The European Habitats Directive prescribes Member States to report on trends in abundance and distribution of protected species. One of these species is Graphoderus bilineatus (De Geer, 1774), a middle-sized predaceous aquatic beetle, listed in Annex II and IV of the Habitats Directive. In the Netherlands, a monitoring scheme for this species has been set up to assess the national trend as well as the trend in the national Natura2000 sites. In this scheme, a selection of 1 km² squares is surveyed in cycles of six years using a standardized field method by professional fieldworkers. In each selected square, five locations were sampled per annum (September-October) by two different observers using hand netting. The data of the first two rounds (2011-2016 and 2017-2022) have been analysed using both an occupancy model and a Poisson GLMM. We found evidence of a declining trend in occupancy as well as in population size. The decreases were stronger outside than within Natura2000 sites and also stronger than those of three other beetle species that are often found together with G. bilineatus. In addition, considerable differences between observers were detected in the data, despite the application of a standardized field method.

From the 85 km² squares in which Graphoderus bilineatus is known, 68 squares were chosen, covering 80% of the known distribution at the time, including all Natura2000 sites (41 squares). The remaining 27 squares were chosen at random from the pool of 44 squares outside Natura2000 sites, plus 12 squares with a high probability. Most selected squares were in the western and northern peat areas, with a few squares in the higher sandy soil region where the species still occurred.

The 1 km² squares have been used to evenly distribute the sample locations across the entire range of the species. Within each 1 km² square, the first observer selected five sampling locations which were visually judged to be potentially suitable for G. bilineatus. This species mainly occurs in dead ends of waterways, 90° angle bends or inlets in the shoreline, particularly between floating organic matter. The preference for these places is probably due to the accumulation of animal remnants on which the adults feed and the availability of shelter while resupplying oxygen. Furthermore, vegetation composition, such as the presence of Stratiotes aloides, Hydrocharis morsus-ranae, and Lemna trisulca, the presence of open water, water clarity, water depth, and natural banks for pupation, determine the suitability of a location for G. bilineatus. After sampling, coordinates and a description of the sampling location were shared with the second observer who surveyed the same location 5-8 days after the first visit. Six years later, the same locations were re-examined twice again.

Each year, 12 squares, representing 60 sampling locations, have been examined. Currently, data are available for 68 squares with 337 locations, each surveyed twice (2011-2016 vs 2017-2022), resulting in a total of 1336 field surveys.