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Study on Eastern Atlantic and Mediterranean Bluefin Tuna migrations using 'pop off satellite tags'.


The aim of this project is to study the migration pattern and the dispersion rates of the Mediterranean bluefiin tuna both in the Mediterranean and in the Atlantic in relation to spawning and nursery grounds and with particular attention to the acquisition of knowledge on the mechanisms of migration and interaction between environmental factors and fish behaviour.

The research project will be built on the following complementary objectives:
a) tagging experiments in the Mediterranean using the "pop off satellite tags" which provide a single location point of the fish. It is like having 100% return from conventional tagging (Block 1996);
b) collection of data on conventional tagging and spawning and nursery areas for bluefin tuna in the Mediterranean;
c) analysis of the data from the tagging experiments using electronic tags and re-evaluation of conventional tagging data in the light of information obtained by satellite on environmental parameters.

These tagging experiments will be useful to verify the applicability of the electronic tags and to prepare the partners to acquire experience for a future greater project regarding the migration pattern and mixing rates of Western and Eastern Atlantic bluefin tuna and other large pelagic fish stocks.

The project is to be included in the priority area 5.1.1 "Effect of environmental factors on fish and fisheries", as it stresses the understanding of how the environmental factors determine the migration patterns and distribution of one of most important commercial species of large pelagic fish.. The project is also relevant to the task specified under paragraph 5.4.1 "Behaviour and strategies in the fishing sector".

As we know, ICCAT has been carrying out for many years the evaluations of Atlantic bluefin tuna stocks, on the hypothesis of the existence of two separated stocks having limited rates of exchange: the Eastern-Atlantic stock including the Mediterranean, and the Western-Atlantic stock.

In the meanwhile, ICCAT has always suggested an increase in the tagging campaigns in order to know better the migration schemes and the fish interexchanges between the two areas, because a good stock assessment and management requires to know the stock structure and limits and therefore it recommends to give priority, in the research planning, to the investigations on the stocks identities. They mainly adviced to use the tagging to study the animal interexchanges between the Western and the Eastern Atlantic (SCRS, 1993).

So far, many bluefin tunas have been tagged on the Atlantic Eastern coast and in the Mediterranean by traditional tags.(Aloncle 1973; Arena et Li Greci 1970; Brethes 1978-1979; Brethes et Mason 1979; C.I.E.S.M. 1972; Cort 1990; Cort et de la Serna 1993; Hamre 1963; Heldt 1927; Lamboeuf 1975; Mather et al. 1973; Rey et Cort 1986; Rodriguez-Roda 1969-1980; Sella 1929-1932; Vilela 1960). Summarized data are reported by Cort et de la Serna (1993) and Cort et Liorzou (1994). However, the interexchange rates between the two areas of the Western and Eastern Atlantic remain unknown, as well as the interexchange rates between the Western and Eastern Mediterranean.

Actually, new suggestions came from ICCAT in November 1994: "Better knowledge of the structure of the stock would be valuable for improving assessment and management... ". " The traditional tagging activities on juvenile as well as on fish should be continued and even increased in order to gain better knowledge on the migration schemes and the interchange between areas. This is particularly true for the Eastern Mediterranean where little tagging is carried out, and consequently little is known about the mixing rate with the Western basin. It is further recommended that all types of "intelligent tags" be evaluated to allow the monitoring the f sh and their movements" (SCRS, 1994).

The intelligent tags mentioned by ICCAT could be the three types of electronic tags following: the above-mentioned "pop off satellite tags" (Block, 1996), "data storage tags" or "archival tags " (Smith and Goodman, 1986; Ekstrom, 1994: Metcalfe et al. 1994a; ), which record some environmental parameters and give information about their influence on the migrations and geographical indications on the trip of bluefin tuna during its life at liberty and "pop off archival satellite tags" (Block, 1996), which combine the characteristics of both the "pop off satellite tags" and "electronic data storage tags".

Since (a) the "data storage tagging" is very expensive because the high cost of both the tags and the bluefin tuna, (b) the success of their use in the tagging experiments depends on the recoveries of the tags, that are generally not very high with a percentage of 4.5% (J.L. Cort and B. Liorzou, 1994), (c) because of the high number of countries within the Mediterranean that are likely to find our tags, a good strategy to begin the research into the Mediterranean seems to be the use of "pop off satellite tags", the information of which are not connected with the recoveries of the tags. Actually, the "pop off satellite tag" is simply a very small satellite transmitter (35g) that will come offonce and float to the surface, indicating the position of the fish. In this way, "pop off satellite tags" will be used to obtain information on the movements and the dispersal of 100% of the tagged fish. The recapture data of the pop off tags will be associated to the environmental parameters of the areas obtained from satellite to gather information on the behaviour of bluefin tuna associated with migration and seasonal changes in distribution.

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Universita degli Studi di Bari
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Via Giovanni Amendola 165/A
70126 Bari

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