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I .:1 ,I f'1'- ~ j fl. ,. \1 The small sample size makes it impossible to accurately estimate stocking survival or current population numbers. Based on preliminary data, early estimates place the Lake Havasu razorback sucker population at 1,600 to 3,600 fish. This would place survival rates between 5 to 12%, which is quite similar to survival rates in Lake Mohave (Marsh, oral communication). Status and Existing Risks [I The flannel mouth sucker is the only native fish species which is thriving in the study area, or for that marter, has improved its status in the lower main stream river. Its numbers have expanded to several thousand; however, recruitment maybe tenuous at best and supplemented by occasionally strong year classes. The factors that contribute to its success are poorly understood and warrant further investigation. However, the fish are prolific, fast growing, and may possess some behavior to avoid predation. It also appears there are relatively low numbers of predators in its preferred habitat which is contributing to their survival. Unfortunately, those conditions are subject to change and are not wide spread. Laughlin Lagoon was dredged and deepened in 2000. Prior to this, it was shallow and daily fluctuations in the river stage caused cycles of flooding and drainage which exposed much of this lagoon into mudflats. Dredging deepened the entrance and created over 1 Ian of deep habitat that is ideal for centrarchids such as bluegill, green sunfish, and largemouth bass. Dredging has also increased the volume of the backwater, resulting in increased volumes of water that flow into and out of the lagoon during the daily filling and drainage cycle. The location of one of the major spawning areas just upstream of the lagoon's entrance suggests newly hatched suckers may be more vulnerable to be entrained into the lagoon which now has higher numbers of predators. Another threat is the inevitable development ofthe area. There are plans to construct a marina inside the lagoon as well as another bridge that would span the river. It is doubtful that native species would benefit from construction, increased boat traffic, and pollution, especially near this lmown spawning area. Based on CPUE data, razorback sucker numbers during the past four years have been declining with time. Unlike Lake Mohave, where fish persisted for nearly 5 decades, suckers can be lost not only through Parker Dam, but also through the Central Arizona Project and Metropolitan Water District's pumping plants. Razorback suckers have successfully passed through these and similar structures, but again, it is not the rate of successful passage that is worrisome, it is the rate of over-all loss (Marsh and Kesner, 2000; this report). Large numbers of suckers are no longer being stocked into Lake Havasu. The migratory behavior of maturing adults is unlmown. For many other species, young adults have more of a tendency to range from their core population (Chart and Bergersen, 1992). If this is actually the nature of maturing razorback sucker juveniles, young adults may be more prone to loss than older, more sedentary adults. Virtually nothing is lmown regarding the status ofbonytail. Only one bonytail was captured in the river, but several have been taken in Lake Havasu. An angler recently reported catching one while fishing with meal worms. Ifbonytail were inhabiting the same habitats as razorback suckers, we would have expected to have captured more than 100 based on razorback sucker returns. It is more likely they are utilizing deeper habitats found in the lower portion of the reservoir where sampling didn't occur or its possible they simply haven't survived. Annual monitoring should be increased to a minimum of two sampling efforts, one conducted in late February Or early May for razorback suckers in the upper reservoir and river and another in lower portions of the reservoir during late May and early April for bonytail. 11 II ;1 f'l te I :1 I ,I :1 ~I I I 19