Friedland et al.: Post-smolt growth, maturation, and survival of two stocks of Atlantic salmon 



657 



(ANOVA). However, the results of this ANOVA 

 are not reported because the interaction term 

 was significant. Instead, a one-way analysis of 

 variance, with stock as the factor, was conducted 

 for each smolt year. 



Circuli spacing patterns and post-smolt 

 growth 



The spacing of scale circuli deposited during the 

 first year at sea was measured from scales of 

 2SW salmon returning to the Penobscot and 

 Connecticut rivers. The first spacing was mea- 

 sured between the first circulus of the post- 

 smolt growth zone and the next circulus, and 

 all successive pairs were measured until the end 

 of the post-smolt growth zone. The end of the 

 zone was recognized when circuli spacings be- 

 gan to widen a second time, indicating the be- 

 ginning of the second sea summer (Fig. 2). All 

 measurements were made along the 360° axis 

 of the scale. 



Growth during the post-smolt period was 

 evaluated from the circuli spacings in three re- 

 gions of the scale. We used the spacing patterns 

 as growth indices because, in salmonids and 

 other fish, scale intercirculi spacing is directly 

 related to growth (Doyle et al., 1987; Barber and 

 Walker, 1988; Fisher and Pearcy, 1990). Three re- 

 gions of the scale were selected to correspond with 

 seasons of the calendar year. The circuli pair spac- 

 ings used to represent the "spring" period, or first 

 entry of salmon into the marine environment, was 

 fixed to pairs "two" through "six" (Fig. 3). The "sum- 

 mer" period, or the period of maximum growth, was 

 represented by the five widest and contiguous mean 

 spacings. The first sea "winter" period was repre- 

 sented by the five narrowest and contiguous mean 

 spacings. The general assumption that regions of the 

 scale are associated with seasons of the year is sup- 

 ported by the analysis of post-smolt scales from 

 tagged fish and ocean recaptures (Friedland et al., 

 1993). The three growth zones were identified from 

 the patterns of mean spacings of circuli pairs from 

 all the data for a smolt year cohort and sea-age. This 

 approach was used because it was not possible to 

 identify seasonal growth zones in the spacing pat- 

 terns for all individuals because of the variability of 

 these patterns. 



Cumulative growth during the post-smolt period 

 was represented by the number of circuli in the post- 

 smolt growth zone and the length of the zone itself. 

 A circuli count was determined for each cohort and 

 sea-age group. The count was based on the mean cir- 

 culi spacing patterns also used to identify growth 



360° Axis 



Post-smolt 



growth 



zone 



Focus 



Figure 2 



A scale from an Atlantic salmon, Salmo salar, with measurement 

 axis and growth zones marked. 



zones. The count consisted of the number of circuli 

 from the end of the freshwater zone to the first cir- 

 culus of the third winter circuli pair. The post-smolt 



