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428 The Southwestern Naturalist vol. 35, no. 4 1989. Sampling in the Green River was conducted every second or third day depending on success of pre- vious sampling effort, and once a week in the lower Yampa River in 1988 and 1989. Captures of razorback suckers in other spring electrofishing and trammel net- ting programs in the lower 72 km of the Yampa River (1987 to 1989) were also included. Sampling was ini- tiated in mid-to-late April (dependent upon river ice- melt, weather conditions, and river access) and ter- minated in June following several attempts with no razorback sucker captures. This period was sampled because of captures of ripe razorback suckers in pre- vious years (Tyus, 1987; C. McAda, pers. comm.). Sex and spawning condition were determined by expression of sex products (milt or eggs) either spontaneously or following manual pressure on the abdomen. Sex was undetermined for fish with breeding tubercles but with- out expressible sex products, and these fish were not considered to be actively spawning (i.e., releasing eggs or sperm). The estimated breeding period was defined by the interval between first and last capture of ripe fish. New captures were tagged with a uniquely num- bered Carlin-dangler tag, and most fish were weighed and measured (total length, TL). Movements and habitat use were investigated from 1987 to 1989 using capture-recapture data and radiote- lemetry. Fish used in the capture-recapture analyses included razorback suckers tagged and recaptured by United States Fish and Wildlife Service (USFWS), individuals tagged by USFWS and recaptured by an- other agency, and fish tagged by another agency and recaptured by personnel of USFWS. Seventeen razor- back suckers were implanted with radio transmitters by R. Valdez and W. Masslich (pers. comm.) in Oc- tober and November 1986 and 1987 and tracked by them during winter. These same fish were tracked by USFWS during late spring and summer. We present USFWS radiotracking data, but locations and dates of initial captures and radio-implantations by R. Valdez and W. Masslich (pers. comm.) were used as reference for our evaluation of spring movements. Habitat descriptors (water temperature, depth, ve- locity, substrate, and general habitat type such as riffle, eddy, shoreline, and flowing or quiet water) were re- corded at the point of fish capture. Water temperature was recorded with hand-held thermometers, velocity was measured at 60% of water depth, depth was mea- sured with a wading rod, and substrate was classified by visual and manual observation. Substrate in the spawning reaches was further described from visual observations taken during periods of low flow. Four thermographs were used to obtain continuous records of river temperatures; two were placed in the lower Yampa River near its confluence with the Green River, and two were placed in the Green River near Jensen, Utah. Radio-implanted fish (Tyus, 1988) were individu- ally identified by a unique signal (frequency and pulse rate) using radio receivers, omni-directional whip and bi-directional loop antennas. Attempts were made to locate the radiotagged fish each week from mid-April to early August 1987 and 1988, using airplanes and boats. Location (km) and microhabitat data (depth, velocity, substrate) were taken at the triangulated sig- nal source if the fish remained in one position for 30 min. After the 30-min period, each fish was disturbed by wading or probing with a rod to confirm its location, ensure that it was alive, and to collect substrate data. Fish movements and recaptures occurring within a spawning reach or remaining near the initial contact were classed as "local," and movements between rivers or spawning reaches were classed as "migrations." Fish movements to and from a spawning reach in the spring were considered spawning migrations. Numbers, cap- ture dates, and locations of native ripe flannelmouth suckers (Catostomus latipinnis), bluehead suckers (Ca- tostomus discobolus), and putative razorback x flan- nelmouth sucker hybrids were recorded in 1989. RESUL Ts-A total of 230 adult razorback suck- ers were captured from 1987 to 1989. Of these, 125 fish received new Carlin-dangler tags, and 70 fish were recaptured on 105 occasions (24 fish were recaptured more than once). Most razor- back suckers (84%, n = 194) were ripe and in- cluded 74% males (n = 144,69 recaptures) and 26% females (n = 50, 18 recaptures). An addi- tional six fish (two recaptures) were tuberculate but did not have expressible sex products. Total lengths of all razorback suckers ranged from 405 to 597 mm. Males averaged 507 mm TL (n = 140, SD = 23.76) and 1,370 g (n = 136, SD = 200.9); females averaged 547 mm TL (n = 45, SD = 20.56) and 1,686 g (n = 43, SD = 296.98). Spawning Habits-Ripe male razorback suck- ers were tuberculate on the anal and caudal fins and along the sides of the caudal peduncle. Some ripe females were lightly tuberculate in these same areas, but most were not. Recaptures within a season indicated that the appearance of breeding tubercles in males was closely followed by a readi- ness to spawn and that tubercles began to fade soon after the fish were spent (four ripe, heavily tuberculate males were recaptured 11 to 35 days later at which time they were not ripe and only lightly tuberculate). Razorback suckers were most readily captured in two areas between mid-April and early June: the Jensen site (km 486.4 to 504.0) and the Yam- pa site (lower 5 km of the Yampa River, Fig. 1). A total of 177 razorback suckers (91% of all breeding captures) was captured at the Jensen site and 14 fish (7% of all breeding captures) were captured at the Yampa site. .. .. "