Length-Based Model for Largemouth Bass with Individual Variation in Annual Growth Increments

We used a length-based model to explore the effects of mortality rate and individual variation in annual growth on population size- and age-structure of largemouth bass.  We measured annual growth in length of individual, PIT-tagged largemouth bass in two Michigan lakes.  As expected, mean annual growth increments were greater for smaller bass, decreasing from 32 mm/y in 225-mm bass to 0 mm/y in 409-mm bass.  For a given length, we observed substantial variation among individuals in annual growth increment: SD was about 12 mm in 240-mm bass, declining to about 6 mm in 400-mm bass.  In a classic Ford-Walford plot, annual growth increment declines linearly with length, and reaches zero at the asymptotic length, consistent with von Bertalanffy growth.  We constructed an individual-based population model in which mean annual growth increment declines linearly with fish length.  The realized annual growth increment for each individual is selected each year from a normal distribution with mean and standard deviation decreasing linearly with increasing fish length, with the constraint that growth increments may not be negative.  For each modeled individual, the probability of annual survival is constant or a function of length if size-based regulations are in effect.  We used an age-based catch curve to estimate a total mortality rate in Woodland Lake of 50% per year for largemouth bass age 4 and older.  Model output shows the interacting effects of size-dependent mortality and variation in growth among individuals on the population’s size structure, age structure, and population biomass in different length classes.