Our guppy experiment is set up to study life-history changes caused by harvesting. This involves monitoring of the populations, and harvesting at regular intervals. Now it is the time for the third harvest.
In practice, harvesting guppies means lots of work: handling and marking the fish, meticulous measurements, and photographing.
We are harvesting one population each day. First all guppies in a tank are caught. Using a mesh, the fish are pre-sorted into 'small' and 'big' ones, and ones that are near the threshold size are control-measured. One fourth of either small or big ones, depending on the treatment (or of all, in case of size-independent harvest), are then destined to be today's catch. Harvested fish are measured and photographed, and preserved for later genetic analyses. A sub-sample of the remaining fish are also measured and photographed. Furthermore, the smallest guppies are also marked, such that we have some idea about the age structure of the population.