L(0=L'exp K [l-exp(-a(*-0)]} 



where L' = length (centimeters) at start of cycle 

 V = age (growth layers) at start of cycle. 



The growth equation for juveniles of <4 growth 

 layers is 



L = 77 exp Q^Qg [l - exp(-0.90980 



The growth equation for males of >4 growth 

 layers is 



FISHERY BULLETIN: VOL. 75, NO. 4 



below in the various hypotheses of rate of accumu- 

 lation of layers. 



It appears that, effectively, in terms of the data 

 yielded by the tooth readings, 1.5 layers are laid 

 down in the first year. One possible explanation 

 alternative to that of actual deposition of 1.5 

 layers/yr is that a single layer is laid down in the 

 first year, but that in some individuals ( about half) 

 there is a strongly developed subsidiary line 

 within the layer that makes the single layer ap- 

 pear like two layers, yielding an average of 1.5 

 layers. This explanation seems unlikely, however, 

 in view of the data on thickness of the first layer. 



L = 156.85 exp 



0.0507 

 0.3765 



l-exp(-0.3765(f-4.11)) 



and for females 



L = 156.85 exp yffff f 1 " ex P(-°- 6354 ^ - 4 - n )) 



The fits of the model to both males and females is 

 excellent, albeit slightly better for the females 

 about the point of convergence of the two curves 

 than for males. 



The equations rearranged and reduced for es- 

 timating age from length are 



6 and 9 < 157 cm 



t = -1.099 ln(6.960 - 1.372 InL) 



d>157cm 



t = 4.113 - 2.656 ln(38.540 - 7.426 InL) 



9>157cm 



t = 4.113 



1.574 ln(59.871 - 11.645 InL). 



Note: These equations should not be used to esti- 

 mate age from actual length data except for 

 grouped samples of smaller animals (about 165 cm 

 or less in females and 170 cm in males), for which 

 growth rate is still large compared with individual 

 variation in length. 



Estimated age in growth layers at 134 cm, the 

 predicted length at 1 yr derived above from ex- 

 trapolative comparison with other delphinids, is 

 1.57 layers. Since, as discussed above, the esti- 

 mate of 134 cm is likely to be a slight overestimate 

 due to some nonlinearity of growth during the first 

 year, the estimate of 1.57 layers is rounded down 

 (to the nearest half layer) to 1.5 layers for use 



738 



The "subsidiary line" hypothesis would predict a 

 subsidiary inflection or plateau in the cumulative 

 percent of first layers in relation to thickness, and 

 such is not apparent (Figure 4). 



We found no correlation between thickness of 

 the innermost growth layer and month of capture 

 (Table 3). It is apparent from the data that the 

 layers are formed rapidly (very few relatively thin 

 innermost layers are seen) and probably through- 

 out the year in the population. 



With no basis for direct calibration, we provi- 

 sionally use three alternative hypotheses of rate of 

 layer deposition (similar to those put forth for the 

 spotted dolphin, S. attenuata — Perrin et al. 1976) 

 in the age-based analyses below, namely: 



I. One and one-half layers per year, or 



TABLE 3. — Thickness of innermost growth layer in teeth of 331 

 eastern spinner dolphins, with 3-12 layers, by month of capture. 



Thickness of innermost layer ■*■ 

 thickness of next youngest layer 



