The Influence of Demography on Population Dynamics of Iteroparous Fish Species

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In many iteroparous fish populations, recruitment is quite variable from year to year. Typically, these populations have a skewed recruitment distribution, with most years being poor and rare years being very high. We used an age-classified matrix projection model to explore the effect of this skewed recruitment distribution on population growth rate. To parameterize our model, we used vital rate estimates from a 20+ year data set for Lake Erie walleye, a population that is important both ecologically to the lake ecosystem and commercially as the primary focus of recreational and commercial fisheries. We developed a deterministic model that used age-specific mean vital rates and we used the longterm (asymptotic) solution to represent our base population. Population growth rate in this model was near 0 (r = 0.009). Elasticity analysis showed that population growth rate was most sensitive to the survival of the younger age classes (ages 0-3). We compared these base model results to results from a demographically stochastic model with age-0 survival rates drawn randomly from the 20+ year data set. Mean population growth rate was an order of magnitude greater than in the deterministic model (r=0.09), suggesting considerable importance of the skewed age-0 survival distribution in determining population dynamics. Because the occurrence of high age-0 survival has been shown to be correlated with a number of environmental variables that are predicted to change with changes in the climate, we also explored the effect of changing the frequency of occurrence of years having high age-0 survival. As expected, frequency of occurrence of good years strongly influence population growth rate. The extent to which rarity of good years was detrimental to modeled populations was influenced by longevity of individuals in the population. Thus, we expect results to differ across species with different life history patterns. In addition, species that are naturally long-lived, and thus somewhat buffered from the effects of increasing rarity of good age-0 survival years, may become more susceptible to collapse if harvest truncates the age-distribution. We will discuss the interactions of a “boom or bust” life history, longevity, climate change, and fishing in their influence on population dynamics.