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



35 



in the hatchery by varying the feeding levels: the group 

 that was fed the most also grew the most and had the 

 most widely spaced scale circuli. Positive correlations 

 between circulus spacing and growth also have been ob- 

 served for nonsalmonid fishes including Tilapia (Doyle 

 et al., 1987; Matricia et. al., 1989; Talbot and Doyle, 

 1992), and walleye (Stizostedion vitreum) (Glenn and 

 Mathias, 1985). 



Circulus spacing is potentially useful for comparing 

 ocean growth rates of salmon in the ocean. Spacing of 

 the first few ocean scale circuli may indicate relative 

 growth rates of juvenile fish immediately after ocean 

 entry. However, in order for spacing of scale circuli to be 

 a practical indicator of fish growth rate, the relationship 

 between the two must be consistent and significant. 

 The relationship between circulus spacing and fish or 

 scale growth rate is determined by the relative rates 

 of growth and circulus formation. If circuli (like tree 

 rings) are formed at a constant rate, then there would 

 be a directly proportional relationship between spacing 

 and growth rate (e.g., a doubling of growth rate would 

 result in a doubling of spacing). Conversely, if the rates 

 at which circuli are formed are directly proportional 

 to growth rates (e.g., a doubling of growth rate would 

 result in a doubling of circulus formation rate), then the 

 spacing of circuli would be constant. Our earlier study 

 of growth rate, circulus formation, and circulus spacing 

 among 82 individually marked juvenile coho salmon 

 growing for a period of 63 days in saltwater tanks indi- 

 cated that neither of these two extremes is the case, but 

 that both circulus formation rate and circulus spacing 

 are positively correlated with fish growth rate (Fisher 

 and Pearcy, 1990). 



Our main objectives in this study are to further as- 

 sess the reliability of circulus spacing as an indicator 

 of growth rate in FL of coho salmon in the ocean, to 

 investigate how growth of coho salmon changes season- 

 ally, and to compare any seasonal changes in growth 

 rate with seasonal changes in the spacing of scale cir- 



culi. If circulus spacing is a reliable indicator of growth 

 rate, then seasonal changes in growth rate should be 

 tracked by changes in the spacing of circuli laid down 

 at the scale margin. We investigated relationships be- 

 tween scale growth rate, fish growth rate, circulus spac- 

 ing, and circulus formation rate for coded-wire-tagged 

 (CWT) adult coho salmon collected in the ocean fisher- 

 ies in years when ocean growth varied widely, including 

 year classes affected by the 1982-83 El Nino, and for 

 juvenile and maturing coho salmon caught in the ocean 

 off Oregon and Washington in research cruises 1981-85 

 and 1998-2002. 



Materials and methods 



Scale and FL data 



Fish fork length (FL) and scale data from a variety 

 of sources were used in this study (Table 1). During 

 research cruises on the Oregon and Washington coastal 

 shelf we collected juvenile and maturing coho salmon 

 in the upper 20-40 m of the water column with purse 

 seines from 1981-85 (Pearcy and Fisher, 1988, 1990) 

 and with a rope trawl from 1998-2002 (Emmett and 

 Brodeur, 2000). Scales samples were removed from the 

 fish from an area equivalent to area "A" described in 

 Scarnnechia (1979). When scales were not available from 

 area "A," we took scales from between areas "A" and "B" 

 in Scarnnechia (1979). (See also Clutter and Whitesel, 

 1956). We also examined scales from the same area 

 from 687 maturing CWT Columbia River and northern 

 coastal Oregon coho salmon caught in the Oregon ocean 

 fisheries between 1982 and 1992. 



Changes over time in FLs of maturing coho salmon 

 caught in research nets and of CWT hatchery coho 

 salmon originating between northern Oregon and north- 

 ern Washington and caught in the ocean fisheries be- 



