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- designed to withstand flooding within an acceptable flood recurrence interval (e.g" 50 years) could provide nonna- tive controls while preserving sportfishing interests, Once the supply of centrarchids is curtailed, their numbers in the river could be expected to dwindle, and mechanical measures, such as netting in shoreline habitat would expe- dite their removaL Green sunfish are widely distributed in shoreline habitats, but reproduce in deeper and more isolated backwaters (Tyus, pers, observ,). These areas should be especially targeted, Finding effective control measures for nonnative fishes adapted to the main channel (e,g" channel catfish; Figure 4) poses an especially difficult challenge, Chemical measures cannot be used because it would harm sympatric endan- gered species, Mechanical removal would be prohibitively expensive, Eddy and other main channel habitats pre- ferred by the catfish are very difficult to sample and the entire river would need to be targeted indefinitely How- ever, one possibility that emerged fro~ th~ workshop is use of intensive fishing to remove larger predators, In gen- eral, channel catfish in the Upper Colorado River basin are not fully piscivorous until they attain a length of about 420 mm (Tyus and Nikirk 1990), Larger catfish are very vul- nerable to fishing pressure and angling has virtually elimi- nated the larger individuals in some locations (e,g" Wyoming; Gerhard~ jlnq Hubert 1991). Commercial fishing for channel catfish ih'the'Missouri (Hesse 1994) and Mis- sissippi rivers (Pitlo 1997) has been so effective that fishing had to be closed or size restrictions imposed to allow the fisheries to recover. However, reduction in fish size, or even collapse of the "fishery" would be a desirable out- come in the Colorado River. Because the preferred com- mercial method of catfishing is baited hoopnets (Hesse and Mestl1989), catch rates of native fishes would pre- sumably be very low and any endangered fish captured would likely be alive for return to the river, Other nonna- tive predators, like pike or carp, also can be caught in the nets and could be removed from the system, Unfortunately, there is no intensive warmwater fishery in the Upper Colorado River basin, and it is doubtful if sufficient fishing pressure can be generated to significantly reduce catfish stocks, A commercial fishery would avoid the high costs of intensive fishing that would be incurred by agencies, but state fish and game agencies have ex- pressed rel1!ctance to allow establishment of commercial fishing due 10 potential regulatory and sociopolitical prob- lems, In addition, there are no established local markets for channel catfish or carp, Nevertheless, markets in the midwestern United States are sufficiently well-developed that a trial program might entice entrepreneurs into the Upper Colorado River basin, especially if suitable incen- tives were offered (L Hesse, River Ecosystems, Crofton, NB, pers, comm.). If successful, economic benefits derived from a commercial program could be used to aid recovery efforts. A pilot program, established in a reach where the existing channel catfish population could be fished com- mercially and monitored accurately, is needed to explore benefits and costs of this novel control measure, September 2000 ENDANGERED SPECIES 11 I , I i ~ Figure 3. Alongshore habitats. such as this large backwater, are pre- ferred habitat of small cyprinids and utilized as rearing areas by young of endangered fishes, Control of northern pike may be relatively easy, provided that steps are taken to prevent escape from reservoirs, Some of the fish are reproducing in coolwater pond areas where they congregate for spawning in late winter, Mechanical removal with seines or stationary nets could provide a very efficient control technique when the individuals are concentrated in a relatively confined habitat, such as low- lying reaches of the Yampa River above Craig, Colorado (John Hawkins, Colorado State University, pers, comm,), Control of common carp (Cyprimls carpio) is made diffi- cult by their ability to invade many different habitats, Almost every conceivable control measure has been applied to carp at one time or another, but mechanical methods are the most widely used (Panek 1987; Wiley and Wydoski 1999). We propose mechanical removal of carp from major source areas, such as the Browns Park Nation- al Wildlife Refuge (NWR), Adult carp have ready access to ideal spawning habitat when the NWR is flooded, and their progeny populate the river reaches, If it proves prac- tical to block access to the NWR, reproduction could be reduced and resident carp could be poisoned, If access cannot be blocked completely, harvesting could reduce Figure 4, Channel catfish are now common in many rocky habitats in the mainstream rivers of the Colorado River basin, Fisheries 21