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88 JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 21 unavailable. They were excluded from analyses other than for assessing usability of structures for age estimation. Forty-five hatchery-produced, pond-reared specimens of known age were provided frozen by DNFH and Willow Beach National Fish Hatchery (WBNFH). Four were of the 1974 year class (age VI when sacrificed), 6 from 1979 (age III), 5 from 1980 (age 11), and 30 from 1984 (age less than I or young-of-the-year [YOY)). Fish of year classes H through VI originated from broodfish caught from Lake Mohave; YOY were mixed offspring from wild-caught and hatchery- reared adults (Toney 1974, Inslee 1982, Harriman 19851. Seven structures were studied for use in age estimation: opercular, cleithral, and branchiostegal bones,; vertebral centra; scales; pectoral fin-rays; and both intact and sectioned otoliths. All were examined from 14 wild-caught and 25 hatchery fish. Age estimates were attempted from sectioned otoliths for all wild-caught specimens. Apparent annular marks were carefully examined at appropriate magnifications, and counted only when a mark passed uninterrupted across an entire structure. All marks questionable in this regard were excluded. Age estimation was considered progressively less reliable as the number of marks excluded by this criterion increased, or obviously so when estimates from more than one example of a structure from an individual indicated different ages. Standard length (SL) is used exclusively unless other- wise noted. Literature values for total length (TL) of adults were converted by the formula (Minckley 1983): SL = TL X 0.783 t 0.012. Values in text and tables are ± one standard error of the mean, where appropriate. Opercular, cleithral, and branchiostegal bones were boiled in water, cleared of soft tissues, and air dried (Galtsaff 1952, Scott 1977, Harrison and Hadley 1979). Presumed annuli on surfaces of intact structures were counted under direct lighting on a darkened background by dissecting light microscopy (LM) (McConnell 1952, Bulkley 1960, Blake and Blake 1978). These consisted of narrow zones of opacity separated by broader, transluscent bands. Cleithra of small specimens were read on lateral surfaces from posterior to anterior margin (Casselman 1974). Cleithra from large fish and all opercula were counted on mesial surfaces, the latter from posterior to articular margin. Putative annuli on branchiostegal rays were counted from proximal to distal. Vertebral centra were read on articular surfaces from center to margin by dissecting LM under reflected light (Appelget and Smith 1951, Galtsaff 1952, MacCrimmon 1979). Scales and sections of pectoral fin-rays were mostly examined at 40X under phase-contrast LM. Scales were removed from the right side between dorsal fin and lateral line, rubbed free of soft tissues, dried, placed on glass slides, and measured from focus to lateral margin (Minckley 1983). The first 2 or 3 anteriormost pectoral fin-rays were dis- articulated, removed, dried, sectioned by jeweler's saw, and smoothed on fine sandpaper (MacCrimmon 1979), Sections were immersed in 70% ethanol under reflected light, and read from focus to posterior margin (Dielder and Williams 19731. Preparations of scales or sections of other structures for viewing by scanning electron microscopy (SEM) were mounted on pegs and spattered with gold (Echlin 1978). Otoliths (sagittae) were excised, cleared of soft tissues, and air dried. Mesial sections were prepared by positioning and securing the structure on a glass slide with thermo- plastic cement and hand grinding on fine sandpaper (Taubert and Coble 1977). They were examined externally under dissecting LM at variable magnification in 70% ethanol and by SEM. Sections were ground to a boomerang shape, and measurements on a microprojector from focus to first angle, then to posterior margin (a third angle was occasionally required), were summed to estimate radius. Since most wild-caught fish were sacrificed or died in winter (January through March), edges of structures were assumed equivalent to a last annulus. Adults from May 1980 and January 1981 were inadvertently mixed and edges were arbitrarily assigned a winter 1980-81 value. Back calculations of SL were by the formula: L' = C + rL C l S', 11 S JJ where L' is SL at annulus formation, C is SL at develop- mental origin of the structure (ca. 10 mm TL, which in razorback larvae of that size is essentially equivalent to SL; Minckley and Gustafson 1982), S' is annulus radius, S is total structure radius, and L is SL at capture (Carlander 1969). RESULTS AND DISCUSSION Evaluation of Structures for Aging Widely-spaced, complete annuli corresponding to known ages at sacrifice were evident on all 7 structures of 5 hatchery-produced, pond-reared razorback sucker older than a year (in part, Figs. 1 and 4). Fifteen hatchery YOY demonstrated no apparent annuli. Presumed annular marks near foci of structures from wild-caught fish, when visible, had configuration and spacing similar to those on hatchery specimens. We consider this to validate use of structures examined for young fish and for early years of life in older specimens (see Beamish and McFarlane 1983). Marks more distant from foci in wild-caught fish were distinct, but close- ly spaced and thus indicative of abruptly slower growth in larger individuals. Wild-caught fish demonstrated little consistency among structures in number of presumptive annuli (Table 1). Despite evident and complete marks across struc- tures, interpretation became increasingly difficult near distal margins of bones due to crowding, and thickening of articular regions commonly masked foci and innermost presumptive annuli. Both conditions resulted in under- estimation of apparent age. Dense and irregular ossification,._ thickening, and opacity of articular ends of opercles, cleithra, and branchiostegal rays were especially pro- nounced in the largest specimens (Fig. 1). Apparent growth rings on vertebral centra were similarly obscured as they became incorporated into a thick, peripheral collar. Similar problems have been encountered in other fishes (McCon- nell 1952, Bulkley 1960, Scott 1977, Blake and Blake 1978,