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Squawfish Population Viability Analysis --July 1993 Page 40 As argued above, the Colorado squawfish in the Upper Basin of the Colorado River is not fragmented. It's distribution is relatively continuous, with no internal breaks. There seems to be nothing equivalent to an "edge effect" that affects these fish. Of course, there are "end effects" created by blocks to the river flow, for example, Lake Powell and Flaming Gorge Reservoir. But these are something different. In the entire Colorado River drainage, there has, of course, been fragmentation created by the major dams. The small fragmented populations of Colorado squawfish below Lake Powell did go extinct. This needs to be explained. It could be from Allee Effects, mentioned above. I tried to construct a metapopulation model of the Colorado squawfish, but I couldn't get close to the biological patterns reflected in the literature and by the comments of the on-river squawfish biologists. So, unlike many other species, the Colorado squawfish is not in its current range governed by extinction/recolonization dynamics. 4.3 SourcelSink Dynamics Source/sink dynamics occur when one region of the population has positive population growth and exhibits net emigration and a different region, with negative growth, sustains its numbers with net immigration from the region with excess growth. I believe this is possible with subpopulations in the Upper Basin of the Colorado River. My hypothesis is shown in Figure 4.1. The basic idea is that the juveniles that drift past the confluence of the Green and Colorado Rivers return upstream as sub adults in a partially random manner. This results in a net subsidization of the Colorado from the Green. This is, of course, speculative.