearbook, 3: 51. Fitzmaurice, P. et al. 2003. Fitzmaurice, P. et al. 2004. Heessen, H.J.L. (ed), 2003. Report of the Delass Project. DELASS - Development of Elasmobranch Assessments. Final Report EU-Study Contract DG XIV 99/055.
Kohler, N.E., Casey, J.G. and Turner, P.A., 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 Tibbo, S.N., 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., Rogan, E., Hernandez-Garcia, V. and Lordan, C., 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. Species
Type
Prionace glauca (m) Prionace glauca (f) Prionace glauca (both) Prionace glauca (both)
Length to round weight Length to round weight Length to round weight Fork length to TL
Intercept
Slope
SE Slope
r2
N
-4.977 -7.899 -5.775 2.605
1.542 2.171 1.739 1.191
0.3654 0.0821 0.5156
0.6 0.794 0.672 0.911
14 183 558 234
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 distance (miles) S.D of distance
1162 735
1665 803
2011 876
2090 858
1956 557
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Table 3. Angling Centres with total number of blue sharks caught with numbers of males, females and unsexed fish at each centre. Angling Centre Achill Ballinskelligs Bay Ballycotton Ballydavid Ballyvaughan Baltimore Belmullet Brandon Burton Port Castletownbere Castletownshend Clew Bay Clifden Clonakilty Cork Harbour Courtmacsharry Crookhaven Crosshaven Derrynane Dingle Doolin Downings Dungarvan Dunmore East Enniscrone Galway Bay Killala Killybegs Kilmore Quay Kinsale Lagos Liscannor Loop Head Lough Swilly Mullaghmore Porcupine Bank Port Na Blagh Roundstone Schull Seafield Shannon Estuary Sligo bay Tralee Bay Union Hall Valentia Island Youghal BLANKS Total Percentages (%)
Male
Female 1
2
46 2
27
147
3 4
3 1 52
2 4 7 6 80 5 2
29 25 6 103 655 39 14 2
5 1
2 2 1 22 3 2 10
88 9 9 5 8 5 362 5 14 37
8
72
2
34
3
13 20
203 1.21
1806 10.75
Unsexed 5 3 1016 274 9 773 10 16 13 10 28 64 612 1987 2329 180 16 9 319 5 460 384 53 308 20 184 3121 379 411 491 7 188 3 17 251 131 8 61 1 76 10 102 448 2 14794 88.04
Total 6 3 1062 278 9 947 10 22 14 66 28 95 641 13 2096 3064 224 32 9 321 5 553 394 9 53 313 30 191 1 3505 387 427 538 7 268 3 17 251 167 8 61 1 76 23 125 448 2 16803 100
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Table 4. Catch, effort and CPUE data for all skippers (1980 value represents a 3 year average between 19781980). Year
Catch
Effort
CPUE
1980
553
211
2.62
1981
169
126
1.34
1982
171
102
1.68
1983
608
163
3.73
1984
386
141
2.74
1985
115
74
1.56
1986
364
140
2.6
1987
275
148
1.86
1988
256
139
1.84
1989
500
238
2.1
1990
907
252
3.6
1991
915
320
2.86
1992
679
277
2.45
1993
1501
359
4.18
1994
1071
339
3.16
1995
1370
498
2.75
1996
1183
337
3.51
1997
1578
411
3.84
1998
749
330
2.27
1999
822
447
1.84
2000
576
362
1.59
2001
832
380
2.19
2002
358
314
1.14
2003
471
284
1.66
2004
128
175
0.73
2005
143
147
0.97
2006
220
140
1.57
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Table 5. Catch, effort and CPUE data for 10 reliable skippers. Skipper A B C D E F G H I J
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 79 170 76 69 50 50 119 90 75 33 33 31 30 5 6 5 9 54 44 10 13 25 11 32 27 11 22 24 18 9 31 11 13 65 21 18 26 39 73 68 41 60 46 59 16 22 15 16 25 5 93 31 28 24 124 16 86 21 40 21 20 14 1 38 39 44 47 97 28 57 70 26 23 16 24 16 9 31 11 4 23 36 28 55 148 144 166 61 55 26 24 2 13 1 16 51 48 33 38 58 41 75 49 33 10 26 27 30 28 6 21 35 53 56 44 12 41 51 60 56 52 45 20 42 12 36 8 21 23 23 19 23 27 1 8 0 5 31 32 9 61 72 52 59 69 14 34 22 29 8 0 11 1 11 288 338 356 329 433 352 574 653 612 316 368 270 291 105 206 64 104
