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JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE CALIFORNIA IMPERIAL COUNTY VOL. 23 ~\NEVADA ~ •~''°~ UTAH c~rr ar. ~ -- _ n.di La ua ~1~A `\ 14fa rNI -- °ba {aNv Cllr ~ll, 1Rb MNiw ~ +~ • Lxy~a° • fly~tdf I La r~la ~~ ° • ~ ~Hw ""~"' ARIZONA .rn.. sruvr ~,~/ ' ~ O.f.~ AREA ~FxaEm c, •o,° our _ rW o ~ •r~a , w.noew KEY MAF' ALL-AMERfCAN CANAL ARIZONA YUMA COUNTY Figure 1. Map of the Coachella Canal and Vicinity. weighted with 2-cm steel chain to ensure conformance with the canal's bottom. Two sets of metal frames (4 m x 4 m), with identical netting, were placed in the downstream check drop. Nets restricted all but the smallest fish from escaping downstream during dewatering. This approach ensured normal fish dis- tribution and allowed for a staged sample in case fish biomass or numbers made it impractical to sample the entire reach. On 5 November 1984; nets were placed, an up- stream check drop was closed, and the canal was allowed to drain to a depth of 20 cm through partial opening of the lower check drop. Fish were collected using 6-mm-mesh bag seines and dip nets, lifted by bucket from the canal by a hydraulic boom truck, and placed in holding tanks for later processing. Species numbers, total lengths, and weights were recorded for a subsample of the catch, with the remaining fish being separated by species, counted, and weighed in bulk. Fish were later released to a ponded section of canal upstream from the study area. After seining, canal banks were walked to count fish not captured. Number of individuals observed were multiplied by average weights for the same species taken earlier to estimate total biomass. MEXICO RESULTS Seining yielded 3,110 fishes (Table 1), consisting of five species. A subsample of 776 channel catfish (Ictalurus punctatus), 35 common carp (Cyprinus carpio), 16 threadfin shad (Dorosoma petenense), and a single largemouth bass (Micropterus salmoides) were individ- ually weighed and measured. The shallow and clear nature of water remaining in the canal allowed for an accurate count of fishes which had evaded capture. In Sections B, C, and D, 330 fish were observed in the canal. Section A con- tained several hundred fishes, which were assumed to have passed downstream when the upstream check gates were accidentially opened following seining; these were excluded from analysis. Channel catfish dominated in both numbers and weight. Total relative abundance exceeded 95% for all sections, with an overall average of 96.6%. Channel catfish appeared to be slightly more abundant imme- dately below the upstream check drop (Section A, 0.136 fish/m2) than further downstream (Section D, 0.022 fish/m2) and had an average density of 0.036 fish/m2. Biomass averaged 3.237 g/m2 for the entire study reach (Table 2).