Can We Better Understanding the Elusive “Competition” and “Recruitment Variability” Using Individual-Based Modeling: A Case Study Using Red Drum

Quantifying competition and predicting fish recruitment remain unresolved. Individual-based modeling offers one approach for better understanding the consequences of competition on survival and ultimately the underlying causes of recruitment variability. We used an individual-based model to simulate red drum spawning and subsequent growth and survival to recruitment (25 mm). The model grid was 200 x 200 25-m² cells, with each cell assigned as open water (sandy bottom) or vegetated. Temperature varied daily, and two zooplankton prey types were simulated on each cell that included consumption effects of the red drum.  Vegetated cells had higher prey and lower mortality rates.  A fixed annual fecundity was divided into fixed-size batches and introduced on fixed time intervals.  Growth, mortality, and movement were simulated hourly for each individual. We simulated all combinations of 1-6 batches spawned in a season, in intervals of 4 to 21 d, and with a range of assumed competition intensities. Batch spawning had a negative effect on recruitment when larvae were intrinsically non-competitive, whereas batch spawning increased recruitment when larvae were competitive. The implications of these results on understanding the interplay between competition and recruitment in red drum, and other species with similar life histories, are discussed.