Fisher and Pearcy: Seasonal changes in growth of Oncorhynchus kisutch off Oregon and Washington 49 



exceptionally low for this year class (Fig. 5A). Circulus 

 spacing revealed two notable trends. First, in 1983 

 the maximum spacing of circuli following the winter 

 annulus was only very slightly lower than in other 

 years, which indicates that spring growth in FL of 

 maturing fish in 1983 was not unusually low. Perhaps 

 maturing coho salmon continued to grow in length in 

 spring 1983, when photoperiod was increasing rapidly, 

 despite low food availability. Bjornsson (1997) found 

 that changes in photoperiod may possibly control the 

 level of pituitary growth hormone (GH), which strongly 

 stimulates skeletal growth in salmonids and that in- 

 creased levels of GH can induce growth in length even 

 during starvation. Second, the spacing of circuli at the 

 scale margin for fish caught in July 1983 was unusu- 

 ally low. similar to the spacing at the scale margin 

 from fish caught in August of most years. This find- 

 ing indicates very slow growth rates for maturing fish 

 by July 1983. Length data 1 for maturing CWT coho 

 salmon from the Oregon side of the Columbia River 

 basin caught in the ocean sport and troll fisheries 

 indicated that between June and September 1983 

 the average length of fish changed very little, which 

 would indicate that somatic growth ceased during the 

 summer. 



Our results confirm the utility of circulus spacing 

 as an indicator of growth rate in FL of coho salmon 

 in the ocean. Correlations between average circulus 

 spacing and estimated average growth rates of groups 

 of fish were significant and positive (Table 4), even 



when growth was measured over long intervals of time 

 (four to five months for juveniles, and over a year for 

 maturing coho salmon), and even when the estimates 

 of growth rate were subject to error In addition, our 

 data indicate large seasonal changes in growth rate in 

 FL of coho salmon in the coastal ocean off Oregon and 

 Washington, a result also suggested by data in Ishida 

 et al. (1998) for coho salmon in the North Pacific (see 

 Fig. 6), and these seasonal changes in growth rate ap- 

 pear to be tracked by seasonal changes in spacing of 

 scale circuli. 



Acknowledgments 



We thank all personnel from the Estuarine and Ocean 

 Ecology Division of the National Marine Fisheries Ser- 

 vice and from Oregon State University who participated 

 either in the research cruises or in processing samples 

 from those cruises. We also thank Lisa Borgerson of the 

 Oregon Department of Fish and Wildlife for supplying 

 scales from coho salmon caught in the ocean fisheries, 

 and the captains and crews of the FV Sea Eagle, FV 

 Ocean Harvester, FV Frosti and the RV Ricker for their 

 expert assistance during the cruises. Ric Brodeur and 

 Edmundo Casillas provided helpful comments on an 

 earlier version of this paper. This study was funded by 

 the Bonneville Power Administration through a grant to 

 the National Marine Fisheries Service and from NMFS 

 to Oregon State University. 



