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ISMP backwater sampling evaluation <br />EXECUTIVE SUMMARY <br />We sampled backwaters in the Grand Valley reach of the Colorado River, CO, to estimate <br />bias and precision of the Interagency Standardized Monitoring Program (ISMP) sampling <br />technique to detect presence and estimate abundance of centrarchid fishes. This was <br />accomplished by sampling backwaters with the relatively low effort ISMP seine-sampling <br />approach, followed by relatively intensive depletion or capture-recapture (DMR) sampling. <br />Presence-absence and abundance data gathered with each technique were then compared to <br />determine bias and precision of ISMP. <br />A total of 46 backwaters were sampled in 1997 and 1998. A total of 108,542 fish were <br /> <br /> <br /> <br /> <br /> <br /> <br />captured in those samples, most of which were non-native cyprinids sand shiner Notropis <br />stramineus (41 %), red shiner Cyprinella lutrensis (26 %), and fathead minnow Pimephales <br />promelas (20 %). Largemouth bass and green sunfish represented 4.9 % of all fishes captured. <br />Most largemouth bass were less than 120 mm total length (TL), but individuals up to 263 mm TL <br />were captured. Green sunfish captured were generally less than 80 mm TL, but individuals up to <br />227 mm TL were captured. <br />Overall, the ISMP sampling approach underestimated the number of backwaters occupied <br />by largemouth bass and green sunfish by about 50 %. In other words, ISMP detected those <br />centrarchid species in only every other backwater in which they occurred. Results of a logistic <br />regression model suggested that the probability of detecting bass and sunfish in backwaters with <br />the ISMP sampling technique was relatively low even when each was relatively abundant. When <br />ISMP sampling detected largemouth bass and green sunfish in Colorado River backwaters, <br />iii