9375 - Laserfiche WebLink

Home Browse Search

as 4 days post-peak in 1992 (low flow year) and as much as 39 days post-peak in 1995 (high flow year). The same relationship was observed above the canyon. Water temperature was a more consistent predictor of chub spawning than river flow. Spawning commenced above the canyon when river temps reached 18°C, in 1992, 1993 and 1994, and at 17°C in 1996. In Westwater Canyon, spawning was initiated at river temps ranging from 17-20°C during the five years YOY were collected there. Growth of Age 0 Chubs Young of year chub daily growth rates (mm /day) were calculated for the months of July and August when sample sizes exceeded 5 fish on two occasions per month. Monthly growth rates ranged from a project low of 0.15 recorded above the canyon during August, 1995, to a high of 0.83 in the canyon during August, 1994. A positive correlation (Rz= 0.59; p=0.02) was found between the monthly growth rates and respective monthly degree days (>20 C) (Figure 6). Chub YOY Cohort Relative Abundance Sampling for YOY occurred throughout the summer, but many of the following analyses considered only July and August collection data. During these months, YOY were most abundant and still preferred low velocity areas opposed to flowing water habitats. This shift in habitat preference typically occurred in September or when the YOY grew to 40 - SOmm TL. [During the September, 1996, fish community trip more YOY were collected electrofishing the shoreline than with seines in low velocity areas.] Young of year chubs were collected in all three sample sub-reaches with 1668 collected above the canyon, 1477 in Westwater Canyon, and 71 below the canyon. Young of year chubs were present in sub-reach samples with the following frequencies of occurrence: 81.5% (n=27) above the canyon, 55.17% (n=29) within the canyon, and 58.3% (n=26) below the canyon. Catch rates (fish / mZ) were significantly (F=3.036; p=0.002) more variable above the canyon than within it. Project mean catch rates per strata were greatest above the canyon (Table 4). Instantaneous catch rates in this uppermost strata ranged from a project high of 2.28 observed on 02 Aug 93 to zero recorded on several occasions (Figure 7). Within the canyon, CPE ranged from a high of 1.42 recorded on 15 Aug 96 to a low of zero recorded on several occasions, particularly in May, June and early July. Below the canyon, catch rates ranged from a high of 0.24 recorded on 29 Ju194 to zero and were significantly less (t=2.413; df=28; p=.022; canyon vs below canyon) than the upper strata catch rates. Table 4. Annual mean catch per unit effort (fish /mz) and standard en or for each of three Colorado River sampling strata: Above Canyon (RK 204 - 199.4); Canyon (RK 199.2- 190.4), and Below Canyon (RK 186.4 - 176.8), Utah. 1992-1996. ABOVE CANYON CANYON BELOW CANYON n+ cpe* S.E. n+ cpe* S.E. n+ cpe* S.E. 1992 4 .034 .0 ] 4 4 .321 .196 4 .017 .008 1993 6 .679 .154 6 .086 .028 4 .024 .009 1994 8 .062 .019 8 .175 .032 6 .034 .016 1995 5 .007 .OOI 7 .012 .005 7 .002 .0007 1996 4 .607 .249 4 .673 .153 4 .018 .006 ~ampung occasions * grand average of total cpe per strata 7