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<br />. '. ~~. <br /> <br />001619 <br /> <br />Roundtail Chub <br />Collection of relatively few adult and juvenile roundtail chub during the 7 -year research period led <br />Ryden (2000a) to conclude that this species did not have a significant mainstem population. <br />Roundtail chub was relatively common in some San Juan River tributaries, such as the La Plata and <br />Mancos rivers, but in recent years their numbers declined in other tributaries, such as the Animas <br />and Florida rivers (Miller and Rees 2000). Based on collections from 1934 and 1961 held in the <br />University of Michigan Museum of Zoology, roundtail chub was relatively common in the <br />mainstem San Juan River below Farmington, New Mexico, prior to 1962, but they have declined <br />dramatically since then. <br /> <br />Reasons roundtail chub declined in the San Juan River are not known. One hypothesis is that a <br />fish-poisoning project in 1962, prior to closure of Navajo Dam, killed roundtail chub downstream <br />as far as Shiprock, New Mexico. Another hypothesis is that Navajo Dam has fragmented their <br />habitat, preventing movement from upstream roundtail chub populations to the mainstem river <br />below the dam. Some biologists believe that roundtail chub numbers were reduced by channel <br />catfish predation (F. Pfeifer, USFWS, Personal Communication). This hypothesis is based on <br />relatively large numbers of roundtail chub in the Gunnison and upper Colorado rivers, where <br />irrigation diversion dams block access of channel catfish, compared with reaches of these rivers <br />below the irrigation dams where channel catfish are abundant and roundtail chub are rare. It is also <br />possible that a combination of these events caused the decline: the population was reduced by the <br />poisoning operation and channel catfish predation kept it from rebuilding. Whatever the reasons, <br />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 determined <br />habitat use based on the habitats where the fish were collected. Roundtail chub YOY were <br />collected primarily in low-velocity glide and pool habitats. Juveniles and adults were typically <br />found in pool habitats with woody debris cover. A few adults were also collected from glides. <br />Vanicek and Kramer (1969) collected roundtail chubs primarily from pools and eddies in the Green <br />River, and Holden and Stalnaker (1975) collected them from a variety of habitats throughout the <br />Upper Basin. This information indicates that pools and eddies are key habitats for roundtail chub. <br /> <br />Flannelmouth Sucker <br />Flannelmouth sucker was the most-abundant fish in San Juan River electrofishing surveys during <br />the 7 -year research period (Ryden 2000a). All life stages were found, and at times the species <br />numerically dominated collections. This 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 <br />to the 7 -year research period. After comparing 1991 to 1997 data with the 1987 to 1989 data, <br />Ryden (2000a) 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 />3-14 <br /> <br />Program Evaluation Report <br /> <br />September 2000 <br />