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ISSUE 2, 1987 AGE ESTIMATION FOR RAZORBACK SUCKER 91 ently deflected medially between 4 to 6 years of otolith age to produce an imperfect cup shape. The incomplete rim became directed mesoventrally to surround a distinct sulcus, the floor of which demonstrated less accretion than other parts. Only sectioned otoliths provided clear, apparent indica- tions of annual growth for razorback sucker. Seventy-four (84.1%) of 88 fish examined showed marks readily inter- pretable as annuli (Fig. 4). An unreadable percentage resulted from breakage during preparation (2.3%), section through other than the structural focus (3.4%), or aber- rantly irregular growth of margins (10.2°/x). A B Figure 3. A. Medial aspect of left sagitta from a razorback sucker caught from Lake Mohave, Arizona and Nevada, 34 years of age by otolith section. B. Lateral aspect of same sagitta, showing inner arch (both 15X). Age Estimation from Sectioned Otoliths Otolith growth, as with that of other bony structures in fishes, is by accumulation of material at periodically variable rates. Accretion is believed under endocrine con- trol, following circadian rhythms of that system, which in turn may be attuned to photoperiod, temperature, or other environmental variations (Jacket 1981). Otoliths are con- sidered more conservative than other bony structures in response (e.g., reabsorption) in times of stress such as lack of food (Marshall and Parker 1982, Campana 1983a), physical exertion (Campana 1983b), or low pH (teen et al. 1985). Seasonal growth is established by greater accretion in summer and less in winter (Irie 1960, Pannella 1971, Williams and Bradford 1974, Liew 1974, Tauber and Coble 1977, Campana and Neilson 1985). Conditions in warm- monomictic reservoirs such as Lake Mohave (Priscu 1978, Paulson et al. 1980, Priscu et al. 19821 should be conducive to otolith annulus formation, as the pristine Colorado River must also have been due to marked seasonal variation in water temperature (Minckley 1979). Postlarval razorback sucker have otoliths visible under dissecting LM when 10 mm long (Minckley and Gustafson 19821. The structures were well developed and a focus sharply defined in YOY from DNFH, as was marginal growth on a generally horizontal plane through the fourth to sixth years in older hatchery fish. However, expansion was greatest anteriorly, posteriorly, and dorsolaterally, and less mesoventrally, to result (as in wild fish) in an incipient cup shape. A fragile inner arch anterior to the sagittal body was visible in YOY, seemingly grew at a lesser rate than the margins, and had circuli unequal in number (usually fewer) to those on the sagittal body. It was typically broken during preparation. Intact and readable sagittae of wild-caught razorback sucker all had 23 or more distinct marks (24 if the margin is considered a last annulus) interpretable as annuli. The greatest apparent age based on unquestioned, continuous marks across the otolith section, was 44 years, considering the margin as the last annulus. If these represent absolute ages, individuals comprising the sample of 70 fish in Figure 6 originated between 1937 and 1958, and 62 (88.6%) hatched prior to or coincident with construction and filling (1942-54) of Lake Mohave. Our data on longevity and years of recruitment are supported by a 52.4 cm SL male collected in 1976 from the lower Colorado River estimated by otolith (asteriscus) examination to be 22 years old (hatched ca. 1954), and another 60 cm long (presumably TL [ca. 47 mm SL], no sex given, collected in 19711 that had 17 annuli (also hatched near 1954) (age determined by the late John E. Fitch, CADFG, fide. James A. St. Amant, in McAda and Wydoski [1980] and in litt. 1976). Evidence of consistently slow growth in large adults in the upper Colorado basin is provided by an absence of detectable growth in a tagged adult (length unreported) over a period of 1.5 years and only a 2.3 mm annual increment in another individual (39.8 mm TL at tagging) over 3.5 years (McAda and Wydoski 1980). Tyus (1987) further demonstrated an average annual growth of only 2.2 mm for 39 tagged and recaptured razorback sucker over a period of 1 to 8 years in Green River, Utah. Rapid initial growth followed by a decline to annual increments of 1.0 cm or less has been reported for many other catostomids (Houser 1969, Beamish and Harvey 1969, Beamish 1973, MacCrimmon 1979, Beamish and McFarlane 1983, Sigler et al. 1985). Cui-ui (Chasmistes cujus) in Pyramid Lake, Nevada, essentially cease to grow after they attain 20 years of age (Scoppettone et al. 1986). Otolith growth in fishes generally parallels that of the body as a whole, which is the basis of their use in age and growth studies (Dunkelberger et al. 1980, Mugiya et al. 1981, Mugiya 1984). Standard lengths were back-calculated