474 



Fishery Bulletin 97(3), 1999 



360° Axis 



B 



Post-smolt 



Growth 



Zone 



360° Axis 



Freshwate 



Growth 



Zone 



Fjrestiwater 



'prowth 



Zone 



Figure 2 



Illustrations of Atlantic salmon scales from a ISW salmon (A) and a postsmolt salmon (B). Focus 

 of each scale is marked; freshwater growth zone extends from focus to start of marine growth; and 

 postsmolt growth zone extends to first winter annuli. The figures are not in scale. 



Postsmolt growth was interpreted from circuli 

 spacing patterns deposited during the postsmolt year 

 Scales were cleaned and mounted between glass 

 slides and the spacings of scale circuli were measured 

 with a Bioscan Optimas image processing system. 

 The first spacing was measured between the first 

 circulus of the postsmolt growth zone and the next 

 circulus and continued with successive circuli pairs. 

 For maturing fish returning to their natal river af- 

 ter at least one winter at sea, the measurements were 

 made through the first seawinter annulus of the scale 

 and thus captured the entire postsmolt growth zone 

 (Fig. 2A). For fish captured as postsmolts, measure- 

 ments were taken to the edge of the scale (Fig. 2B). 

 Thus, only part of the data collected from maturing 

 fish was needed for comparison with the postsmolts. 

 Measurements were made on a single scale from each 

 specimen at a pixel resolution of 0.004 mm along the 

 360° axis of the scale. 



Return rates by sea-age and fraction of the smolt 

 cohort maturing after one seawinter ( ISW) were com- 

 puted for the three stocks used for comparison. Re- 

 turn rates are simple percentages of the number of 

 returns-at-age to the number of smolts released. 

 Return rates for the Connecticut and Penobscot 

 stocks originally reported in Friedland et al. ( 1996b) 

 are updated here. Return rates for the Saint John 

 stock were computed similarly and are based on smolt 

 releases of 172, 145, and 206 thousand smolts for 

 the years 1982-84, respectively. Releases over the 

 period 1974-92 averaged approximately 200 thou- 

 sand smolts annually in the Saint John system. The 



fraction of the smolt cohort maturing after one 

 seawinter, the ISW fraction, was computed with the 

 formulae of Friedland et al. ( 1996b). Likewise, these 

 data are updated for the Connecticut and Penobscot 

 stocks and are newly reported for the Saint John 

 stock. 



Circuli spacing data for Gulf of St. Lawrence 

 postsmolts were compared in three ways. In the first 

 comparison, data for the postsmolt scales were com- 

 pared for the period 1982-84. The spacings of the 

 first ten circuli pairs were compared with ANOVA. 

 This and subsequent statistical analyses were re- 

 stricted to the first ten circuli pairs because most 

 postsmolt scales did not have circuli beyond the tenth 

 pair. As a consequence, spacing patterns for circuli 

 pairs beyond pair number 10 were not well estimated. 

 In the second comparison, for two of the seasons, 1982 

 and 1983, where sufficient samples of postsmolts 

 were available, samples were poststratified by col- 

 lection date. The poststratification was done to 

 achieve nearly equal numbers of samples in each 

 stratum. In 1982, three within-year strata were cre- 

 ated: 8 to 17 August composed of 126 samples, 18 

 August to 5 September composed of 12 1 samples, and 

 7 September to 18 October composed of 125 samples. 

 In 1983, two strata were created: 24 to 30 Septem- 

 ber composed of 73 samples and 1 to 11 October com- 

 posed of 81 samples. Circuli spacing of the first ten 

 circuli pairs for the poststratified groups were com- 

 pared with ANOVA. For the third comparison, cir- 

 culi spacing data from Gulf postsmolt scales were 

 compared with the circuli spacing data for the three 



