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<br />Squawfish Population Viability Analysis --July 1993 <br /> <br />Page 30 <br /> <br />. <br /> <br /> . <br />..... . <br />c:: <br />Q) <br />e <br />..... <br />.,..; <br />2 . . <br />u <br />~ <br /> <br /> <br /> <br />. <br /> <br />o <br /> <br />1 <br /> <br />2 <br /> <br />Spawning biomass (millions of tons) <br /> <br />Figure 3.1. Recruitment of North Sea herring vs. spawning stock biomass, <br />1947-1973~ fitted by Ricker curve R = aN exp(-bN) (taken from Colin W. <br />Clark Bioeconomic Modelling and Fisheries Management, 1985) <br /> <br />A quite different kind of density dependent behavior occurs only at low <br />densities. It is variously termed an Allee Effect or depensation. It implies <br />a kind of failure of some sort of group beneficial mechanism. What it <br />means is that below a critical density it can become impossible for the <br />recruitment to balance adult mortality and that the population enters a. <br />deterministic process of collapse to extinction. For terrestrial vertebrates <br />such Allee Effects can involve such mechanisms as group defense from <br />predators, team hunting, aggregate group thermoregulation, or the ability <br />to find mates during seasonal breeding periods. <br /> <br />3.2 Colorado Squawfish Carrying Capacities <br /> <br />Prior to harvest by Homo sapiens, the Colorado squawfish must have had <br />some average number of adults per unit length of river. For proper <br />comparison, this number would have to be adjusted by the size distribution <br />of the adults. Call this number K. Today, Tyus (1991) guestimates 14 <br />