J4 



Abstract — In this study we present 

 new information on seasonal variation 

 in absolute growth rate in length of 

 coho salmon (Oncorhynchus kisutch ) in 

 the ocean off Oregon and Washington, 

 and relate these changes in growth 

 rate to concurrent changes in the 

 spacing of scale circuli. Average spac- 

 ing of scale circuli and average rate of 

 circulus formation were significantly 

 and positively correlated with average 

 growth rate among groups of juvenile 

 and maturing coho salmon and thus 

 could provide estimates of growth 

 between age groups and seasons. 

 Regression analyses indicated that 

 the spacing of circuli was proportional 

 to the scale growth rate raised to the 

 0.4-0.6 power. Seasonal changes in 

 the spacing of scale circuli reflected 

 seasonal changes in apparent growth 

 rates offish. Spacing of circuli at the 

 scale margin was greatest during the 

 spring and early summer, decreased 

 during the summer, and was lowest in 

 winter or early spring. Changes over 

 time in length offish caught during 

 research cruises indicated that the 

 average growth rate of juvenile coho 

 salmon between June and Septem- 

 ber was about 1.3 mm/d and then 

 decreased during the fall and winter 

 to about 0.6 mm/d. Average growth 

 rate of maturing fish was about 2 

 mm/d between May and June, then 

 decreased to about 1 mm/d between 

 June and September. Average appar- 

 ent growth rates of groups of matur- 

 ing coded-wire-tagged coho salmon 

 caught in the ocean hook-and-line 

 fisheries also decreased between June 

 and September. Our results indicate 

 that seasonal change in the spacing 

 of scale circuli is a useful indicator 

 of seasonal change in growth rate of 

 coho salmon in the ocean. 



Seasonal changes in growth of coho salmon 

 (Oncorhynchus kisutch) off Oregon and 

 Washington and concurrent changes 

 in the spacing of scale circuli 



Joseph P. Fisher 



William G. Pearcy 



College of Oceanic and Atmospheric Sciences 



Oregon State University 



104 Ocean Admin. Building 



Corvallis, Oregon 97331-5503 



E-mail address (for J. P. Fisher) |fisheng>coasoregonstateedu 



Manuscript submitted 20 September 

 2003 to the Scientific Editor. 

 Manuscript approved for publication 

 8 September 2004 by the Scientific Editor 



Fish. Bull. 34-51(2005). 



Large interannual and decadal varia- 

 tions occur in the abundance and pro- 

 ductivity of North Pacific salmonids. 

 These fluctuations, which affect har- 

 vestable biomass, are influenced by 

 survival rates, ages at maturity, and 

 somatic growth (Beamish and Bouil- 

 lon, 1993; Mantua et al., 1997; Hare 

 et al. 1999; Pyper et al., 1999; Hobday 

 and Boehlert, 2001). 



The growth of smolts after ocean 

 entry — growth that is critical to 

 production — is also thought to be 

 an important determinant of their 

 survival. As for juvenile and larval 

 fishes in general, size-selective mor- 

 tality may occur (Miller et al., 1988; 

 Bailey and Houde, 1989; Litvak and 

 Leggett, 1992; Sogard, 1997) with 

 the result that faster growing sal- 

 monids experience less mortality 

 from predators than slower growing 

 salmonids (Parker, 1971; Bax, 1983; 

 Fisher and Pearcy, 1988; Holtby et 

 al., 1990; Jaenicke et al., 1994; Wil- 

 lette, 1996, 2001). This size-selective 

 mortality may explain much of the 

 interannual variability in survival 

 of juvenile salmonids and the sub- 

 sequent abundance of different year 

 classes. However, other investigators 

 have not found a strong relationship 

 between growth of juvenile salmon 

 and mortality (Fisher and Pearcy, 

 1988; Mathews and Ishida, 1989; 

 Blackbourn, 1990). 



Intercirculus spacing of scales has 

 been used to estimate early ocean 



growth rate of juvenile salmon and 

 has been linked to differential sur- 

 vival rates. For example, Healey 

 (1982) used the spacing of the first 

 five circuli to demonstrate intensive 

 size-selective mortality in juvenile 

 chum salmon (Oncorhynchus keta) as 

 they migrated offshore. Holtby et al. 

 (1990) correlated early ocean growth, 

 based on intercirculus spacing, with 

 marine survival of age 1+ coho (O. 

 kisutch) smolts. The spacing of early 

 ocean circuli from the scales of ma- 

 turing Atlantic salmon (Salmo salar) 

 has been used to estimate juvenile 

 growth rates, which are correlated 

 with survival and age at maturity, 

 and to identify stocks (Friedland et 

 al., 1993; Friedland and Haas, 1996; 

 Friedland and Reddin, 2000; Fried- 

 land et al., 2000). 



Correlation between circulus spac- 

 ing and growth rate was reported 

 by Fisher and Pearcy (1990) for age 

 0.0 coho smolts reared for 60 days in 

 salt water tanks. In addition, posi- 

 tive correlations between the spacing 

 of scale circuli and fish growth rate 

 have been observed for rainbow trout 

 (O. mykiss) (Bhatia, 1932), and sock- 

 eye salmon (O. nerka) (Fukuwaka and 

 Kaeriyama, 1997), and between the 

 spacing of circuli and feeding ration 

 and growth for sockeye salmon (Bil- 

 ton and Robins, 1971; Bilton, 1975). 

 Bigelow and White (1996) were able 

 to manipulate the spacing of scale 

 circuli of cutthroat trout (O. clarkii) 



