This dialog box is dedicated to fishing effect on populations.
First, at the bottom, a check box let you activate fishing: do not forget to check it after tuning the fishing effort.
Here, you can specify the minimum and maximum fishing sizes, and the number of fish to be sampled per reach. This number can be absolute, or calculated per hectare. It is also possible to make the sampling spatially variable by using the Strahler order of the reaches.
Keep in mind that this will activate fishing regularly, once a year, in June to be precise. It is always possible to generate punctual fishing using the ad hoc intervener within the simulation.