9539 - Laserfiche WebLink

Home Browse Search

other than backwaters, particularly in flooded vegetation. Inherently high variation in NNC abundance and changing habitat conditions probably contribute to difficulties in data interpretation. ~ The results of this study suggest that depletion of NNC is attainable within individual backwaters on one-day sampling occasions (one trip), but depletion among sampling occasions (trips) is eventually offset by reproduction, recruitment, and immigration of NNC into newly depleted backwaters. Overall, ! treatment reaches in the Green River tended toward lower NNC relative densities than control reaches, but lack of depletion among trips and increases in NNC CPE during the final sampling trips suggest that removal did not cause these differences. Conversely, results from the Colorado River indicate that, while ~ no treatment/control difference was detected, CPE did vary among removal trips, although not in the anticipated negative fashion. These findings suggest that effects of nonnative fish control may not be detectable by looking solely at results from removal efforts (i.e., for evidence of depletion over time) or ~ treatment/control comparisons, but rather that results from the two approaches should be considered together as they may suggest conflicting interpretations. Although some reduction in abundance of nonnative cyprinids was observed among the first few removal efforts in treatment reaches in 1998 and 1999 in the Green River, particularly of adult fish, the reduction was very temporary and was quickly offset by reproduction and growth of larval fishes into a size vulnerable to the sampling gear. Small fish were frequently observed to escape through the seine, ' which was selected to exclude fish less than 25 mm. As reproduction and growth occurred, the number and relative proportion of subadult fishes increased during the sampling period each year. Subadults less than 25 mm TL early in the spring likely recruited into the adult category and vulnerability to the gear type later in the season. The effects of NNC reproduction on removal efforts are even more pronounced in the Colorado River during 1998 and 1999. Positive CPE trends were due primarily to increases in subadult fishes, but 1 adult fish CPE increased as well. This latter could be a result of growth of subadults into adults as well as immigration, particularly in 1998 when the final trip was conducted four weeks after the previous trip. The four-week hiatus likely allowed subadult fish to grow to adult size and larval fish to become fully recruited to the gear. The increase in catch in 2000 was primarily composed of subadult fish. The low water and '~ -15- 1