Laserfiche WebLink
substituting the trapping technique, (2) maintaining the current sampling protocol and adding <br />additional sampling (trapping or electrofishing, whichever is most effective) on a regular or as-needed <br />basis, (3) mark-recapture population estimates, (4) age analysis of captured fish, and (5) collecting <br />abundance and biological data on the entire fish community. <br />Humpback Chub <br />Both Black Rocks and Westwater Canyon continue to support large populations of humpback <br />chub. Size of the adult population in Black Rocks (the only part of the population measurable with <br />trammel-net data) appears basically unchanged from 1988 to 1991, although CPE was somewhat <br />higher in 1991. However, the population structure appears to have shifted towards larger fish by <br />1991. In Westwater Canyon, catch rates for humpback chub increased somewhat during the course of <br />ISMP, with highest catches reported in 1991. The most obvious change in Westwater Canyon is <br />increased abundance of roundtail chub in the upper canyon. CPE for roundtail chub was about six <br />times higher in 1991 than it was earlier and their relative proportion of the Gila community increased <br />from about 33 % in 1988 to 62 % in 1991. The size structure in Westwater Canyon and Black Rocks <br />indicates continued recruitment to adult populations of both roundtail and humpback chubs. A large <br />number of juvenile humpback chub were collected from Westwater Canyon, but relatively few were <br />collected from Black Rocks. <br />The sampling schedule as currently outlined for ISMP may not be adequate to monitor the two <br />populations of humpback chub. The 3-year sampling intervals could allow substantial population <br />changes to occur without being detected. Further, using only trammel nets restricts sampling to the <br />adult population. Monitoring should also include gears expected to capture juvenile and YOY chubs <br />such as seines, traps, and small-mesh trammel nets. The additional sampling gear would allow <br />monitoring all age classes of chubs in the two populations. In addition, sampling frequency should be <br />increased to every 2 years. However, ISMP in Westwater Canyon should be temporarily suspended <br />because a more intensive study of that population is currently underway. ISMP should be reinitiated <br />after that study is completed. ISMP can be.considered for other humpback chub populations as well <br />when intensive studies of those populations are completed. <br />ix <br />