| Abstrakt: |
AbstractTo evaluate the dynamics of fishing and natural mortality, we used a catch–age cohort analysis model of the population dynamics of walleye Sander vitreusin Escanaba Lake, Wisconsin, during 1956–2009. The catch–age model was based on angling catches at age from a compulsory creel census, abundance at age from mark–recapture studies, and recruitment at age 0 from mark–recapture studies. Harvest of age-3 and older (age-3+) walleyes averaged 12.5 fish/ha and varied 55% among years during 1956–2002, before the imposition of a 71-cm (28-in) minimum length limit that eliminated harvest. The population density of age-3+ walleyes averaged 34.8 fish/ha and varied 51% among years during 1956–2009. Annual exploitation of age-3+ walleyes averaged 34% during 1956–2002 and varied 35% among years during that period. The variation in population density explained only 0.5% of the variation in annual exploitation during 1956–2002. Natural mortality was best described by a model with decreasing age-specific vulnerability and a negative relationship with angling exploitation. Relative vulnerability to natural mortality declined gradually with age from 100% at age 0 to 83% at age 3, 50% at age 9.4, and 24% at age 17. Relative vulnerability to fishing mortality increased sharply from less than 1% at age 0 to 98% at age 3, peaked at 100% at age 4, and declined gradually to 66% at age 17. Fully vulnerable natural mortality (M) declined from 0.70 to 0.10 (mean = 0.41) as annual angling exploitation (u) on age-3+ walleyes increased from 0.0 to 0.65 (mean = 0.30). Our findings suggest that natural mortality declines with age and is inversely related to fishing mortality, so that it varies among ages and years. We conclude that stock assessment models for walleye fisheries should evaluate the effects of year-specific and age-specific natural mortality on model predictions.Received September 4, 2010; accepted February 24, 2011 |
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