The incidental capture of seabirds by Spanish drifting longline fisheries in the western Mediterranean Sea *

Bycatch by drifting longlines is considered an important threat for the conservation of several seabird species (Brothers et al., 1999). Bycatch by longliners in Antarctic waters is probably the most important threat to albatross populations (Tasker et al., 2000). Scientific data on bycatch in the western Mediterranean Sea are scarce (Cooper et al., in this volume). The western Mediterranean Sea is an important fishing area where the Spanish drifting longline fleet target swordfish (Xiphias gladius), bluefin tuna (Thunnus thynnus) and albacore (T. alalunga). Incidental captures by drifting longliners include several species of seabirds. The International Action Plan of the FAO (IPOA/Seabirds) regarding incidental capture of seabirds by longline fisheries started in 1999. FAO members agreed to develop National Action Plans to assess incidental catch by country (Anonymous, 1999). The Spanish Oceanographic Institute (IEO) developed a monitoring programme of incidental SCI. MAR., 67 (Suppl. 2): 65-68 SCIENTIA MARINA 2003


INTRODUCTION
Bycatch by drifting longlines is considered an important threat for the conservation of several seabird species (Brothers et al., 1999).Bycatch by longliners in Antarctic waters is probably the most important threat to albatross populations (Tasker et al., 2000).Scientific data on bycatch in the western Mediterranean Sea are scarce (Cooper et al., in this volume).The western Mediterranean Sea is an important fishing area where the Spanish drifting longline fleet target swordfish (Xiphias gladius), bluefin tuna (Thunnus thynnus) and albacore (T.alalunga).Incidental captures by drifting longliners include several species of seabirds.
The International Action Plan of the FAO (IPOA/Seabirds) regarding incidental capture of seabirds by longline fisheries started in 1999.FAO members agreed to develop National Action Plans to assess incidental catch by country (Anonymous, 1999).The Spanish Oceanographic Institute (IEO) developed a monitoring programme of incidental  MÍNGUEZ, D. ORO, E. DE JUANA and A. MARTÍNEZ-ABRAÍN (eds.)The incidental capture of seabirds by Spanish drifting longline fisheries in the western Mediterranean Sea* captures by drifting longline fisheries working in the western Mediterranean since 1976.Surveys onboard commercial drifting longliners were carried out from 1999 to 2000 within the project EMTP, a European project on bycatch funded by IEO and EU (DG Fisheries).In this paper, we present the first data on seabird bycatch by Spanish drifting longlines in the western Mediterranean in order to assess the potential importance for seabird conservation.

MATERIAL AND METHODS
The study area includes the fishing area of the Spanish drifting longline fleet in the western Mediterranean.Data on fisheries bycatch were collected from July 1999 to August 2000.Scientific observers worked onboard 18 longliners targeting swordfish, bluefin tuna and albacore from five different base ports (Cartagena, Águilas, Garrucha, Carboneras and Motril).Data collection and survey strategies were divided into two strata by type of fleet and four strata by type of drifting longline gear.A monthly research programme was undertaken along the whole fishing area.Data on 557 fishing sets were collected following standardised fishing parameters: -Detailed description of types of gear used: swordfish, bluefin tuna and albacore drifting longlines and swordfish semipelagic longline.
-Fishing strategies, scheme and fleet dynamics.
-Technical characteristics of fishing sets, including geographic position, duration, depth, baits, type of setting, etc.
-Environmental factors and meteorological conditions, including measures of surface temperatures, and fishing depth and temperature by specific sensors attached to gears.
-Detailed data on commercial and incidental catches of marine species.
Total annual fishing effort by longline type (swordfish, bluefin tuna and albacore) were not available during the analysis as a consequence of the number of landing ports, fishing periods and heterogeneous fleets.
The Spanish drifting longline fleet consists of 72 licence vessels from 4 to 22 m in length.The fishing grounds include a large area of the western Mediterranean between 36º and 44ºN and -02ºW and 05ºE (Camiñas and De la Serna, 1995;Camiñas and Valeiras, 2001).The most important fishing effort is carried out around the Balearic Islands and in the Ibiza Channel.The fleet is very dynamic and often changes fishing methods as well as gear structures.The type of gear used depends on season, target species, fishing yields and fishing area (Valeiras and Camiñas, 2001).
The fleet use 4 types of drifting longlines with some differences in structure and target species: longline for swordfish, longline for bluefin tuna, longline for albacore tuna and semipelagic longline for swordfish.Fishing gears are made up for a main line, from 19 to 60 km in length, equipped with 1,500-4,000 branch lines.Floats keep the main line near the surface.Hooks are baited with fish (e.g.mackerel, sardine, scabbard fish) and shortfin squid (Illex spp.).Semipelagic longlines are set with weights which maintain the gear deeper.Longlines for swordfish are set late in the afternoon and remain overnight.Hauling starts in the first hours of the morning and finishes usually by dawn.Longlines targeting bluefin tuna and albacore can be set during both night and day hours.

