SIZE FREQUENCY COMPOSITION OF THE BLUEFIN TUNA CATCHES IN THE TYRRHENIAN SEA AND IN THE STRAITS OF SICILY IN THE PERIOD

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1 SCRS/2002/046 Col. Vol. Sci. Pap. ICCAT, 55(1): (2003) SIZE FREQUENCY COMPOSITION OF THE BLUEFIN TUNA CATCHES IN THE TYRRHENIAN SEA AND IN THE STRAITS OF SICILY IN THE PERIOD A. Di Natale, A. Mangano, A. Asaro, M. Bascone, A. Celona, A. E. Navarra, M. Valastro, and B. Zava 1 SUMMARY Major changes that happened in the bluefin tuna fishery after the adoption of the quota system in 1998 strongly affected the ability to collect good statistical data: fishermen are more afraid and the development of the tuna farming has reduced the availability of fish for the purse-seine fishery. For this reason, all the data collected are becoming more important for the statistical system. These bluefin tuna data collected, with great difficulty, are from one of the most important areas in the Mediterranean, and they can provide a comparison between the previous data sets and the current situation. RÉSUMÉ Les changements importants survenus dans la pêcherie de thon rouge après l adoption du système de quotas en 1998 ont fortement affecté la capacité à recueillir de bonnes données statistiques : les pêcheurs sont plus inquiets et l essor de l élevage thonier a réduit la disponibilité du poisson pour le sennage. C est pourquoi toutes les données qui sont collectées deviennent de plus en plus importantes pour le système statistique. Recueillies avec de grandes difficultés, ces données sur le thon rouge proviennent de l une des principales zones de la Méditerranée. Elles peuvent fournir une comparaison entre les jeux de données antérieurs et la situation actuelle. RESUMEN Tras la adopción del sistema de cuota, en 1998, se han producido importantes cambios en la pesquería de atún rojo que han afectado a la capacidad de recopilar buenos datos estadísticos: los pescadores tienen más miedo y el desarrollo de la cría de túnidos ha reducido la disponibilidad de peces para la pesquería de cerco. Por ello, todos los datos recopilados son cada vez más importantes para el sistema estadístico. Estos datos del atún rojo, recopilados con gran dificultad, provienen de una de las zonas más importantes del Mediterráneo y pueden proporcionar material para una comparación entre los anteriores conjuntos de datos y la situación actual. KEY WORDS Thunnus thynnus, bluefin tuna, Mediterranean fishery. 1 Aquastudio Research Institute, Via Trapani 6, Messina, Italy. 171

2 1. INTRODUCTION The adoption by ICCAT of the quota system for the bluefin tuna in the Mediterranean made a dramatic change in all the fishery: the national governments enforced the quota, establishing criteria for the management of the situation, the fishermen changed the usual behaviour to face the new regulation, the market adopted new strategies. At the same time, the tuna farming and the prices on the Japanese market induced fishermen and enterprises to develop this activity in the Mediterranean, with several floating cages in various Countries (Miyake et al., 2002). The purse seine fishery become the most important provider for the live tunas (minor quantities are provided by some tuna traps), and fishes are transferred at sea from the purse seine to floating cages. In the past, huge quantities of bluefin tuna size data were collected from the purse seine fishery and their component is quite important in the ICCAT data bank, used for the stock assessment of this species. In the last two years the situation shown a dramatic change, sometimes with no more tunas landed by purse-seiners in Sicily or in the Southern Tyrrhenian areas, because all the catches were moved to floating cages. This fact, besides of the efforts and the difficulties to follow the new situation, is raising several problems and created a big hole in the availability of data, cutting the historical series. At the same time, the progressive adoption of the driftnet ban (fully enforced by Italy since January 1, 2002), reduced in a significant manner the good quality data comin g from this fishery In parallel, even the data from the longline fishery became more difficult to collect, due to the adoption of different strategies to land tunas in some important fleets (adoption of collective carrying vessels, new landing places to minimize controls, etc.), while the movements of the fleets increased a lot compared to the past, sometimes including foreign landing ports. To face these difficult problems, Aquastudio increased its capacity to collect data on other fisheries (like the traditional hand line fishery in the Strait of Messina), and the results are included in this paper. This work was partially funded by the Italian Direction General for Fisheries and Aquaculture (Law 41/72), the EC General Directorate for Fishery (projects 97/0029, 97/0046, 97/0074, 98/0034 and 99/0032) and by Aquastudio. 2. METHODS The methodology adopted to collect the size frequencies of the bluefin tuna (Thunnus thynnus) along the Sicilian coastline in the Strait of Sicily and along all the coasts of the southern and central Tyrrhenian Sea (Italian Regions: Sicily, Calabria, Basilicata, Campania and Lazio) was basically the same adopted in the past in various projects (Di Natale, 1990; Di Natale et al., 1998, Cau et al., 1999; Di Natale, 1999). The basic structure of the landing ports was kept in the same manner (Ponza in the central Tyrrhenian Sea, Lipari, Milazzo and Palermo in the southern Tyrrhenian Sea and Favignana, Marsala and Mazara del Vallo in the Straits of Sicily). In addition to these traditional landing points used for sampling, Aquastudio decided to include the Straits on Messina, with a sampling point in the small 172

