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i 74
<br />The Southu~eslern Naftnalisf
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<br /> 75 I 76 I I 77 III 78 I V 79 V - 00 V I
<br /> YEGR OF LIFE
<br />vol. 28, no. 2
<br />Frc. 8.-Comparisons of growth of known-age razorback suckers (numbers indicate sample
<br />sizes), with growth estimates back-calculated from scale annuli Ior 20 fish sacrificed in 1975.
<br />Horizontal lines are means, bars represent tl standard error, and vertical lines represent thr range
<br />of lengths (darkened -known-age fish; open -estimated from annuli).
<br />investigators could not be achieved for more than the first six apparent
<br />annular marks on scales. After [hat point dense circuli were over-lapping,
<br />obscure, or obviously lacking, with scales often appearing to have grown
<br />uniformly except for random "checks" that could not be interpreted as
<br />annuli. Most scales were deeply worn on the margins, and regenerated scales
<br />(excluded from further consideration) were common. Minimum estimates for
<br />ages of fish with possible annuli were ] 1 to ]4 years, and maxima ranged to
<br />>20 years. Estimates of growth rates for the first six years of life mmpare
<br />favorably, however, with growth o[ known age fishes under hatchery and
<br />laboratory conditions (Fig. 3).
<br />Variation in length o[ individuals from the hatchery cohort is remarkably
<br />high, and this is shared by estimates of growth made from scales of wild-
<br />caught adults. Multiple or protracted spawning in nature may result in a
<br />similar size distribution, but the hatchery cohort all were spawned the same
<br />day in 1974. A similar size range (7.1 to 13.5 cm TL at 5 months of age) in
<br />hatchery produced fish reared in ponds at Dexter, News Mexico, in 1981 (J.
<br />E. Johnson, pers. comm.) provides further evidence {or inate variability in
<br />this life-history character. Size variation in the 1914 cohort resulted in some
<br />hatchery individuals becoming sexually mature at six years of age and 35.39
<br />crtt TL, white siblings <35 cm TL had no sexual development.
<br />McAda and L1'ydoski (1980) similarly found little relationship between
<br />scale radius and body lengths of razorback suckers. They assigned ages of 4
<br />to 9 years to fish 49 to 61 cm TL, but doubted accuracy of their determina-
<br />tions. Evidence against validity of their data was a report by James St.
<br />Amant (in McAda and t•'1'ydoski, 1980) of a 66.2 cm TL male from the lower
<br />Colorado River estimated to be 22 years old and another large fish (length
<br />unknown) that was 17 years old, both on the basis of sagitta analysis.
<br />b4cAda and Wydoski also noted a fish recaptured 1.5 years alter original
<br />tagging in the upper river had not grown at all, and a second specimen
<br />(50.8 cm TL) had grown only 8 mm when recaptured 3.5 years later.
<br />I am of the opinion that razorback suckers currently in Lake Mohave
<br />hatched when the reservoir was filling in the early 1950s. If this is so, mean
<br />growth between 1956 and 1963 would have slowed from ca. 7.0 cm in their
<br />seventh year o[ life (Fig. 3) to less than a cm per year in 1964 (Fig. 2). Males
<br />May 1989 Mintkley-Stains of Razorback Sucker 175
<br />may have essentially ceased to grow after 1968 or before (Fig. 2), tending to
<br />suppress the apparent growth rate of the over-al! population. Death of
<br />larger, faster growing fish, and slow growth by smaller fish toward the mean
<br />[or the cohort(s), might also serve to explain the remarkably slow average
<br />growth rate in recent years. An average o[ 7.3 carcasses;'km o[ large (>50 cm
<br />TL) razorback suckers were along the shoreline of Lake Mohave in spring
<br />1979. Sagittae have now been obtained from a representative sample of
<br />razorback suckers from Lake Mohave, so accurate aging will hopefully soon
<br />be accomplished.
<br />Reproduction.-McAda and Wydoski (1980) recently summarized informa-
<br />tion on reproduction by razorback suckers. Before large dams, razorback
<br />suckers migrated in early spring, evidently to spawn (Hobbs and Miller,
<br />]953). Jordan (1891) reported such a migration info the Animas River, Colo-
<br />rado, and Chamberlain (1904) cited early repoxts that they congregated in
<br />tributaries to larger streams. Minckley and Carothers (1979) collected a
<br />gravid female razorback sucker and observed two other individuals in the
<br />Paria River, Arizona, in June ]978. Concentrations in the Salt River of Ariz-
<br />ona in the 1920s (Ellison, 1980) were presumably spawning aggregations.
<br />Douglas (1952) specifically described spawning by razorback suckers in
<br />shallow coves of Lake Havasu in March ]950. Water depths ranged from
<br />0.25 to 1.5 m and temperatures were between 14° and 16°C. lndividua]
<br />females were accompanied by 2 to 6 males as they swam in small circles over
<br />the bottom. Mates remained close to responsive females and the fish occa-
<br />sionally settled to the bottom and vibrated their bodies rapidly. Gamete
<br />emission was not visible because of silt disturbed by the fish. Jonez and
<br />Sumner (1959) observed razorback suckers spawning in Lake Mead between
<br />1 March and 15 April 1953, and described extensive shoreward movements at
<br />that time. There was a tendency [or the fish to concentrate near in[lowing
<br />rivers. Water temperatures were between 12° and 18°C. Spawning was
<br />widespread along grave] shorelines in water 0.6 to 5 m deep. My extensive
<br />observations corroborate those o[ Douglas (1952). I have most often observed
<br />spawning razorback suckers in shallow (<5 m) waters along gravelly, sub-
<br />merged terraces in bays and inlets, and once along canyon walls and on a
<br />gravel-cobble bar among boulders in current about 1.6 km below Hoover
<br />Dam. The last fish were concentrated in an area of inflowing hot springs
<br />(Gustafson, 1975a).
<br />McAda and Wydoski (1980) collected razorback suckers in spawning con-
<br />dition from water about 1 m deep and at the upstream ends of gravel bars in
<br />the upper Colorado River basin. Substrate was predominately cobble, and
<br />water velocity was about 1 m/sec. Females captured over bars were ripe, but
<br />those from other places were not; al] males over or near bars were ripe. They
<br />also observed behavior resembling that described by Douglas (1952) in the
<br />gravel pit at Walker Wildlife Area, but no successful reproduction was indi-
<br />cated. Linda Ulmer and Loudermilk (pers. comm.) have confirmed spawn-
<br />ing activity and hatching of eggs on small, gravelly, wave-cut terraces of
<br />Senator Wash Reservoir, California.
<br />Holden (in McAda and Wydoski, 1980) collected a few subadult fish in the
<br />upper Colorado River basin before 1977, and Holden (1978) reported tenta-
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