The effect of fish predation on benthic density and community structure has been studied intensively in the past. The results of these experiments, however, were inconsistent. While some experiments resulted in dramatic changes of benthic biomass or density other experiments did not. It has been argued that methodological differences such as duration of the experiment, mesh size or fish species are responsible for these differences.

We suggest that the intensity of predation determines the kind of predation effects. While massive predation seems to induce a strong biomass decline, moderate predation seems not to affect total benthic biomass. In this case carrying capacity is not relocated by the fish predator. Moderate predation is expected to change species composition. This is because vulnerable species will be reduced by the predators (i.e. their losses exceed their production). The free resources however will immediately be used by substitute species less vulnerable to predation. Because these will be most likely smaller and fast growing species, total secondary production will increase in such systems. We represent here a case study for moderate predation and its effects. In a reach-scale field study a stretch containing two species of benthivorous fish is compared with a fishless reference stretch.