42 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 1. — Fork lengths (in cm.) of blue fin tuna taken in the vicinity of Cape Cod, for each number of annuli 

 INumbers In parentheses estimated from regression of length on caudal spread or on weight] 



by depressions in the surface and also by variations 

 in color, which were accentuated when the verte- 

 brae were soaked in water, or when they were 

 stained (Galtsoff, 1952). We examined the verte- 

 brae either witli the naked eye or with the aid of a 

 wide-field binocular microscope. We believe that 

 these annuli on scales and vertebrae are probably 

 formed during winter or early spring. 



Scales were legible for most fish weigliiiig 50 

 pounds or less, but rarely for larger ones. As scales 

 could be more readily collected than vertebrae, we 

 used scales for most of our age determinations of 

 small fish, resorting to vertebrae for larger speci- 

 mens. The material from each specimen was 

 usually examined independently, and often also 

 by our colleague Donald Allen. When readings 



Table 2. — Average foil, length of htuefin tuna taken in the 

 vicinity of Cape Cod, for each year of age from readings 

 of annuli on scales and vertebrae 



differed, material was reexamined. If the differ- 

 ence between extremes remained greater than 

 2, the specimen was discarded. For differences 

 of 2 or less, the average value or the unit closest 

 to it was used. Actually, there were few disagree- 

 ments in readings for fisli up to 50 or 60 pounds. 

 Legible scales and vertebrae were found for 28 

 fish and counts of annuli on scales agreed with 

 those on the vertebrae. Readings for fish of 70 

 to 270 pounds often differed by 1 year; those 

 for larger fish sometimes differed by 2 years or 

 more. Lengths (table 1) are from annuli counted 

 from scales or vertebrae, or both; length-fre- 

 quency distributions (fig. 3) are for each year of 

 age ; average length (table 2) is for each year. 



ANALYSIS OF LENGTH FREQUENCIES 



Another method of estimating age and growth 

 is by following the seasonal progression of dom- 

 inant size groups. This is especially useful for 

 species that spawn over a fairly short season and 

 grow rapidly. Evidently tlie bluefin tuna meets 

 these conditions, as even casual observers notice 

 the regularity with which catches of small tuna 

 can be ranked in size categories by eye. Moore 

 (1952) and Postel (1954) analyzed size frequencies 

 to determine the ages of yellowfin tuna in the 

 Pacific and the tropical eastern Atlantic, re- 

 spectively. Aikawa and Kato (1938) used the 



