Development of abundance and size structure of young-of-the-year perch populations using three methods

The reliable assessment of fish populations, which can vary in their spatial and demographic structure, assumes that the results are independent of the assessment method used. To test this assumption for the size structure of the young-of-the-year (YOY) age cohort of perch, three gravel pit lakes and four shallow ponds were monitored using three sampling methods from May to October. While bongo nets were used for early juveniles in the pelagic zone, electrofishing and multi-mesh gillnets were used later in the year when perch had moved to the littoral zone. Since bigger perch (post-larvae) switch from the pelagic to the littoral zone during ontogenesis, bongo net catches during June in the pelagic area of the lakes sampled only the smaller perch, while simultaneous electrofishing in the littoral zone caught bigger perch. Later in the season in the littoral zone, smaller perch were caught only by electrofishing and the bigger ones with gillnets. Monthly samples caught by electrofishing and gillnets in the experimental ponds from June to September showed even larger differences between the sizes of perch. Because the size distribution of the YOY perch cohort in the ponds had broadened considerably, there was sometimes no overlap in the length-frequency distributions between the two methods used, clearly demonstrating that using a single method is not sufficient for drawing a complete picture of the population size structure. This was verified by the removal of fish from the experimental ponds in October. Our results give clear evidence (and thus confirm previous studies) that using one method alone would result in an incomplete picture of the development of the size structure of the YOY perch population, due to the facts that (1) not all perch switch simultaneously between different habitats during ontogenesis and (2) that swimming performance, habitat-specific occurrence and activity change with size, thus affecting the method-specific catchability. Consequently, at least two appropriate methods must be used in an overlapping/parallel sampling design in order to draw a reliable picture of the development of the YOY perch population in any given body of water.