FISHERY BULLETIN: VOL. 83. NO. 4 



depends upon the representativeness of the sample, 

 the accuracy of the readings, the assumptions that 

 1 GLG equals 1 yr, and the adequacy of the growth 

 model. 



The sample analyzed w^as a simple random sam- 

 ple, stratified only by sex, taken from all specimens 

 collected betw^een 1973 and 1978. These were them- 

 selves a sample of the animals killed incidental to 

 fishing operations, which were a sample of the 

 population. Although Powers and Barlow (fn. 2) have 

 shown a bias towards a higher proportion of calves 

 killed in purse seine nets, we have no reason to 

 believe that the relationship between length and age 

 is different in our sample than in the population. 



Teeth were read as accurately as possible Reilly 

 et al. (1983) have investigated the precision of the 

 readings but, without known-age animals, it is not 

 possible to validate their accuracy. 



Our assumption that 1 GLG equals 1 yr is based 

 on captive, tetracycline-marked Hawaiian spinner 

 dolphins (Myrick et al. 1984) a distantly related 

 species. Known-age, captive or marked spotted 

 dolphins are not available for direct GLG calibration. 

 If differences are found between GLG in spotted and 

 spinner dolphin teeth when data from known-age 

 spotted dolphins are available, these estimates of 

 growth based on GLGs will need to be revised. 



It is often assumed that when one GLG is 



deposited each year the first GLG begins with the 

 neonatal line immediately at birth and ends precisely 

 1 yr later. Yet it is possible, as suggested by Perrin 

 et al. (1977) for S. longirostris, that the first GLG 

 is not always complete Teeth from young, known- 

 age dolphins from the same stock born at different 

 times during the year are needed to address this 

 question. 



It is also possible that the neonatal line may not 

 be deposited immediately at the time of birth. In 

 bottlenose dolphins, stranded on the mid-Atlantic 

 coast of the United States, variability has been found 

 in the time of deposition of the neonatal line in 18 

 neonatal specimens (Hohn unpubl. data). These 

 specimens were identified as neonates because they 

 lacked the umbilicus (indicating that the calf was not 

 stillborn) and their dorsal fin and flukes were fold- 

 ed (Ikvolga and Essapian 1957). Some of these 

 stranded specimens showed no neonatal line while 

 others had part of a neonatal line deposited. Similar- 

 ly, in our sample of offshore spotted dolphins from 

 northern areas there is not neonatal line in some 

 postnatal specimens so that the amount of time since 

 birth is unknown. This difference in timing of 

 neonatal line deposition may be due to individual 

 variation in tooth growth and mineralization or small 

 difference in gestation time 



The precise timing of the deposition of the first 



180 



Figure 7.— Length-age [GLGs, (growth 

 layer groups), pooled mean estimates] data 

 for northern offshore spotted dolphins up 

 to 3.0 GLGs, and the fitted Gompertz model 

 of growth. 



2.0 



AGE (years) 



3.0 



4.0 



564 



