7872 - Laserfiche WebLink

Home Browse Search

1 1 1 6-hour period. This quantity was estimated by conducting laboratory studies in which red shiners were offered a surplus of chironomid larvae prey. Mean maximum consumption of Colorado squawfish larvae in a 6-hour period was approximately i 0% of red shiner body mass. The number of larvae used in each trial varied from 15 for the lazgest larvae to 227 for the smallest. In alternative prey trials, an equal mass of Colorado squawfish larvae and chironomid larvae were used such that the total mass of prey did not exceed 90% of red shiner maximum ration. Generally, 25 to 100 chironomid larvae (0.005 to 0.01 g each) were added. At the conclusion of a trial, predators were captured and remaining larvae and alternative prey were recovered from pools by washing sediment and straining pool contents through fine-mesh nets. Recovered fish larvae were counted, and chironomid larvae- were weighed to determine red shiner consumption. Preliminary trials showed that in 5 of 5 cases, the removal protocol recovered all 25 10-mm fish larvae and chironomids released into mesocosms. The response variable measured for each mesocosm trial was number of larvae eaten. To estimate the number of attacks that occurred in a trial, this quantity was divided by its respective size-dependent capture probability from equations 2 and 3. For example, if ten larvae were eaten and probability of capture was 0.25, then 10 • (0.25)'' = 40 attacks must have occurred. The estimated total number of attacks in each trial was then sealed to (number of attacks• m2)~(larva• red shiner day)-' where day was defined as being 14-hours long which approximates the daylight period during summer in the Colorado River Basin. Data from 69 pool trials was analyzed with a general linear model that assumed a Poisson distribution of the responses because most observations were at or neaz zero with relatively few large values. The data were used to estimate a statistical model that predicted loge(attack rate) as 14