Density dependent growth and size specific competitive interactions in young fish

Body size is a major determinant affecting an individual's performance. In this study, four factors affecting an organism's competitive ability as rel ated to body size: the attack rate, the maximum growth capacity. the metabo lic demands and the size spectrum of available resources, were investigated for small stages of Eurasian perch (Perca fluviatilis) and roach (Rutilus rutilus). The size dependence of the attack rate on a 0.5-mm cladoceran zoo plankton of larval and juvenile perch was estimated and compared with the r esults from similar experiments for roach. At equal body sizes, roach alway s had a higher attack rate than perch. In contrast the reverse was the case for maximum growth capacity. The maintenance requirements at the same size were higher for perch than for roach. Based on the above data we were able to gain a mechanistic understanding of the outcome of two field enclosure competition experiments, one between perch lan;ae and 1-yr-old roach and on e between perch larvae and roach larvae. In the first experiment, we found strong intraspecific density dependent growth in perch larvae. while the pr esence of l-yr-old roach had no effect on perch larvae growth. High parch d ensities had strong negative effects on the zooplankton resource. Due to th e larger size of l-yr-old roach and consequently higher metabolic demands, 1-yr-old roach showed a stronger negative response in growth to high perch larvae densities than perch despite roach's higher attack rate on zooplankt on. In the second experiment, perch larvae were negatively affected by high densities of roach larvae and had a reduced growth at high intraspecific d ensities. In contrast, the growth of roach larvae was not affected by perch larvae or high intraspecific densities. This difference between species co uld be related to the simultaneous lower attack rate and higher growth capa city of perch, leading to a higher sensitivity in growth of perch than of r oach to decreasing resource levels. Temporal variation in competition inten sity was present in both experiments. This variation could be related to th e foraging efficiencies and different growth capacities of the competitors and the species composition and size structure of the zooplankton resource. Our study points to the potential for both intra- and interspecific compet itive interactions in fish larvae in freshwater systems. Our results also s uggest that species specific differences in how foraging, growth and food p rocessing capacities relate to body size are of vital importance for intera ctions in ecological communities.