Laserfiche WebLink
<br />c:':.) <br />Cl <br />...... <br />(X) <br />00 <br />CD <br /> <br />Roundtail Chub <br />Collection of relatively few adult and juvenile roundtail chub during the 7 -year research period led Ryden <br />(2000a) to conclude that this species did not have a significant mainstem population. Roundtail chub was <br />relatively common in some San Juan River tributaries, such as the La Plata and Mancos rivers, but in recent <br />years their munbers declined in other tributaries, such as the Animas and Florida rivers (Miller and Rees <br />2000). Based on collections from 1934 and 1961 held in the University of Michigan Museum of Zoology, <br />roundtail chub was relatively common in the mainstem San Juan River below Farmington, New Mexico, <br />prior to 1962, but they have declined dramatically since then. <br /> <br />Reasons roundtail chub declined in the San Juan River are not known. One hypothesis is that a fish- <br />poisoning project in 1962, prior to closure of Navajo Dam, killed roundtail chub downstream as far as <br />Shiprock, New Mexico. Another hypothesis is that Navajo Dam has fragmented their habitat, preventing <br />movement from upstream roundtail chub populations to the mainstem river below the dam. Some biologists <br />believe that roundtail chub numbers were reduced by channel catfish predation (F. Pfeifer, USFWS, <br />Personal Communication). Tbis hypothesis is based on relatively large numbers of roundtail chub in the <br />Gunnison and upper Colorado rivers, where irrigation diversion dams block access of channel catfish, <br />compared with reaches of these rivers below the irrigation dams where channel catfish are abundant and <br />roundtail chub are rare. It is also possible that a combination of these events caused the decline: the <br />population was reduced by the poisoning operation and channel catfish predation kept it from rebuilding. <br />Whatever the reasons, roundtail chub is not common in the San Juan River. <br /> <br />Miller et al. (1993) found roundtail chub in several San Juan River tributaries and detennined habitat use <br />based on the habitats where the fish were collected. Roundtai1 chub YOY were collected primarily in low- <br />velocity glide and pool habitats. Juveniles and adults were typically found in pool habitats with woody <br />debris cover. A few adults were also collected from glides. Vanicek and Kramer (1969) collected <br />roundtail chubs primarily from pools and eddies in the Green River, and Holden and Stalnaker (1975) <br />collected them from a variety of habitats throughout the Upper Basin. Tbis information indicates that pools <br />and eddies are key habitats for roundtail chub. <br /> <br />F/anne/mouth Sucker <br />Flannelmouth sucker was the most-abundant fish in San Juan River electrofishing surveys during <br />the7-year research period (Ryden 2000a). All life stages were found, and at times the species <br />numerically dominated collections. Tbis suggests that habitat is not limiting for this species in the <br />San Juan River. Flannelmouth sucker abundance increased in Reach 6, the upper San Juan River, <br />but declined in the remainder of the study area during the course of the 7 -year research period, <br />raising concern for the status of this species. Ryden (2000a) hypothesized that flannelmouth sucker <br />declined because of high numbers offish with low condition factors resulting from drought prior to the 7- <br />year research period. After comparing 1991 to 1997 data with the 1987 to 1989 data, Ryden (2000a) <br />hypothesized that the decline may be a cyclic phenomenon in flannelmouth sucker <br />populations. It is also possible that habitat changes from Navajo Dam's reoperation to mimic a <br />natural hydrograph (higher spring flows and lower base flows) reduced the amount of habitat for <br /> <br />September 2000 <br /> <br />3-14 <br /> <br />Program Evaluation Report <br />