9533 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I the principal identifiable food items. Digestive tracts of 22 (5%) red shiner adults contained fish larvae; 9 from Shafer Canyon (4%), 2 from Lathrop Canyon (8%), 5 from Millard Canyon (18%), and 6 from the Bonita Bend area (3%). All fish larvae consumed were identified as cypriniforms and most were catostomids (species undetermined because larvae were too digested for accurate identification). The only other study in the Colorado River basin to document pr~ation by red shiner on fish larvae was that conducted by Ruppert et al. (1993). They exami~~d t~ diets of 184 red shiner (36-79 mm TL), 47 sand shiner Notropis stramineus (30-65), 4~thead minnow Pimephales promelas (32-60), 176 redside shiner Richardsonius balteatus (36-77), and 17 channel catfish Ictalurus punctatus (51-144) collected during 30 June-25 July 1991 from ephemeral shoreline embayments near the confluence of the Yampa and Green rivers, Colorado. Fish larvae were found in 15% of the red shiner but not in any other species. Of the 58 fish larvae consumed, all were cypriniforms (mostly catostomids); seven were identified as bluehead sucker. Insects, including adult or immature caddisflies, mayflies, beetles, and water striders, were the principal food items in digestive tracts of all fishes except fathead minnow, which contained mostly algae and organic debris. Ruppert et al. (1993) concluded that because red shiner are very abundant in nursery habitats used by young native fishes, they may be an important predator on native fishes in the Colorado River system, especially in habitats with low invertebrate forage during spring and early summer. Evaluation of Adult Red Shiner Exclosures: 1995 (Objective 2) Sampling outside and inside each exclosure (one inside Millard Canyon and one in a flooded wash at Bonita Bend) demonstrated that adult red shiner were successfully excluded from the exclosed areas, but larval fish (including suckers) easily passed through the block netting. The integrity of the exclosure inside Millard Canyon was successfully maintained until late June when it was breached by rising water levels, whereas, the integrity of the exclosure at Bonita Bend was maintained throughout the sampling season. Results suggest that use of exclosures to block access of nonnative fishes into important native-fish nursery habitats could be adapted for broader-scale use in the Recovery Program. In a recent review of options for selective control of nonnative fishes in the upper Colorado River basin, Lentsch et al. (1996) concluded that mechanical control of many of the smaller non natives in critical nursery habitats for endangered fishes might be achieved by blocking their access into portions of these habitats. Marking Otoliths in Larval Suckers: 1995 (Objective 3) Otoliths from all flannelmouth sucker larvae treated with TC had lucid or bright fluorescent marks, suggesting that marking of otoliths with fluorescent chemicals could be applied in field mark-capture studies on razorback sucker larvae to facilitate investigation of their early biology and recruitment. Number of distinct growth increments in otoliths cqrresponded directly to the number of days between marking and subsampling, demonstrating that otoliths of larval f1annelmouth sucker deposit daily growth increments. If flannelmouth sucker larvae are appropriate surrogates for razorback sucker in determining patterns of otolith growth, then daily deposition of growth increments could be assumed for wild razorback sucker larvae. Results of a recent laboratory study conducted by the LFL on growth of otoliths in captive larval razorback sucker confirmed daily deposition of growth increments. With these field and laboratory validations of otolith growth, wild-caught razorback sucker larvae can be 27