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Pre- and Post-Regulation Abundance of Selected Nonnative Fishes in Backwaters Visual pattern detection 1. Comparison of stocking (all known stocking events) densities vs. backwater fish densities Figure 16 shows known stocking densities of largemouth bass and densities of largemouth bass in backwater sample locations within the intensive study area of the Colorado River. The sizes of the symbols reflect the relative density of largemouth bass stocked or sampled. Visually, the large symbol (triangle) near the center of the figure suggests that there may be a concentration of largemouth bass stocking events. There are also some relatively high densities of largemouth bass found in the Colorado River near these stocking locations. A visual relationship between known stocking densities and backwater densities for fathead minnow was not observed (Figure 17). In general, fathead minnow appear to be widespread and prolific and their riverine distribution appears to be independent of stocking events. Green sunfish is not known to have been stocked at all in western Colorado, but as previously stated, it too appears to be widespread and prolific. 2. Comparison of treated ponds to backwater fish densities Largemouth bass in the ISA tended to show high densities in proximity to ponds that were slated for treatment to control nonnative fish (Figure 18). With few exceptions, however, densities remained relatively high in close proximity to these ponds after they had been treated (Figure 19). Green sunfish showed high densities throughout the backwater sample locations both distant and proximal to ponds that were to be treated (Figure 20). High densities of green sunfish remained in the sampled backwaters within the ISA even after ponds had been treated (Figure 21). Fathead minnow exhibited patterns very similar to green sunfish, and did not reveal any further potentially interesting visual spatial pattern. 3. Comparison of nonnative fish densities in ponds vs. backwaters High densities of green sunfish in the ISA appear distributed throughout the river as indicated above, and show no obvious relationship to ponds with high densities of the same species (Figure 22). This pattern holds true for both largemouth bass and fathead minnows as well. To summarize our findings using visual pattern detection, in general there were no strong spatial patterns observed. However, identification of clusters of stocking events or high density locations for nonnatives can be useful in targeting field efforts or future work. Examining this pattern with a complete data set for nonnative, nonsahnonid fishes could prove valuable. It may be worth following up on largemouth bass stocking events to determine whether or not these particular sample locations are connected via stream courses to the main channel of the Colorado River. Unfortunately, we did not have good enough data on stream connectivity to accomplish this during this study, but connectivity can override spatial location for obvious reasons. If good connectivity exists between stocking locations and/or ponds containing nonnative species, the 16 1 1 n 1