Skipper A B C D E F G H I J
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 41 32 28 28 19 19 39 25 18 18 17 16 15 6 5 4 10 16 11 9 5 6 6 6 5 3 10 13 11 7 14 9 8 25 8 11 11 16 19 24 20 32 27 18 16 8 15 15 14 5 14 14 11 10 24 8 25 10 16 18 11 18 3 26 19 16 17 11 11 17 25 11 13 7 14 11 12 12 14 5 9 14 23 23 43 36 35 24 27 15 10 3 11 1 6 13 19 14 10 10 16 20 18 12 10 9 14 17 11 8 15 19 27 25 21 21 21 17 20 17 20 21 14 18 11 14 5 6 9 10 21 14 13 4 6 5 3 7 11 4 9 9 9 14 13 6 8 15 7 6 6 10 3 3 122 100 149 121 124 124 179 179 165 133 185 139 132 100 102 77 73
Skipper A B C D E F G H I J
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 1.9 5.3 2.7 2.5 2.6 2.6 3.1 3.6 4.2 1.8 1.9 1.9 2.0 0.8 1.2 1.3 0.9 3.4 4.0 1.1 2.6 4.2 1.8 5.3 5.4 3.7 2.2 1.8 1.6 1.3 2.2 1.2 1.6 2.6 2.6 1.6 2.4 2.4 3.8 2.8 2.1 1.9 1.7 3.3 1.0 2.8 1.0 1.1 1.8 1.0 6.6 2.2 2.5 2.4 5.2 2.0 3.4 2.1 2.5 1.2 1.8 0.8 0.3 1.5 2.1 2.8 2.8 8.8 2.5 3.4 2.8 2.4 1.8 2.3 1.7 1.5 0.8 2.6 0.8 0.8 2.6 2.6 1.2 2.4 3.4 4.0 4.7 2.5 2.0 1.7 2.4 0.7 1.2 1.0 2.7 3.9 2.5 2.4 3.8 5.8 2.6 3.8 2.7 2.8 1.0 2.9 1.9 1.8 2.5 0.8 1.4 1.8 2.0 2.2 2.1 0.6 2.0 3.0 3.0 3.3 2.6 2.1 1.4 2.3 1.1 2.6 1.6 3.5 2.6 2.3 0.9 1.6 2.1 0.3 1.3 0.0 1.7 4.4 2.9 2.3 6.8 8.0 5.8 4.2 5.3 2.3 4.3 1.5 4.1 1.3 0.0 1.1 0.3 3.7 2.4 3.4 2.4 2.7 3.5 2.8 3.2 3.6 3.7 2.4 2.0 1.9 2.2 1.1 2.0 0.8 1.4
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2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1985 1984 1983 1982 1981 1980 1979 1978 1977 1976 1975 1974 1973 1972 1971 1970 80
60
40 Numbers
1200
1000
800 Numbers
Numbers Tagged 2000
1800
1600
1400
600
400
200
0
Year
Numbers Recaptured 120
100
20
0
2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1985 1984 1983 1982 1981 1980 1979 1978 1977 1976 1975 1974 1973 1972 1971 1970
Year
Figure 1. Number of blue sharks tagged and recaptured by year.
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Figure 3. Location of tagging of blue sharks by Irish Central Fisheries Board
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Figure 4. Location of recaptures of blue sharks tagged by Irish Central Fisheries Board, by sex.
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Figure 5. Variation in days at liberty after tagging of recaptured blue sharks tagged by Irish Central Fisheries Board.
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Figure 6. Location of recaptures of blue sharks tagged by Irish Central Fisheries Board, by month of recapture.
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4.5 4 3.5
CPUE
3 2.5 2 1.5 1 0.5 0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 Year
Figure 7. CPUE (blue shark per boat per day) for all skippers 1980-2006 (1980 value represents a 3 year average between 1978-1980).
700 600
Blue Shark..
500 400 300 200 100 0 1989
1991
1993
1995
1997
1999
2001
Year
Figure 8. Catch (number of blue shark caught per boat) for 10 reliable skippers.
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2003
2005
200 180 160 140 Days
120 100 80 60 40 20 0 1989
1991
1993
1995
1997
1999
2001
2003
2005
2003
2005
Year
Figure 9. Effort (number of days spent shark fishing) for 10 reliable skippers.
4.0 3.5 3.0
CPUE.
2.5 2.0 1.5 1.0 0.5 0.0 1989
1991
1993
1995
1997
1999
Year
Figure 10. CPUE (blue shark per boat per day) for 10 reliable skippers.
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2001
