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206 The Southwestern Naturalist vol. 49, no. 2 TABLE I-Ages (years) of Colorado pikeminnow (Ptychocheilus lucius) estimated from scales, sectioned otoliths (s), whole otoliths (w), and vertebrae re- moved from each of 8 fish from the Green River basin, Colorado and Utah. Fish Total Verte- Otolith Otolith num- length bral (s) (w) Scale ber (mm) age age age age 1 324 5 5 5 5 2 445 4 5 5 6 3 453 5 4 6 7 4 471 7 5 6 5 5 478 8 7 7 6 6 492 5 6 8 6 7 518 8 9 5 6 8 602 10 10 10 7 between ages estimated from vertebrae and whole otoliths (iI = 0.368; n = 9) or between vertebral and scale estimates (iI = 0.417; n = 22). A final analysis produced low correlation between the 2 otolith techniques (iI = 0.399; n = 10). We found similar relationships in 8 fish (es- timated ages 5 to 10 years) from which all 4 structures could be evaluated (i.e., scales, ver- tebrae, and 2 otoliths; Table 1). Ages obtained from reading sectioned otoliths were in close agreement with vertebral estimates, having av- erage discrepancies of less than ::t 1 year. In ad- dition, the variance of TL explained with ver- tebrae and sectioned otoliths ages differed by only 12%. Ages estimated from whole otoliths and scales showed less agreement with verte- bral ages, and ages obtained by 2 otolith meth- ods differed as much as 4 years. First 5 years of growth were detected in read- ing whole otoliths from 13 hatchery-reared fish (ages 0, 1,2,4, and 5 years). The otoliths had appropriate numbers of annuli, marked by slow growth (closely-spaced circuli) in winter and faster addition of bone (wider-spaced cir- culi) in summer. Annuli were present in fish older than their first winter of life and were similar to putative annuli of wild-caught indi- viduals. DISCUSSION-Previous attempts to age adult Colorado pikeminnow have been made with scales, but in our study, fish age was underes- timated when using scales compared to verte- bral estimates from older fish. Although scales are typically used for aging fishes that have them (Jearld, 1983; Carlander, 1987), it has been demonstrated repeatedly that the scale method has limitations. Scales do not form for some period of time in young fish (Carlander, 1982), they can be subject to peripheral ero- sion, and annuli might not be produced or might be too impacted to read in older fish (Soupir et aI., 1997; GiIlanders et aI., 1999), all of which increase reader bias (Boehlert and Yoklavich, 1984; Kimura & Lyons, 1991). Sim- ilarly, a previous Colorado pikeminnow scale study reported the first annulus was often ab- sent, the first 3 or 4 annuli were difficult to distinguish, false checks and eroded scales oc- curred, and 25% of scales were regenerated (Hawkins, 1992). We were not able to retrieve and age scales, vertebrae, and 2 otoliths from each fish used in this study because of restrictions imposed by agencies, and problems with retrieving and processing otoliths. However, we were able to compare ages estimated with 4 structures taken from 8 individuals. Results from aging these fish supported our findings that ages obtained from vertebrae and otolith sections were strongly correlated and seemed to provide un- biased estimates of TL. Our presumption that TL would be a rea- sonable estimator for growth of Colorado pi- keminnow would be suspect without further justification. Vanicek (1967) and Vanicek and Kramer (1969) reported a steady increase in mean TL and weight with increasing annual age of Colorado pikeminnow (n = 182). Haw- kins (1992) demonstrated a close relationship between Colorado pikeminnow age and TL for all of the fish captured in the Yampa River (n = 148) and Green River (n = 116), and for fish ages to 10 years in the Colorado River (n = 14) and White River (n = 48). Osmundson et aI. (1997) reported continued Colorado pi- keminnow growth in TL for even the largest (oldest) fish they captured (i.e., 800 to 899 mm; ages> 32 years, n = 8). Thus, we consider our use of TL as an estimator of Colorado pi- keminnow growth was appropriate. Our finding that vertebral centra are just as useful for aging Colorado pikeminnow as sec- tioned otoliths has important implications, be- cause vertebrae are relatively large, sturdy, and easy to age. In comparing historic fish remains, otoliths would be more easily lost from skele- '"