The European Standard EN 14757 recommends gillnet mesh sizes that range from 5 to
55mm (knot-to-knot) for the standard monitoring of fish assemblages and suggests adding
gillnets with larger mesh sizes if necessary. Our research showed that the recommended
range of mesh sizes did not provide a representative picture of fish sizes for larger species
that commonly occur in continental Europe.We developed a novel, large mesh gillnet which
consists of mesh sizes 70, 90, 110 and 135mm (knot to knot, 10m panels) and assessed its
added value for monitoring purposes. From selectivity curves obtained by sampling with single
mesh size gillnets (11 mesh sizes 6 – 55mm) and large mesh gillnets, we identified the
threshold length of bream (Abramis brama) above which this widespread large species was
underestimated by European standard gillnet catches. We tested the European Standard
gillnet by comparing its size composition with that obtained during concurrent pelagic trawling
and purse seining in a cyprinid-dominated reservoir and found that the European Standard
underestimated fish larger than 292mm by 26 times. The inclusion of large mesh
gillnets in the sampling design removed this underestimation. We analysed the length-age
relationship of bream in the Římov Reservoir, and concluded that catches of bream larger
than 292mm and older than five years were seriously underrepresented in European Standard
gillnet catches. The Římov Reservoir is a typical cyprinid-dominated water body where
the biomass of bream > 292mm formed 70% of the pelagic trawl and purse seine catch.
The species-specific relationships between the large mesh gillnet catch and European
Standard catch suggested that the presence of carp (Cyprinus carpio), European catfish
(Silurus glanis), tench (Tinca tinca) or bream warrants the use of both gillnet types.We suggest
extending the gillnet series in the European Standard to avoid misinterpretation of fish
community biomass estimates.