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Fishery Bulletin 103(1) 



correlated with instantaneous growth rates (in length) 

 of juvenile coho salmon (Beckman et al., 2004), the 

 finding that plasma IGF-I is also correlated with the 

 spacing of circuli at the scale margin of juvenile coho 

 salmon is further evidence that circulus spacing and 

 growth rate are positively related for coho salmon. 



Our data suggest that growth rate in FL of matur- 

 ing coho salmon is usually highest between early or 

 mid-April and late June. This is a period of increasing 

 photoperiod and often rising sea-surface temperature 

 (SST) at 50°N in the northeastern Pacific Ocean, but is 

 well before the maximum SST in late August (Fig. 10). 

 Both increased day length and temperature stimulate 

 growth in salmonids (Brett, 1979; Bjornsson, 1997). The 



decreases in apparent growth rate in length of maturing 

 coho salmon after the summer solstice could be associ- 

 ated with a number of factors. One possibility is that 

 there is a shift during the summer away from skeletal 

 growth to growth in weight (with a resultant increase 

 in condition) or to gonadal development. Data in Ishida 

 et al. (1998) for coho salmon caught in research nets 

 in the North Pacific tend to support this proposition 

 (their Appendix Table 6). Their data indicate that the 

 rate of growth in FL of maturing coho salmon decreased 

 from 1.45 mm/d between April and May to 0.49 mm/d 

 between July and August. (See also Fig. 6, present 

 study). Over the same time period the condition index 

 (weight (g)x(10 7 /FL[mm] 3 )) of the fish they sampled 

 increased from 113.3 to 143.8, an increase of 27%. 

 Thus, skeletal growth slowed over the summer, but 

 the condition of the fish increased. 



In contrast to growth rates of Columbia River co- 

 ho salmon, which decreased greatly between early 

 and late summer, and were quite low (s0.5 mm/d) 

 by August and September, the growth rates of fish 

 from the Grays Harbor basin, although also declin- 

 ing during the summer, remained high well into 

 September and early October (-0.7-1.4 mm/d), al- 

 lowing the Grays Harbor fish to attain a significantly 

 larger final average FL. Several factors may result in 

 the differing growth patterns of maturing fish from 

 these two groups. Many of the fish from the Columbia 

 River are early spawners, and peak spawning occurs 

 from late October to early November, whereas the 

 Grays Harbor fish are mainly late spawners, and 

 peak spawning occurs from mid-November to late- 

 December (Weitkamp et al., 1995). Because of their 

 later spawning the Grays Harbor fish may shift from 

 somatic to gonadal growth later in the summer or 

 fall than do the earlier spawners from the Columbia 

 River. Maturing coho salmon from the Grays Harbor 

 drainage also have a much more northerly distribu- 

 tion than do maturing fish from the Columbia River 

 (Weitkamp and Neely, 2002) and, therefore, the two 

 groups encounter very different ocean conditions (e.g., 

 temperature, salinity, prey fields, prey distributions, 

 and potential competitors for food) while feeding in 

 coastal waters. The different environmental condi- 

 tions experienced by the Columbia River and Grays 

 Harbor fish may also contribute to their differing 

 temporal growth patterns. 



Because of the poor conditions for growth of fish 

 associated with the 1983 El Nino, adult coho salmon 

 in 1983 were exceptionally small off Oregon and 

 were in poor condition (Pearcy et al., 1985; Johnson, 

 1988). Our scale analysis indicates that the small 

 size of fish in 1983 was largely due to a failure of 

 growth of maturing fish after formation of the winter 

 annulus. Although the average scale radius between 

 OE and the winter annulus was slightly smaller for 

 the 1982-83 year class than for other year classes, 

 the average scale radius between the winter annu- 

 lus and the scale margin, representing the growth 

 of maturing fish in spring and early summer, was 



