8170 - Laserfiche WebLink

Home Browse Search

years. Neither peak flows nor habitat availabili could be related to fall eaz class stren of tY Y ~ Colorado pikeminnow. However, fall catch rates of Colorado pikeminnow in both reaches followed the same trends for the five years of this study, and were positively (non-significantly) correlated (r~ = 0.76, p = 0.13). This indicates that the overall response of Colorado pikeminnow to the flow regimes was similaz, despite the shifting effects of peak and base flows, habitat availability, temperature, larval production, nonnative densities and other unknown factors. '~ The large desert river system in which we work is inherently variable. These long-lived fish evolved in this unusual environment and, as a species, could presumably survive the more extreme events of flood and drought in the unregulated system. This does not mean that extreme events create particulazly favorable conditions. A moderate year, in which peak flows aze near average, temperature fluctuations aze reliable and habitats aze stable, is likely to offer the most `~ favorable conditions for both good production and subsequent recruitment into the age one group. However, the nonnatives which have become so plentiful in the rivers also thrive under moderate flow conditions. It is recommended to manage the river system as naturally as possible, in order to take advantage of the positive aspects of both high and moderate flow scenarios, while avoiding extremely low flows which may favor nonnatives over the Colorado pikeminnow. Sampling Efficiency Evaluation of sampling efficiency showed that ISMP sampling is representative of the species composition and relative abundance of the more common fish present in a given section of backwater. However, subsequent seining can produce 1-4 additional species per backwater. For example, during the Apri11995 sampling effort five species were present in the river which did not appeaz in any of the first seine hauls. The ISMP sampling would have missed these species; flannelmouth sucker, bluehead sucker, channel catfish, black bullhead and speckled dace. However, the species missed on the first haul are generally incidental species, which do not include Colorado pikeminnow in the sections of river tested. Colorado pikeminnow, if present, were collected on the first haul on 82% of occasions. Sampling efficiency was lowest in areas with obstructed substrates (vegetation or rocks), but did not seem to be affected by depth of water, depth of mud, or total backwater size. The t variation observed in catch rates and capture probabilities cannot be attributed to these factors. Catch rates in the same section of the same backwater taken at different times and days varied greatly. The second sample catch rate was variously higher, lower, or the same. There was no observed relationship of catch rates or capture probability to time of day or temperature. Also, repeated sampling did not significantly reduce the catch rates or the number of fish present on subsequent occasions, although there was some mortality of nonnative fish during the sampling procedures. The propensity of these species to assemble in integrated schools affects their distribution within backwaters. Fish distribution is not homogenous or random. Instead, they occur in disjunct groupings. A seine haul pulled in any one spot has an initial catch rate based more on whether or not fish aze schooling at that location than anything else. Therefore repeated sampling within backwaters is necessary to obtain a representative sample. ISMP akeady accounts for this by requiring two seine hauls in each backwater. f~ xiii