40 



Fishery Bulletin 103(1) 



circulus spacing and circulus formation rate are inde- 

 pendent indicators of growth rate — both tending to in- 

 crease with increasing growth rate but not necessarily 

 together in the same fish or in the same group or year 

 class. At least when averaged over periods of months or 

 more than a year, differences in average growth rate 

 may be expressed by differences in average spacing of 

 circuli, differences in average rate of circulus formation, 

 or differences in both. 



Regressions of circulus spacing and formation rate 

 on growth rate 



We expressed average spacing of circuli and rates of 

 circulus formation as power functions of the scale growth 

 rates, equivalent to linear regressions of ln-ln trans- 

 formed data. These regressions are shown in Figures 

 3 and 4 for year classes of juvenile fish and groups 

 of maturing fish, respectively. Because scale growth 

 rate and fish growth rate were very strongly corre- 

 lated (Table 4), we show only the regressions with scale 

 growth rate. 



Change in average spacing of circuli and in average 

 rate at which circuli form was proportionally smaller 

 than the change in average scale growth rate. Aver- 

 age spacing of circuli was proportional to the average 

 scale growth rate raised to the 0.6 power (juvenile fish, 

 Fig. 3A) or the 0.5 power (maturing fish, Fig. 4A). If 

 these relationships hold over a wider range of scale 

 growth rate and circulus spacing, then a doubling of 

 scale growth rate would be associated with only a 1.5- 

 fold (2 - 6 ) or 1.4-fold (2 05 ) increase in circulus spac- 

 ing. Similarly, average rate of circulus formation was 

 proportional to the average scale growth rate raised to 

 the 0.5 power (juvenile fish, Fig. 3B) or the 0.8 power 

 (maturing fish, Fig. 4B). 



Seasonal changes in circulus spacing and fish growth rate 



Seasonal changes in average circulus spacing were con- 

 sistent among the different year classes and release 

 times of CWT coho salmon (Fig. 5, A-E). During the 

 first year in the ocean, average spacing of scale circuli 

 increased rapidly after OE (usually in May) to aver- 

 age peak values of about 0.050 mm-0.055 mm, then 

 gradually decreased to average minimum values of 

 about 0.031 mm-0.040 mm in the annual ring. By late 

 September 1999-2002, spacing at the margin of scales 

 from juvenile fish had decreased from peak values (Fig. 

 5E), indicating that the gradual decrease in spacing of 

 circuli which forms the annual ring begins as early as 

 the late summer of the first ocean year. For some year 

 classes (e.g., 82-83, 85-86, 90-91, 91-92) the annual 

 ring was a distinct narrow zone of very closely spaced 

 circuli (Fig. 5, A and C), whereas in other years the 

 annual ring was broad and subtle, with more widely 

 spaced circuli (e.g., 83-84, 86-87, and 84-85 for the 

 March released fish; Fig. 5, A and B). 



After the annulus (black dots, Fig. 5), the spacing 

 of circuli increased sharply to peak values of about 



Table 4 



Correlations (r) between average circulus spacing (mmi, 

 average estimated scale growth rate (mm/d), average 

 estimated fish growth rate (mm/dl, and average esti- 

 mated circulus formation rate (circuli/d) between ocean 

 entry and late summer for nine year classes of juvenile 

 coho salmon and during the entire ocean growth period 

 for 17 groups of CWT maturing coho salmon (see Table 

 2). All correlations were significant (P<0.05), except were 

 noted ("n.s"). 



Comparison 



Circulus spacing 

 vs. scale growth rate 



Circulus spacing 

 vs. fish growth rate 



Circulus spacing 



vs. circulus formation rate 



Scale growth rate 

 vs. fish growth rate 



Scale growth rate 

 vs. circulus formation rate 



Fish growth rate 



vs. circulus formation rate 



0.055 mm-0.060 mm and remained high for a vari- 

 able distance. Compared to the peak spacing, spacing 

 of circuli at the scale margin was relatively high for 

 maturing fish caught in late June or July 1982, 1984, 

 1985, 1986, 1987, 1991, and 2000, whereas, spacing at 

 the scale margin was quite low compared to the peak 

 spacing for fish caught in July 1983, 1989, 1990, and 

 1992 (Fig. 5, A, C, and D). Spacing at the scale margin 

 was very low among unmarked maturing fish caught in 

 late September 2001 (Fig. 5D). 



Compared to the large interseasonal variation in 

 spacing of circuli in the pre- and postannulus zones, 

 from about 0.03 mm in the annual ring to about 0.06 

 mm for the most widely spaced circuli, interannual 

 variation the peak and minimum spacing of circuli was 

 quite small. The peak spacing of circuli was similar 

 among year classes, even when total growth differed 

 greatly (e.g., the 82-83 vs. the 81-82 and 83-84 year 

 classes, Fig 5A). The unusually small postannulus scale 

 growth of fish caught during a strong El Nino in July 

 1983 (Fig. 5A) was characterized by a much narrower 

 region of widely spaced circuli and more closely spaced 

 circuli at the scale margin than in other years. 



In general, pre-annulus scale growth was greatest for 

 the fish released in March (Fig. 5B), was slightly less 

 for the fish released in late April or early May (Fig. 5A), 

 and was smallest for the fish released in late May or 

 early June (Fig. 5C). These data indicate that date of 

 release may strongly affect the amount of growth at- 



