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<br />Hybridization of Three Species of Chub in a Hatchery
<br />
<br />Three species of chub coexist in the Green and
<br />Colorado rivers: bony tail chub (Gila elegans), hump-
<br />back chub (G. cypha), and roundtail chub (G.
<br />robusta). This Gila complex in the upper Colorado
<br />River basin has often been troublesome to taxonomists.
<br />Deacon et aI. (1979) listed the bony tail chub as an
<br />endangered species in Arizona, California, Colorado,
<br />Nevada, Utah, and Wyoming, and the humpback
<br />chub in Arizona, Colorado, Utah, and Wyoming.
<br />Both are listed as endangered species by the U.S.
<br />Department of the Intcrior (1980).
<br />Holden and Stalnaker (1970) found intergrades of
<br />the round tail and bony tail chubs and of bony tail and
<br />humpback chubs in the upper Colorado River basin;
<br />intergrades of bony tail and humpback chubs were the
<br />more numerous. Smith et al. (1979) suggested that
<br />many specimens referred to by Holden and Stalnaker
<br />as intergrades were, in fact, pure species. The purpose
<br />of this study was to determine whether the bony tail
<br />chub could hybridizc with the roundtail chub and
<br />humpback chub.
<br />Brood stock consistcd of five bony tail chub females,
<br />eight round tail chub males, and five humpback chub
<br />males. Bony tail chubs were collected from Lake
<br />Mohave, near Cottonwood Cove, 30 April to I May
<br />1979 and 29 April to 3 May 1980. The round tail and
<br />humpback chubs werc collected from the Colorado
<br />River, near Black Rocks, 5 November 1979. Total
<br />lengths and weights of the collected specimens were
<br />457 -559 mm and 850-1,300 g (bony tail), 298-352 mm
<br />and 240-420 g (roundtail), and 304-330 mm and
<br />275-385 g (humpback).
<br />Because spawning temperatures range from 18 to
<br />2loC (U.S. Fish and Wildlife Service 1979), incoming
<br />hatchery water (12-130 C) was heated with a liquid
<br />propane gas water heater (45,000 BTU/h), capable of
<br />heating 23 L/min of incoming water to 20-2Ioe. The
<br />heated water entered two ad jacen t concrete tanks (6 x
<br />0.75 x 0.75 m) that scrved for recirculation, settling of
<br />suspended matter, and aeration. Water was pumped at
<br />132 L/ min from onc tank into the other by a 1/ 20-hp
<br />fresh-f1o pump, and returned to the first tank by
<br />gravity. Excess water escaped over a standpipe 70 cm
<br />tall and 7.5 cm in diameter. Water was pumped from
<br />these tanks with a I fJ-hp sump pump through 1.73-cm
<br />polyvinyl chloride pipe with brass valves at a rate of
<br />8 L/min to egg incubators and at 5 L/min to the two
<br />broodstock tanks (6 x 0.75 x 0.75 m). Adult chubs
<br />were held in the first tank, which contained (in the
<br />upper half) cobblestones ranging in size from 50 to
<br />
<br />140
<br />
<br />75 mm in diameter, through which water percolated to
<br />create an artificial spawning environment. The cobble
<br />was not used by the fish, however.
<br />The egg incubators consisted of three alumlllum
<br />troughs (3.5 x 0.55 x 0.30 m); two troughs received
<br />heated water and the third received river water
<br />(12-l30C). Hatching trays (53)( 53 cm) were covered
<br />with lA-mm-mesh hardware cloth and slanted at a 300
<br />angle into the incoming water.
<br />Ripening of sex products was induced by hormone
<br />injection (Ihering 1937; Ball and Bacon 1954; Clemens
<br />and Sneed 1962). Acetone-dried carp pituitary was
<br />mixed with distilled water (40 mg/1O mL for the
<br />bony tail x round tail cross) or with a solution of
<br />oxytetracycline hydrochloride (40 mg(1O mL for the
<br />bony tail x humpback cross). Before the intra-
<br />peritoneal injections of 4 mg/ kg body weight, each
<br />fish was anesthetized with tricaine methanesulfonate
<br />at I :20,000 to I: 10,000 (Leitritz 1960). Injections were
<br />repeated at 24-h intervals until the fish were ripe.
<br />When eggs could be expelled with slight pressure,
<br />the female was anesthetized and wiped dry and the
<br />eggs were stripped into a plastic pa n. Milt from three
<br />to four males was added to the eggs and water was then
<br />added. When the eggs began to clump and adhere, they
<br />were washed for 60 min, then separated. After samples
<br />were taken to determine size, volume, and numbers,
<br />the eggs were placed on screen trays for incubation and
<br />hatching.
<br />The day after swim up, fry were transferred to a cage
<br />(I x I x I m, covered with 0.5-mm saran filter) in an
<br />outside recirculating raceway. The fry were then fed a
<br />starter trout diet and the natural zooplankton (pre-
<br />dominately cIadocerans) that were present in the
<br />raceway.
<br />
<br />Bony tail chub x roundtail chub. All round tail males
<br />produced milt of a low viscosity after two injections.
<br />One bony tail female was ripe at the time of capture
<br />and two became ripe 24 h after the first injection. Egg
<br />diameters varied from 1.5 to 2.0 mm; the volume was
<br />103 eggs/ mL. Fecundity was 15,862 and 20,806 in the
<br />two injected females and 7,216 in the uninjected
<br />female. Eggs from the uninjected female were infertile.
<br />Eggs placed on slanted trays at 130 C developed very
<br />slowly. Embryos formed by 96 h, but died after I 10 h.
<br />The low water temperature is believed to have contribu-
<br />ted to or caused the mortality.
<br />Eggs placed on slanted trays at 200 C developed
<br />more rapidly. The first cleavage was at 3.6 hand
<br />
<br />Prog. Fish-Cult. 43(3), July 1981
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