By-catches and discards of the Greek swordfish fishery



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By-catches and discards of the Greek swordfish fishery
Panagiota Peristeraki1, Nectarios Kypraios1,

George Lazarakis1, George Tserpes1



SUMMARY
Data on by-catches and discards of the Greek fleets targeting swordfish, collected by observers on board from 2004 to 2006 were analyzed. In terms of weight, the landed swordfish represents the 84% of the total catches, while 14% and 2% correspond to commercial by-catches and discards respectively. The second more abundant species in the catch, in terms of weight, was oilfish (6.1%) followed by bluefin tuna (5.3%) and sharks (1.9%).
RÉSUMÉ
Le présent document analyse les données sur les prises accessoires et les rejets des flottilles grecques ciblant l’espadon, lesquelles ont été recueillies par des observateurs embarqués sur des navires de 2004 à 2006. En termes pondéral, l’espadon débarqué représente 84% des prises totales, tandis que 14% et 2% correspondent à des prises accessoires commerciales et des rejets, respectivement. La deuxième espèce la plus abondante présente dans la capture, en termes pondéral, était le rouvet (6,1%), suivi du thon rouge (5,3%) et des requins (1,9%).
RESUMEN
Se analizaron los datos sobre capturas fortuitas y descartes de las flotas griegas que se dirigen al pez espada, recopilados por observadores a bordo desde 2004 hasta 2006. En términos de peso, el pez espada desembarcado representa el 84% de las capturas totales, mientras que el 14% y el 2% corresponden a capturas fortuitas y descartes comerciales, respectivamente. La segunda especie más abundante en la captura, en términos de peso, fue el escolar (6,1%), seguido del atún rojo (5,3%) y los tiburones (1,9%).
KEYWORDS
Swordfish, by-catch, discards, Mediterranean

1. Introduction
Though several monitoring projects of the Greek longline swordfish fishery have been realized since 1987, information on by-catches (both commercial and discarded) is limited (Megalofonou et al. 2005, Tatamanidis et al. 2006, Tserpes et al. 2006b), mainly due to lack of regular scientific on board observations. Since 2003, in the frames of the National Data Collection Project (NDCP), regular monitoring of the Greek swordfish fishery, including scientific observations on board of commercial longliners, has been implemented. In the present study we attempt to analyze data collected from on board observers, in order to identify the species composition of the catch, as well as, catch and discard rates.
2. Materials and methods
In the frames of the NDCP, data on commercial by-catches and discards of the main Greek fleets targeting swordfish (Tserpes et al. 2006a) are collected on a seasonal basis by observers on board of commercial longline vessels. In the present study, we analyse data from observations covering 196 fishing days (sets) on board of 18 different longline vessels during three fishing periods (2004-2006). Collected data include information on fishing effort, weight and number of captured species. Mean catch per unit effort rates in terms of number (CPUEn=number of fish/1000 hooks) and weight (CPUEw= Round weight in kg /1000 hooks) are estimated for the main commercial and discarded species.

3. Results and discussion
The total fishing effort of the observed trips was 139,710 hooks representing around to 0.7% of the total effort. The mean number of hooks set per fishing day was equal to 715.

The main bulk of commercial by-catches of the fishery consist of bluefin tuna, oilfish and sharks. Other commercial species included in the catch are: dolphin fish, albacore, black skipjack and spearfish. Among sharks the most common is the blue shark (Prionace glauca) representing in terms of number 75% of the total shark catch. Other shark species caught are the shortfin mako (Isurus oxyrhinchus), the thresher shark (Alopias vulpinus), the tope shark (Galeorhinus galeus) and the white shark (Carharodon carharias).


The most common discards are the undersized swordfish and the blue stingray, while incidental catches of sea turtles, ocean sunfish, blackfish and atlantic pomfret are also discarded. A full list of the captured species is given in Table 1.
The catch volume by species in terms of weight and number as well as the corresponding relative percentage in the total catch are shown in Table 2. Swordfish represent the majority of the catches (85% in terms of weight and 72% in terms of number), while oilfish is the second more abundant species (6.1% in terms of weight and 12.6% in terms of number). The second target species of the fishery, the bluefin tuna, is the third more abundant species in terms of weight (5.3%). Sharks represent about 2% of the total catch, significantly lower comparing with the Atlantic swordfish fishery (Mejuto et al. 2006). Differences in the composition of by-catch species are also observed between eastern and western Mediterranean longline swordfish fisheries, with most important the higher proportion of the oilfish in the eastern Mediterranean catches (Castro et al., 1999; Mejuto et al., 2002; Macias et al., 2004).
The discarded swordfish were mostly of lower jaw fork length (LJFL) less than 80cm; thus they can be considered as “zero-class” fish. The majority of them were discarded dead. The discarded swordfish estimated to represent 0.8 % of the total catch in terms of weight and 5.7 % in terms of number.
Only the number of the caught sea turtles was recorded, as the animals were not brought on board to be weighed, but immediately discarded as they were alive in almost all cases. Sea turtle by-catches were observed almost exclusively in three areas: S. Crete, Ionian Sea (SW off Peloponeese) and the area around Kastelorizo.
The mean CPUE rates by species, or group of species -in the case of scarcely caught species- are given in Table 3. The category “sharks” includes all shark species, the category “sea-turtles” include all sea turtle species, and the category “other” include the black skipjack, the spearfish, the ocean sunfish, the blackfish and the atlantic pomfret.
In conclusion, our data indicate that the most abundant species in terms of weight is swordfish, followed by oilfish, bluefin tuna and sharks. Bluefin tuna is a secondary targeted species of the fishery, while oilfish and sharks represent incidental catches. Comparing with previous data, the bluefin tuna catch rates are notably lower, probably due to the decline of the bluefin tuna stocks. In 1998-1999 the bluefin tuna catch rates were reported to be equal to 73 kg/1000 hooks and 1.22 fish /1000 hooks (Tserpes et al., 2000). Those figures are around 5 and 3 times, respectively, higher than those currently estimated.

