VI. MIGRATIONS AND STOCK IDENTIFICATION 



A. INTRODUCTION 



Identification of Atlantic blue- 

 fin tuna stocks is regarded as one of 

 the most important prerequisites to 

 the efficient management of its fish- 

 eries. The studies of stock identifica- 

 tion and migrations are so closely 

 related as to be almost inseparable. 

 Some methods, such as the visual or 

 electronic tracking of fish, and the 

 analysis of the seasons and localities 

 of catches, relate primarily to migra- 

 tions. Others, such as biometric and 

 biochemical studies, are more directly 

 concerned with stock identification. 

 Tagging is one of the most positive 

 methods of studying both problems. 

 In difficult cases, however, it is nec- 

 essary to use all available means to 

 achieve either objective. This is es- 

 pecially true of the bluefin tuna, 

 whose long, rapid and variable mi- 

 grations make their migratory pat- 

 terns and populations especially dif- 

 ficult to identify. 



Until 1954, when sustained tag- 

 ging of the Atlantic bluefin was initi- 

 ated, scientists were limited to de- 

 ductive, or indirect, methods of study- 

 ing its migrations. Likewise, until the 

 development of biochemical meth- 

 ods at about the same time, they de- 

 pended mainly on biometric studies 

 to identify its populations. Even with 

 the aid of these and other advanced 

 techniques, the migratory patterns of 

 bluefin tuna are not completely un- 

 derstood and the stocks have not been 

 positively identified. 



In this section we will consider 

 the migrations and stocks (without a 

 priori implication that they are sepa- 

 rate) in the Mediterranean-eastern 

 Atlantic area and in the western At- 

 lantic. Finally, we will consider the 

 implications of trans-equatorial and 

 transatlantic migrations and present 

 our conclusions in regard to the iden- 

 tity of stocks. 



B. METHODS AND MATERIALS, 

 AND DEFINITIONS 



\. Methods and Materials 



a. Deductive Methods 



Deductions in regard to migra- 

 tions and the identity of stocks have 

 been based on observations or infor- 

 mation of the following types: 



( 1 ) The times of appearances and dis- 



appearances of the fish in fishing 

 areas and the observed move- 

 ments of the fisheries for it. 



(2) The size or age composition of 

 the catches. 



(3) The sex ratio of the catches. 



(4) The observed behavior of the fish 



in specific areas and seasons. 



(5) The apparent preferences of the 

 fish for environmental conditions, 

 and their seasonal changes. 



(6) Deductions as to the localities 

 where specially constructed 

 hooks and lures found in bluefin 

 tuna caught in traps had origi- 

 nated. 



b. Methods Used Mainly for 

 Identifying Stocks 



Just as many of the above meth- 

 ods have been used to identify stocks 

 as well as to study migrations, the 

 following ones, which have been used 

 primarily for stock identification, 

 have also been applied to the study 

 of migrations: 



(1) Anatomical comparisons. 



(2) Biometric comparisons of mor- 

 phological characters. 



(3) Comparisons of biochemical 

 properties. 



(4) Comparisons of areas and sea- 

 sons of spawning. 



(5) Comparisons of growth rates. 



c. Methods Used Mainly for 

 Studying Migrations 



Most of the methods listed be- 

 low were developed to study migra- 

 tions, but tagging is equally useful 

 for identifying stocks: 



( 1 ) Visual tracking from aircraft. [Vi- 

 sual observations from vessels or 

 shore come under item a(4).] 



(2) Tagging. 



(3) Tracking with sonar. 



(4) Tracking with sonic tags. 



d. Development of Bluefin 

 Tuna Tagging 



Tagging is the only positive 

 method of determining that an indi- 

 vidual fish has gone from one place 

 to another. The desirability of tag- 

 ging Atlantic bluefin tuna was rec- 

 ognized long ago (Sella 1912b), but 

 the technical difficulties were such 

 that it was not accomplished on a 

 continuing basis until 1954 (Mather 

 1960, 1963). Sella (1927) tagged 20 

 bluefin tuna weighing from 4 to 20 

 kg off Gailipoli (southern Italy) in 

 1912 with bands around the caudal 

 peduncle, but no returns resulted. His 

 (1927, 1929a) deductions of migra- 

 tions from hooks and lures found in 

 tuna caught in traps, however, 

 aroused new interest in the problem. 

 Methods of marking were discussed 

 extensively at the "Conference of ex- 

 perts — '■ (Anonymous 1932b), and 

 the tagging of bluefin tuna was 

 strongly recommended. Large and 

 small bluefin tuna were tagged off 

 Portugal in the years 1931-1935 and 

 1960 (Frade and Dentinho 1935, 

 Heldt 1938, Vilela 1960). Some large 

 individuals were also tagged off Tu- 

 nisia and Morocco (Heldt 1943). In 

 addition Heldt (1932) distributed 

 marked hooks to tuna fishermen at 

 Groix, France, on the Bay of Biscay 

 in 1927, 1928 and 1929. Many of the 

 hooks and lures retrieved by Sella 

 (1929a) from tuna which had been 

 caught in Mediterranean tuna traps 



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