Line Fishery for Bluefin Tuna in the Italian Seas. Old And

LINELINE FISHERYFISHERY FORFOR BLUEFINBLUEFIN TUNATUNA ININ THETHE ITALIANITALIAN SEAS.SEAS. OLDOLD ANDAND NEWNEW DATADATA

ADDINGADDING ANOTHERANOTHER ELEMENTELEMENT TOTO THETHE PUZZLEPUZZLE OFOF THETHE NATURALNATURAL HISTORYHISTORY OFOF THISTHIS SPECIESSPECIES

Di Natale A.(*), Mangano A.(*), Piccinetti C.(°), Ciavaglia E. (°), Celona A.(*) (*)Aquastudio Research Institute, Messina, Italy (°) Laboratorio di Biologia Marina e Pesca – Università di Bologna in Fano - Italy TWOTWO OPPORTUNITIESOPPORTUNITIES TOTO BETTERBETTER UNDERSTANDUNDERSTAND THETHE BLUEFINBLUEFIN TUNATUNA IN IN THETHE MEDITERRANEANMEDITERRANEAN SEASEA

TheThe traditionaltraditional handhand lineline fisheryfishery inin thethe StraitsStraits ofof Messina.Messina. TheThe newnew longlong--lineline fisheryfishery inin thethe northnorth-- centralcentral AdriaticAdriatic Sea.Sea. THETHE MOSTMOST IMPORTANTIMPORTANT ““TUNATUNA GATEGATE”” INSIDEINSIDE THETHE MEDITERRANEANMEDITERRANEAN SEASEA

There is only one narrow passage along the courses followed by the bluefin tuna in the Mediterranean: the Straits of Messina, about 3 km wide. It links the most important traditional spawning areas in the Mediterranean, the southern Tyrrhenian Sea, to the south Mediterranean basin. A traditional hand line fishery is carried out in the Straits all the year round, targeting on bluefin tuna. Several data sets are available from old papers and more recent research activities. OLDOLD DATADATA SETS:SETS: TRACKSTRACKS OFOF THETHE TRADITIONALTRADITIONAL HANDHAND--LINELINE FISHERYFISHERY FORFOR BLUEFINBLUEFIN TUNATUNA

Fig. 1 - Autumn-Winter Bluefin Tunas from the hand-line Fig. 2 - Autumn-Winter Bluefin Tunas from the hand-line fishery in the Strait of Messina fishery in the Strait of Messina no= 2302 Sep-Mar 1932-35 no = 6573 Sep-Mar 1948-54

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30 10 20 10 5 0 0 <15 50 100 150 200 250 300 350 400 >400 <4,4 9.4 15 24 39 57 75 94 119 144 169 199 234 299 334 WEIGHT CLASSES (kg) WEIGHT CLASSES (kg

A fishery carried out by trolling close to the surface or fishing near to the bottom, in the depth. NEWNEW DATADATA SETS:SETS: UPDATINGUPDATING OO UR UR KNOWLEDGEKNOWLEDGE OFOF THETHE HANDHAND--LINELINE FISHERYFISHERY FORFOR BLUEFINBLUEFIN TUNATUNA

Fig. 3 - Autumn-Winter Bluefin Tunas from the hand-line Fig. 4 - Spring-Summer Bluefin Tunas from the hand-line fishery in the Strait of Messina - no = 398 fishery in the Strait of Messina no = 398 Sep-Mar 2000/2003 no = 426 Apr-Aug 2000/2003 16,00 16,00

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A clear view of a “resident” portion of the stock, certainly not limited to juveniles but having a major component of adult bluefin tuna, staying in the Mediterranean before the arrival of the migrants and after the spawning season. THE OPPORTUNITY TO CONFIRM OLD AND RECENT DATA IS PROVIDED BY A NEW LONG- LINE FISHERY FOR BLUEFIN TUNA IN THE NORTH-CENTRAL ADRIATIC SEA

Figure 6 - Length frequencies of BFT taken by the long-line fishery in the north-central Adriatic Sea between November 2002 to April 2003. no = 153

LENGTH CLASSES (cm) Another clear view of a “resident” portion of the stock, even here having a major component of adult bluefin tuna, present all the year round in the shallowest part of the Adriatic Sea, but where the food chain is particularly rich. OVERLAPPING VIEWS IN TWO FISHING SEASON (FALL-WINTER AND SPRING-SUMMER) AND IN TWO DIFFERENT AREAS

Figure 7 - Length frequency of BFT taken by the long-line fishery in the Figure 8 - Length frequency of BFT taken by the long-line fishery in the north-central Adriatic Sea between May to October 2003. no = 232 north-central Adriatic Sea between November 2003 to April 2004. no = 181

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Bluefin tunas are living all the year round in the Mediterranean Sea and the size frequencies in two different areas confirms that this fact is not necessarily related to the age. A STABLE TREND IN AVERAGE WEIGHT OVER A PERIOD OF 70 YEARS IN THE SAME FISHERY

Figure 8 - Mean weight of "residents" BFT taken by the hand line fishery in the Strait of Messina "RESIDENTS" in autumn-winter trend "residents"

MEAN WEIGHT (kg) 30

0 1932 1933 1934 1935 1948 1949 1950 1953 1954 2000 2001 2002 2003 THETHE MIRRORMIRROR OFOF AA COMPLEXCOMPLEX SITUATIONSITUATION BUTBUT ALSOALSO AA CLEARCLEAR IMPORTANCEIMPORTANCE OFOF THETHE ““RESIDENTRESIDENT”” TUNASTUNAS WITHINWITHIN THETHE STOCKSTOCK STRUCTURESTRUCTURE

It is sufficiently obvious that the catches obtained by the hand line fishery in the Straits of Messina in autumn-winter (September to March) mainly consist of adult “resident” tunas, because they happen in a period when usually adult tunas are still to enter into the Mediterranean or had left this basin after spawning. These “resident” tunas, generally older than the year of first maturity, are not necessarily fishes living in the Mediterranean for their entire life. It is likely that a large part of these “resident” tunas had spawn in the previous season. They can stay in the Mediterranean waters for various periods of their life, at least for more than one year up to a non-defined and unknown number of years, spending their time here travelling and possibly reaching the eastern part of the Mediterranean or moving westward. The fraction of tuna stock fished in spring-summer appears even more complex, if possible. It includes tunas entering from the Atlantic Ocean into the Mediterranean Sea in spring to spawn, and the “resident” tunas, often coming at the surface for spawning. THETHE MIRRORMIRROR OFOF AA COMPLEXCOMPLEX SITUATIONSITUATION BUTBUT ALSOALSO AA CLEARCLEAR IMPORTANCEIMPORTANCE OFOF THETHE ““RESIDENTRESIDENT”” TUNASTUNAS WITHINWITHIN THETHE STOCKSTOCK STRUCTURESTRUCTURE

According to the recent data obtained from the hand line and the long-line fisheries, it seems that the most important component of these spring-summer catches is made of resident tunas. The size composition is similar to the catches obtained in the opposite period.

The old catch data, confirmed by the most recent ones in the Straits of Messina and in the north-central Adriatic Sea, demonstrate that medium and large tunas, other than the young classes, live all the year round in the Mediterranean. These fishes are not preys of the currently most used fishing gear because they mainly don’t live in surface waters. We cannot say how relevant should be the number of specimens coming from the Atlantic Ocean to enter into the Mediterranean for spawning. But we can also affirm that the “resident” component is very important. The stable trend in mean weight during the last 70 years of the “resident” tunas demonstrate that the relevant level of Mediterranean catch over the last 6 years had never evidently affected this component of the stock.