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Squawfish Population Viability Analysis --July 1993 Page 31 adults per river kilometer for the Green River. This is not a very high number. And it is certainly much reduced from the period prior to human harvest. Early historical evidence suggests the presence of much larger adult fish and greater densities. These were large enough that for at least ashort period of time the population could support a commercial harvest. Based on historical accounts of catch, squawfish densities have been much greater. For sake of argument, say they were 10 times greater. What would this reduction be due to? We know that there are a number of things that have negatively impacted the Colorado squawfish: harvesting, a reduced prey base, exotic competitors and predators, modified flow regimes, and so forth. If one had a carrying capacity function ('carrying capacity' in this context really means the equilibrium density in the presence of a set of ecological forces) of the following form: K(harvest, flow regime, exotics, barriers, depensation effects,...) and if one had a quantification of the different impacts, then one could possibly allocate to the different disturbances their relative contribution to the endangerment of the Colorado squawfish. This would be straightforward if only a single factor had any real importance. Multiple causation makes the allocation of blame less precise, and it also allows for nonlinearities and interaction effects. That is, ,a flow regime alteration could have caused a 10% decline and exotics could also caused a 10% decline if each were considered in isolation. But jointly there could be an interaction that caused an overall 40% decline. Knowledge of the K(.) function would be invaluable for recovery planning. If some goal were established for increased abundance, it would be possible to adjust independently the variables to achieve the most cost effective solution. Figure 3.2 shows the reduction of total population density due to human disturbance. It is assumed that there is a function of the form: N(t+l) = R(N(t)) + surv*N(t) = F(N(t)), where R(.) is the density dependent recruitment function, surv is the per time step adult survival rate, and under this relationship the equilibrium obtains for densities at which N(t+ 1) = N(t).