Assessment of the pelagic fish populations using CEN multi-mesh gillnets: consequences for the characterization of the fish communities

The contribution of CEN standard pelagic nets to the assessment of fish communities is tested by comparing three metrics (species composition, species abundance, and size structures) measured in accordance with the standard (i.e. using benthic nets only) to those calculated from the total effort (i.e. including pelagic nets). Hydroacoustic surveys were used simultaneously to assess fish densities in the pelagic habitat. The results show that in most cases the pelagic nets did not provide any extra information about these three metrics. However, their inclusion in the calculation of CPUE and size structures may affect the picture of the fish communities, especially in lakes containing salmonid populations. This study highlights the need to sample pelagic fish when assessing fish communities in order to determine lake quality

Stability and precision of the fish metrics obtained using CEN multi-mesh gillnets in natural and artificial lakes in France

International audienceThe stability and the precision of fish metrics obtained using CEN multi-mesh benthic nets are compared between twenty-seven natural and artificial French lakes. We show that the natural or artificial origin of the lake has no impact on the precision of the fish metrics provided by the method and it provides more precise catch data in French lakes than in the Nordic countries. The precision of abundances and biomass depends on fish density, and thus on the trophic status of the lake sampled. We also show that the sampling effort advocated by standard is appropriate in French lakes despite its underestimation of species number and disregard of marginal habitats. Nevertheless in deep lakes, the sampling effort could be reduced in the deeper layers which are never informative