The importance of density-dependent mechanisms in determining cohort survival in largemouth bass

Understanding factors important to recruitment in fishes has proven to be a difficult and challenging problem. Largemouth bass Micropterus salmoides, a popular recreational sport fish, is an excellent model species to explore mechanisms of recruitment, because both density-dependent and independent processes have been identified as important in regulating populations. Timing of hatching can influence growth and survival of largemouth bass. Individuals hatched relatively early have a period of growth prior to the swim-up of later-hatched cohorts allowing them to switch to piscivory at an earlier date due to release of gape limitations. Otolith and microsatellite data suggest that earlier hatched fish contribute the most to the final population of age-1 bass recruits. Reduced recruitment of the smaller, later cohorts is currently believed to be a density-independent process; however, individual based models have suggested that density-dependent processes may also contribute. Asymmetrical competition can also explain this phenomenon if early cohorts exploit prey resources prior to later cohort swim-up or through interference competition. In a manipulative pond experiment, I reduced the early cohort in 4 of the 8 ponds. Pond in which the early cohort was reduced had greater growth rates in later cohorts and an overall increase in recruitment to the end of the summer, higher Ephemeroptera densities, and largemouth bass consumed more energetic diets. Through this experiment, I was able to determine that early cohorts reduce survival of later cohorts through asymmetrical exploitative competition. Although this study is the first to confirm that early arriving individuals have a competitive advantage in largemouth bass, the phenomenon, priority effects, has been observed in many other kinds of organisms. Unlike previous studies, I was able to document asymmetrical exploitative competition as the primary mechanism of survival. Because evolutionary processes favor early spawning and nest angling targets the larger, early spawning males, further research is needed to identify how increased contribution of later cohorts affects the long-term population dynamics of largemouth bass