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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3 <br /> <br />Lasker 1975; Diana 1995). Density dependent processes such as competition are rarely as <br /> <br /> <br />important until juvenile/adult stages, and then are often reflected in reduced growth rather <br /> <br /> <br />than mortality (Ricker 1954; Schoenherr 1977; Houde 1987). Identification of extrinsic <br /> <br /> <br />factors affecting survival can provide a common base for comparing management <br /> <br /> <br />strategies across a variety of species suffering from similar environmental constraints. <br /> <br /> <br />Mortality in young fish, although always high, typically creates recruitment <br /> <br />bottlenecks during well defined "critical periods" (Hjort 1914; May 1974) suchas during <br /> <br /> <br />the transition from endogenous to exogenous nutrition. Recruitment bottlenecks occur <br /> <br />for two major reasons: predation and starvation (Lasker 1975; Hunter 1981). Predation <br /> <br />is typically a terminal cause; any factor influencing predation tends to influence the <br /> <br />intensity of a critical period. Poorly fed larvae, with limited reserves and energy, grow <br /> <br />slower and are eaten long before they have the chance to starve. Conversely, the more <br /> <br />rapidly a larvae grows, the earlier it exceeds the "predation threshold." <br /> <br /> <br />Population size can thus depend on how quickly individuals pass through a <br /> <br /> <br />window of vulnerability to predators. Braun (1978), for example, evaluated food <br /> <br /> <br />conditions versus hatching dates for whitefish (Coregonus clupeaformis). Individuals <br /> <br />hatched in March did not survive due to limited food, while in April larvae were born into <br /> <br /> <br />higher zooplankton densities and survived. Spatial variation was also important, as <br /> <br /> <br />larvae survived in March if they encountered dense patches of plankton (Braun 1978). In <br /> <br /> <br />pelagic fishes, food patchiness is a major cause of mass starvation during critical periods <br />