660 



Fishery Bulletin 91(4), 1993 



•& 



a. 

 S 



60 65 70 75 80 85 90 95 100 



Observed recapture lengths (cm) 



Figure 6 



Predicted versus observed recapture lengths of tagged south 

 Pacific albacore. The three panels depict recapture lengths 

 predicted on the basis of models fitted to length-frequency 

 data assuming annual cohorts, vertebral-ring-count, data as- 

 suming an annual formation rate, and length-frequency data 

 assuming semestral cohorts. The lines cover the points corre- 

 sponding to a perfect match between predicted and observed 

 lengths. 



eters. In this study, the VB model was preferred to 

 alternatives because it was used in available software 

 such as MULTIFAN. There were no indications from 

 the data that the VB model was not appropriate for 

 modelling albacore growth, although a Gompertz model 

 (Ricker, 1979) appeared to provide an equally good fit 

 to the vertebral-ring-count data. 



In fishes, rings are thought to form in bony tissues 

 as a series of growth checks separated by zones, usu- 

 ally more opaque, that are associated with growth 

 (Casselman, 1983). The usual inference that rings are 

 formed yearly is based on the observation that growth 

 rate, particularly for temperate species, is frequently 

 seasonal as a result of its dependence on temperature. 

 Growth checks are thought to occur during the period 

 of the year when the water is coldest; high growth 

 rates occur during periods of warmer water tempera- 

 tures. Length-frequency analysis provided evidence that 

 juvenile albacore growth is strongly seasonal; there- 

 fore this seasonality could be the basis of ring forma- 

 tion in vertebrae. However, the relationship of water 

 temperature with this seasonality is unknown. Alba- 

 core, while being predominantly a temperate species, 

 are also found in tropical waters, particularly as adults. 

 In common with other Thunnus species, they occupy a 

 three-dimensional habitat within wide temperature lim- 

 its (14-20°C) and are capable of physiological ther- 

 moregulation, maintaining body temperatures of up to 

 15°C warmer than that of the surrounding waters 

 (Morrison et al., 1978). It is possible that ring forma- 

 tion in the bony tissues of albacore is influenced by 

 these biological characteristics and by more complex 

 biological cycles, such as feeding and reproduction, as 

 well as the temperature of the water that they occupy. 



As a working hypothesis, the length-frequency modes 

 and vertebral-rings were assumed to be formed each 