RESULTS
A large number of seabirds approach boats during fishing operations.The following species were observed during the onboard surveys attending fishing operations, but no bycatches were recorded: the lesser black-backed gull (Larus fuscus), Audouin´s gull (L.audouinii), the Balearic shearwater (Puffinus mauretanicus), terns (Sterna sp.) and the Euro- Results and bycatch rates by type of drifting longline gear observed (i.e.number of birds/1000 hooks) are given in Table 2. Drifting longlines targeting swordfish had a maximum bycatch rate of 0.0076 birds/1000 hooks.Albacore longline showed a bycatch rate of 0.0234 birds/1000 hooks.Bluefin tuna and semipelagic swordfish gears did not record any incidental catch.

DISCUSSION
Differences in bycatch rates can be explain by differential selectivity of the fishing gears.The gear targeting albacore has smaller hooks and bait (sardine) than the others.Swordfish longlines sink faster and operate deeper than the albacore gear, causing a reduced availability of baited hooks for scavenging seabirds.Furthermore, the fishing strategy targeting albacore includes more day time during setting and hauling operations.Bluefin tuna and semipelagic swordfish gears operate deep due to heavy components and fishing strategy.
Spatial distribution of bycatches was influenced by distance to the coast.Yellow-legged gull bycatches occurred close to the shore, whereas Cory's shearwater bycatches were distributed in pelagic waters often far from the coast (Table 3).These results agree with the different habits known for these two species, but the small number of seabird bycatches do not allow further conclusions on the spatial pattern of bycatch.
Temporal distribution of bycatch shows larger bycatch rates of gulls during the winter months.Fishing effort during March, in southern waters offshore the Balearic Islands, resulted in most of the yellow-legged gull bycatches (i.e. six birds during a single week).Meteorological conditions, rough seas and fishing operations near the shore during March 2000 explain this event, but migratory patterns must also be taken into account as an important factor affecting seabird abundance in time.
Most of the seabirds (n=19) drowned after taking the bait and becoming hooked during setting operations.Two yellow-legged gulls were hooked during line hauling and released alive with slight injuries to their bills.
Preliminary data indicate that seabird incidental catch rates by Spanish drifting longline fisheries in the western Mediterranean targeting tuna species and swordfish are lower than in other fisheries studied.Bycatch rates (birds / 1,000 hooks) in tuna longline fisheries reach 0.360 in Southern African waters SEABIRD BYCATCH BY MEDITERRANEAN DRIFTING LONGLINE 67 and 0.379 in Australasian waters (Ryan and Boix-Hinzen, 1998).Japanese tuna longline fisheries around Australia have caught 0.15 birds / thousand hooks (Gales et al., 1998).Bottom longlining in the Northeast Atlantic reaches bycatch rates of 1.05 birds / 1,000 hooks (Tasker et al., 2000).Comparisons between studies are for guidance only and standardisation of the catch data is necessary to assess differences.Assessment of drifting longline annual total bycatch needs more information on fleet dynamics, fishing periods and target species.
Population impact of bycatches on seabirds is poorly known at the moment.No mitigation measures to reduce seabird mortality were used in the study fishery.Data from Belda and Sánchez (2001) on seabird mortality on longlines around the Columbretes Islands (Spain) suggests that bycatch rates could be very variable at the local level, due to seabird distribution and also to the dynamics of the drifting longline fishing fleet.Owing to the existence of several large breeding colonies of seabirds in the western Mediterranean, seabird bycatch could be a cause for concern.Although our data are preliminary, we suggest the need to improve the monitoring of incidental bycatch of seabirds in the western Mediterranean.

TABLE 1 .
-Observed fishing sets and observed fishing effort (thousands of hooks) by year.

TABLE 2 .
-Total incidental catch and bycatch rates by species (number of birds / 1000 hooks).