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



45 



700 



600 



500 



400 



300 



200 



1 30 mm/d 



5/30 7/19 9/7 



1 1 1 



10/27 12/16 2/04 



Date 



-r 



T" 



3/25 5/15 7/03 



— I 1 — 



8/22 10/11 



Figure 6 



Average lengths (±2 SE) of juvenile and maturing coho salmon (O. kisutch) 

 caught during research cruises off Oregon and Washington in different 

 months and years (gray and white symbols. The dashed lines are linear 

 regressions and indicate apparent growth rates in FL between the differ- 

 ent catch periods. The late April 2000 sample of maturing coho salmon 

 was from a single trawl off the mouth of the Columbia River (Robert L 

 Emmett, NMFS/NWFSC/HMSC, 2030 S Marine Science Drive, Newport, 

 OR 97365. personal commun.). The small open circles are average lengths 

 (±2 SE) of coho salmon from Ishida et al. (1998) (their Appendix Table 6) 

 plotted against the 15 th day of the months in which they were sampled. 



decreased greatly by mid-August (Fig. 8, A and B). For 

 three periods, 20 May-29 June, 29 June-8 August, and 

 8 August-27 September, median apparent growth rates 

 were 1.43 mm/d (ra=19), 0.64 mm/d (re=24), and 0.24 

 mm/d (n=27), respectively. 



Growth rates of fish from coastal Washington rivers 

 also decreased over the summer, but the decrease was 

 not as great as for the Oregon and Columbia River fish, 

 and the apparent growth rates of the Washington fish 

 were higher at comparable times during the summer 

 (Fig. 9, A and B). The apparent growth rates of Gray 

 Harbor basin fish were over 2 mm/d from late June 

 to mid- July and remained comparatively high (about 

 1.0 mm/d) into late October (Fig. 9B). Washington fish 

 generally were not caught in the fisheries until mid- 

 or late June, about a month after the first catches of 

 the Oregon and Columbia River fish. For three peri- 

 ods 19 June-29 July, 29 July-7 September, and 7 Sep- 

 tember-27 October, median apparent growth rates of 

 the coastal Washington fish were 1.23 mm/d (n=13), 

 0.92 mm/d (n = \Q), and 1.06 mm/d (n=9), respectively. 



The growth data for CWT fish from the sport and 

 troll fisheries, especially those for the coastal Oregon 



and Columbia River stocks, were consistent with the 

 growth data from the mixed stock catches of coho 

 salmon in research nets off Oregon and Washington in 

 that both data sets indicated a substantial decrease in 

 growth rate (FL) of maturing coho salmon between the 

 May-June period and the August-September period. 

 The decreases over the summer in circulus spacing at 

 the scale margin (Fig. 7B) and in apparent growth rates 

 of maturing CWT coho salmon of known origin (Fig. 8B) 

 is further evidence that scale circulus spacing and fish 

 growth rate are correlated seasonally. 



Discussion 



Our data indicate that the seasonal cycle of chang- 

 ing ocean circulus spacing on scales of juvenile and 

 adult coho salmon mirrors a similar seasonal cycle in 

 the growth rate of these fish. We lack direct data for 

 coho salmon collected between late September of the 

 first calendar year of ocean residence and mid-May 

 of the second calendar year, but growth rate during 

 part of the fall and winter may be as low as 0.5mm/d 