3 fishermen village of Ganzirri (Messina), where the traditional hand line fishery and the harpoon fishery are still practiced. In most of the places, the landing control was carried out regularly (two weeks per month), while in other places (Trapani, Messina) it was on opportunistic basis, trying to cover all the positive fishing days. The length data were grouped per gear and area, like for the previous statistics, to make them comparable with the older. 3. THE DATA In 1998, it was possible to obtain 5690 length data, 97.5% from the Straits of Sicily and 2.5% from the central and southern Tyrrhenian Sea. Most of the data (80.8%) are from the tuna purse-seine fishery, while the other 19.2% are from various gears (hand lines, driftnets, longlines and tuna traps). Some fishes from the Tyrrhenian Sea were below the minimum size (due to hand lines used in nonprofessional fishery and driftnets), but they reached 1.04% of the total sample. Due to the large quantity of bluefin tuna samples from the tuna purse seine fishery, in 1998 the average size (LF) was cm (Tables 1-8). In 1999, the total number of length data decreased to 3256 samples, 91.6% from the Straits of Sicily and 8.4% from the central and southern Tyrrhenian Sea. Most of the data (73.5%) are from the tuna purse seine fishery, while the other 26.5% are from various gears (hand lines, driftnets, longlines, small purse seines and tuna traps). Even this year, some fishes from the Tyrrhenian Sea were below the minimum size (largely due to small purse seines, but also to hand lines used in non-professional fishery, longlines and driftnets), and they reached 8.2% of the total sample. Due to the large quantity of bluefin tuna samples from the tuna purse seine fishery, partially balanced by the small fishes, in 1999 the average size (LF) decreased to cm (Tables 9-16). In 2000, the total number of length data was about the same of the previous year, 3290 samples, 71.9% from the central and southern Tyrrhenian Sea and 18,1% from the Straits of Sicily, due to some catches by the tuna purse seines in the traditional Tyrrhenian area. Most of the data (52.5%) are still from the tuna purse seine fishery, while the other 26.5% are from various gears (hand lines, driftnets, longlines, small purse seines and tuna traps). Some fishes from the Tyrrhenian Sea are below the minimum size (1.09% of the total sample), but only due to driftnets, because the juveniles were far from the coastal areas. Due to the large quantity of bluefin tuna samples from the tuna purse seine fishery, in 2000 the average size (LF) was cm (Tables 17-22). In 2001, due the fact that all the purse-seine catches were moved to floating cages and also to the lack of research funding, the total number of length data decreased to only 597 samples, all from the central and southern Tyrrhenian Sea. Most of the data (77.9%) are from the traditional hand line fishery in the Strait of Messina, while the other 22.1% are from driftnets and longlines. Only a very few fish are below the minimum size (0.59% of the total sample) because the juveniles were far from the coastal areas. Due to the lack of all bluefin tuna samples from the tuna purse seine fishery, in 2001 the average size (LF) decreased to cm (Tables 23-24). 4. DISCUSSION From the above-mentioned data sets, it appears quite clear that the situation of the data collection is deteriorating quite quickly in this area, due to the following factors: Important reduction of the purse-seine landings; Tuna farming transhipments and fattening activities; 173

4 Driftnet ban; High migratory behaviour of the longline fleet; Landing in foreign harbours; Significant reduction of research funding. At the same time, the influence of biological factors, like the different patterns of the juvenile bluefin tuna distribution, is clearly affecting the size distribution of the sample. According to the several observations made at sea and to the fishermen s reports, the presence of the bluefin tuna in the central Mediterranean Sea appears quite relevant in the last five years while, according to the eastern Atlantic (including the Mediterranean) population model outputs, this fact was not expected, even if the model was not geographically oriented. Besides of these statements, examining the last four years data it appears that the bluefin tuna length composition of the stock in the area is almost stable, with a high variability from sample to sample and a more limited variation from year to year, due to the various types of gear used and the origin of the samples. BIBLIOGRAPHY CAU A., G. Cavallaro, G. De Metrio, A. Di Natale, G. Marano, L. Orsi Relini, C. Piccinetti Revision of recent bluefin tuna catches in Italy. SCRS/98/89, Genova, September 1998, ICCAT Coll. Vol. Sci. Pap., XLIX: DI NATALE, A., Bluefin tuna (Thunnus thynnus L.) and Albacore (Thunnus alalunga Bon.) fishery in the Southern Tyrrhenian Sea: surveys. FAO/GFCM - ICCAT Expert Consultation on large pelagic species, Bari, Coll.Vol. Scient.Pap., XXXIII: DI NATALE A., A. Mangano, E. Navarra, M. Valastro Osservazioni sulla pesca dei grandi Scombroidei nei bacini tirrenici e dello Stretto di Sicilia. In. Le Ricerche sulla Pesca e sull'acquacoltura nell'ambito della Legge 41/82. Parte 1. M.P.A., Roma: DI NATALE A., La ricerca sulla pesca: I Grandi Pelagici. In. Le Ricerche sulla Pesca e sull'acquacoltura nell'ambito della Legge 41/82. Parte 4: Relazioni. M.P.A., Roma, 1999; Biol. Mar. Medit., 7(4): MIYAKE P.M., J. M. de la Serna, A. Di Natale, A.Farrugia, I. Katavic, N. Miyabe, V. Ticina General review of bluefin tuna farming in the Mediterranean area. ICCAT/GFCM Expert Consultation, Malta, in press: 6p. 174

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