References
Castro, J., J.M.de la Serna, D. Macias and J. Mejuto. 2000. Estimaciones cientificas preliminares de los desembarcos de especies asociadas realizados por la flota española palangre de sureficie en 1997 y 1998. Collect. Vol. Sci. Pap. ICCAT, 51(6): 1882-1893.
Macias D, MJ. Gomez-Vives and JM. de la Serna. 2004. Desembarcos de especies asociadas a la pesqueria de palangre superficie dirigido al pez espada (Xiphias gladius) en el Mediterraneo durante 2001 y 2002. Collect. Vol. Sci. Pap. ICCAT, 56(3): 981-986.
Megalofonou P, C. Yannopoulos, D. Damalas, G. De Metrio, M. Delflorio, J.M. de la Serna, D. Macias. 2005. Incidental catch and estimated discards of pelagic sharks from the swordfish and tuna fisheries in the Mediterranean Sea. Fisheries Bulletin 103:620-634.
Mejuto J, B. Garcia-Cortes, J.M. de la Serna. 2002. Preliminary scientific estimations of by-catch landed by the spanish surface longline fleet in 1999 in the Atlantic Ocean and Mediterranean Sea. Collect. Vol. Sci. Pap. ICCAT 54:1150-1163.
Mejuto J, B. Garcia-Cortes, JM. de la Serna, A. Ramos-Cartelle. 2006. Scientific estimations of by-catch landed by the spanish surface longline fleet targeting swordfish (Xiphias gladius) in the Atlantic Ocean: 2000-2004 period. Collect. Vol. Sci. Pap. ICCAT 59: 1014-1024.
Tatamanidis G, G. Tserpes, P. Peristeraki. 2006. Spatiotemporal variations in shark-by catches of the Greek swordfish fishery in the Eastern Mediterranean. 8th Hel. Symp. Oceanogr. & Fish. HCMR, Thessaloniki, p 226.
Tserpes, G., P. Peristeraki, C.Koutsikopoulos, G. De Metrio, A. Di Natale. 2000. Final Report of the study project 97/046 “The Swordfish Fishery in the Central and Eastern Mediterranean Sea”.
Tserpes G, DK. Moutopoulos, P. Peristeraki, G. Katselis, C. Koutsikopoulos. 2006a. Study of swordfish fishing dynamics in the eastern Meditarranean by means of machine-learning approaches. Fisheries Research 78:196-202.
Tserpes G, G. Tatamanidis, P. Peristeraki. 2006b. Oilfish and shark by-catches of the Greek swordfish fishery in the E. Mediterranean; a preliminary analysis applied to “presence-absence” data. Collect Vol. Sci. Pap. ICCAT 59: 987-991.


Table 1. Commercial by-catch (BC) and discarded (D) species of the Greek swordfish fishery during the 2004-2006 fishing periods.



Scientific name

Common name

Commercial importance

Thunnus thynnus

Bluefin tuna

BC

Tunnus alalunga

Albacore

BC

Ruvettus pretiosus

Oilfish

BC

Coryphaena hippurus

Dolfinfish

BC

Tetrapterus belone

Spearfish

BC

Euthynus alleteratus

Black skipjack

BC

Prionace glauca

Blue shark

BC

Isurus oxyrhinchus

Shortfin mako

BC

Alopias vulpinus

Thresher shark

BC

Galeorhinus galeus

Tope shark

BC

Carharodon carharias

White shark

BC

Dasyatis violacea

Blue stingray

D

Mola mola

Ocean sunfish

D

Centrolophius niger

Blackfish

D

Brama brama

Atlantic pomfret

D

Careta careta et oth.

Sea turtles

D

Xiphias gladius

0-class swordfish

D



Table 2. Total weight (W = round weight in kg) and number of the fish caught during the trips with observers on board (2004-2006) and percentage rate of the total caught weight and number. (L = landed, D = discarded).

Species name

Total weight (Kg)

Number of fish

% Total W

% Total N

Swordfish (L)

25473.5

1195

84.1

66.5

Bluefin tuna

1807.63

53

5.3

3.0

Albacore

137.4

23

0.4

1.3

Oilfish

2079.1

226

6.1

12.6

Sharks

628.8

23

1.9

1.3

Dolphin fish

172.45

37

0.5

2.1

Swordfish (D)

264.65

102

0.8

5.7

Blue stingray

281.7

115

0.9

6.4

Turtles




22




1.2



Table 3. Mean CPUE by weight and number for the commercial by-catch and discarded species of the Greek swordfish fishery (2004-2006). (L = landed, D = discarded).

Species name

Mean CPUEw (Kg/1000hooks)

StDev

Mean CPUEn (number/1000hooks)

StDev

Swordfish (L)

209.1

121.5

8.8

5.0

Bluefin tuna

14.4

57.7

0.4

1.1

Albacore

0.8

3.1

0.2

0.5

Oilfish

16.1

36.3

1.7

3.7

Sharks

4.2

19.1

0.2

0.6

Dolphin fish

1.2

5.3

0.3

1.1

Swordfish (D)

2.2

4.1

0.7

1.5

Blue stingray

2.0

5.1

0.8

1.9

Turtles







0.2

0.5

Others

0.6

3.6

0.2

0.6




1 Hellenic Centre for Marine Research, P.O. Box 2214, 71003 Iraklion, Greece. E-mail:notap@her.hcmr.gr





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