Fisher and Pearcy: Seasonal changes in growth of Oncorhynchus kisutch off Oregon and Washington 



39 



sured in any 10-day catch period. The average FLs 

 were averaged across all years of data, yielding grand- 

 average FLs for each 10-day catch period. The grand 

 average FL for each 10-day catch interval comprised 

 1-27 years of data, but those periods with fewer than 5 

 years of data were discarded. In all, FLs from 149,718 

 fish were used in the analysis. Grand average FLs and 

 the apparent growth rates in FL between each 10-day 

 catch period were plotted against date and compared 

 with the seasonal changes in circulus spacing at the 

 scale margin of the fish in our scale sample. 



Results 



Growth and scale statistics for juvenile and maturing fish 



Average growth rates and circulus formation rates were 

 greater for juvenile fish during their first ocean summer 

 than for maturing fish during their entire ocean life 

 probably because maturing fish experience slow growth 

 in the winter (Table 3). During their first summer in 

 the ocean, juvenile fish grew an average of 1.33 mm/d 

 and formed circuli at the rate of 0.188/d (one every 5.3 

 days): whereas, during their entire ocean life maturing 

 fish grew an average of 1.11 mm/d and formed circuli 

 at the rate of 0.131/d (one every 7.6 days). The highest 

 average growth rate (1.52 mm/d) among the eight year 

 classes of juvenile coho salmon was about 28% higher 

 than the lowest average growth rate (1.18 mm/d). The 

 percentage range in growth rate of maturing fish was 

 similar (31%). Average spacing of circuli was similar for 

 both juvenile and maturing coho salmon (0.0460 mm vs. 

 0.0463 mm), probably because scales from the maturing 



fish contained both more narrowly spaced circuli formed 

 during the winter and more widely spaced circuli formed 

 during the second ocean summer (see below). The varia- 

 tion among groups in average circulus spacing (CV=4.9% 

 and 4.4%) was lower than the variation in fish or scale 

 growth rates (CV=6.7% to 8.3%), although estimation 

 error may have increased the coefficients of variation 

 of the growth rates. 



Correlations between scale characteristics 

 and growth rate 



Circulus spacing was strongly correlated (r=0.89 and 

 0.82, respectively) with scale and fish growth rates 

 among the nine year classes of juvenile coho salmon 

 (Table 4). Circulus spacing was also significantly cor- 

 related with scale and fish growth rates among the 

 17 groups of maturing fish, but the correlations were 

 weaker (r=0.57 and 0.55, respectively) than those for 

 the juvenile fish. Conversely, correlations between the 

 rate of circulus formation and the scale and fish growth 

 rates were slightly higher for the maturing fish (r=0.85 

 and 0.75, respectively) than for the juvenile fish (r=0.76 

 and 0.81, respectively). These results suggest that when 

 growth is averaged over several seasons, during which 

 growth rate varies greatly and may even cease for vary- 

 ing periods of time, differences in growth among year 

 classes or groups may be reflected more clearly by dif- 

 ferences in the numbers of circuli laid down on the scale 

 than by differences in the average spacing of circuli. 



Although the average spacing of circuli and the aver- 

 age rate at which circuli form were both correlated with 

 scale and fish growth rates, they were not correlated 

 with each other (Table 4). This finding indicates that 



