Density Dependent Growth and Size Specific Competitive Interactions in Young Fish

Abstract

Body size is a major determinant affecting an individual's performance. In this study, four factors affecting an organism's competitive ability as related to body size: the attack rate, the maximum growth capacity, the metabolic demands and the size spectrum of available resources, were investigated for small stages of Eurasian perch (Percu fluviatilis) and roach (Rutilus rutilus). The size dependence of the attack rate on a 0.5-mm cladoceran zooplankton of larval and juvenile perch was estimated and compared with the results from similar experiments for roach. At equal body sizes, roach always 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 larvae and I-yr-old roach and one between perch larvae and roach larvae. In the first experiment, we found strong intraspecific density dependent growth in perch larvae, while the presence of I-yr-old roach had no effect on perch larvae growth. High perch densities had strong negative effects on the zooplankton resource. Due to the larger size of I-yr-old roach and consequently higher metabolic demands, I-yr-old roach showed a stronger negative response in growth to high perch larvae densities than perch despite roach's higher attack rate on zooplankton. In the second experiment, perch larvae were negatively affected by high densities of roach larvae and had a reduced growth at high intraspecific densities. In contrast, the growth of roach larvae was not affected by perch larvae or high intraspecific densities. This difference between species could be related to the simultaneous lower attack rate and higher growth capacity of perch, leading to a higher sensitivity in growth of perch than of roach to decreasing resource levels. Temporal variation in competition intensity was present in both experiments. This variation could be related to the 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 competitive interactions in fish larvae in freshwater systems. Our results also suggest that species specific differences in how foraging, growth and food processing capacities relate to body size are of vital importance for interactions in ecological communities.