WSP01460 - Laserfiche WebLink

Home Browse Search

C~) c} ..... CD c:~ f"J The experimental stocking of 939 razorback sucker provided an opportunity to assess the adequacy of habitat for subadults and adults of this species. Ryden (2000b) concluded that survival of these fish was "quite good compared to" other stocking efforts, an indication that habitat was sufficient. Population estimates of the number of stocked fish that survived have not been made. Although the stocked fish initially lost weight (approximately 5 to 10% of their stocked weight), they grew relatively rapidly after the first year or so in the river, and they generally maintained their position in the river, indicating that habitat was not a limiting factor. In addition, some spawning occurred and larvae were formd, indicating at least one acceptable spawning site was found. All of this information suggests that habitat for juvenile and adult razorback sucker is available in the San Juan River for the numbers of stocked fish that survived. The abundance of flannelmouth sucker, bluehead sucker, and speckled dace, and the abundance of habitats that these species use and apparently select, suggests that habitat is not limiting for these three native species. Most biologists familiar with roundtail chub and the San Juan River believe that there is adequate habitat for this species in the mainstem river. As noted above, factors other than habitat may be the primary reasons why this species has not been able to sustain mainstem populations. The question of whether larval razorback sucker or Colorado pikeminnow will use inundated vegetation, backwaters, or some other habitat in the San Juan River will have to wait until a sufficient number of adults are spawning so that larval habitat use can be studied. It appears that larval razorback sucker habitat will be limiting in the San Juan River, but many biologists thought similarly about yay Colorado pikeminnow prior to stocking, and they found adequate habitat. At this time it is not possible to determine how many adult razorback sucker or Colorado pikemilU10w may be able to find adequate habitat in the San Juan River. Habitat will be limiting at some point, since the amount of acceptable habitat is one factor that limits populations in any situation. The lack of key razorback sucker bottomland habitat, and the lack of early and recent collection specimens (only one has been collected in the San Juan River), suggest that only a relatively small population of this species may be possible in the San Juan River. On the other hand, historical and more-recent collections of young and adult Colorado pikeminnow, and available habitat information, suggest larger populations of this species are possible. As razorback sucker population augmentation continues and the stocked Colorado pikeminnow grow to adulthood, habitat limitations for adults will be clarified. Habitat Quality Quality is another component that may make habitat limiting. Habitat quality refers to the habitat characteristics that determine how good a habitat is for a fish species. Not all backwaters, or eddies, or spawning bars are equally useful to the endangered fish species. For example, deeper backwaters were used by yay Colorado pikeminnow in the Green River (Holden 1977) more than shallow backwaters, and larger and deeper backwaters were selected over smaller and more shallow backwaters in the Green and Colorado rivers (Trammell and Chart 1999, Tranunell et al. 1999). Backwaters also need to be productive to provide small food organisms for young fish. In addition, spawning bars need to be clean (have open spaces between the cobbles) so that eggs are not September 2000 3-30 Program Evaluation Report