III. AGE AND GROWTH 



A. INTRODUCTION 



Knowledge of the age-size rela- 

 tionship is not only important from 

 the biological viewpoint but is also 

 essential for effective management 

 of a fishery. Until recently, there has 

 been general agreement on the sizes 

 of Atlantic bluefin tuna at ages up to 

 12-14. Preliminary results for ages 

 up to 26 have been presented in 1975 

 and 1976, but these are controver- 

 sial. 



B. METHODS OF 

 DETERMINING AGE AND 

 GROWTH AND THEIR 

 APPLICATION TO BLUEFIN 



TUNA 



1. Size Frequencies 



Age groups show up as modes 

 when the sizes of sufficient numbers 

 of bluefin tuna are plotted. These 

 modes are usually distinct for small 

 individuals (up to 125-150 cm long), 

 but become less distinguishable with 

 larger fish. The length-weight ratio 

 of large bluefin fluctuates greatly with 

 the seasons (Tiews 1963). Therefore 

 length measurements are more suit- 

 able for size frequency analyses than 

 weights. Growth may be estimated 

 fi-om the progression of modes in 

 plots of size data for consecutive time 

 intervals. 



Size-frequencies were evidently 

 used, although the methodology was 

 not described, by d'Amico (1816, in 

 Heldt 1930) and Bourge (1908, in 

 Roule 1917), in estimating the sizes 

 of juvenile Mediterranean bluefin 

 tuna in their first four months of life. 

 Piccinetti and Piccinetti-Manfrin 

 ( 1 970) presented a more detailed and 

 precise study of their growth through 

 this period, describing the methods 

 they had used. 



Westman and Gilbert ( 1 94 1 ) and 

 Westman and Neville (1942) were 



evidently the first to study the growth 

 of larger bluefin tuna by this method. 

 They traced the growth of young in- 

 dividuals taken off Long Island, New 

 York, in the summers of 1938 and 

 1941 by analyzing their length fre- 

 quencies. They established the age in 

 years of each size group by counting 

 annuli on scales. 



Buser-Lahaye and Doumenge 

 (1954) and Doumenge and Lahaye 

 (1958) likewise analyzed length fre- 

 quencies to estimate the ages of small 

 bluefin tuna caught off the Mediter- 

 ranean coast of France during 1953 

 and 1954. They used Sella's (1929a) 

 data, however, to establish the esti- 

 mated age of each size group. 



More extensive studies of the 

 summer growth of western Atlantic 

 bluefin tuna, using counts of annuli 

 on hard parts as well as size frequen- 

 cies, were presented by Mather and 

 Schuck(l960). 



Furnestin and Dardignac (1962) 

 were the first authors to trace the 

 growth of 7". ihynnus ihynniis through 

 most of the first two years of its life. 

 They used size frequencies of young 

 fish taken off the Atlantic coast of 

 Morocco, where, after attaining a 

 length of about 32 cm, they are avail- 

 able throughout the year. 



Tiews (I960) used the frequen- 

 cies of eye diameters, which he as- 

 sumed to be related to the age of the 

 fish, as well as to its length, to esti- 

 mate the ages of large bluefin tuna 

 caught in the North Sea in the sum- 

 mer and fall of I95<). 



2. Counts of Marks on Hard 

 Parts 



Ages of bluefin tuna have been 

 estimated by counting marks, usu- 

 ally called annuli. which were thought 

 to have been laid down annually, on 

 certain hard parts of the fish. The 

 parts used included scales, vertebrae, 

 and otoliths (Figure 3). 



The relative merits of these meth- 

 ods were extensively discussed at the 

 "conference d' experts pour I'examen 

 des melhodes scientifiques et tech- 

 niques ^ appliquer a I'etude des 

 poissons de la famille des Thonides" 

 (hereafter referred to as "Conference 

 of experts") held at Madrid and Cadiz, 

 Spain, 16-22 May 1932 (Anonymous 

 1932b), and by others as cited here. 



a. Scales 



Age determinations from scales 

 are based on counting the "checks" 

 or areas where two or more circuli 

 are close together, instead of being 

 more widely and evenly spaced as 

 they are on most of the scale's sur- 

 face. 



Corson (1923a, 1923b) presented 

 the first age determinations for Atlan- 

 tic bluefin tuna of which we have 

 knowledge. These were based on 

 scales taken from a small number of 

 young fish caught off Long Island, 

 New York, in September 1923. He 

 used scales from the posterior part of 

 the fish, having found those from the 

 shoulder to be unreadable because of 

 streaks and globules of oil within 

 them. 



At the "Conference of experts" 

 (Anonymous 1 932b), F. de Buen and 

 Sella said that scales did not furnish 



Figure 3. Bluefin tuna vertebra 



showinsi two annuli. 



