Staandwantmonitoring IJssel- en Markermeer 2020 = Gillnet monitoring in Lake IJsselmeer and Lake Markermeer, 2020

The gillnet monitoring started in 2014 with the aim of getting a more accurate picture of the population structure of fish stocks in Lake IJsselmeer (IJsselmeer) and Lake Markermeer (Markermeer) in addition to annual monitoring with beam trawls and market samplings. Therefore, since 2014, a monitoring program with gillnets with different mesh sizes has been conducted every autumn. This monitoring focuses on the stock structure of mainly four commercial fish species; perch (Perca fluviatilis), roach (Rutilus rutilus), bream (Abramis brama) and pikeperch (Sander lucioperca). In particular, the gillnet monitoring is supposed to sample larger, adult individuals more representative, while juvenile fish are sampled as representatively as beam trawl monitoring. The gillnet monitoring uses a combination of a Nordic gillnet (CEN 2005) and five additional panels. The Nordic gillnet consists of twelve attached short panels of 2.5 meters long and 1.5 meters high with a range of small mesh sizes between 10 - 110 mm stretched mesh. The five additional panels are 100 meter long panels with large mesh sizes of 101, 140, 160, 190 (2x) mm. The European standard for fishing with gillnets has been used as a guideline in setting up this gillnet monitoring. It should therefore be possible to compare the data collected with gillnet monitoring elsewhere in the Netherlands or Europe. To make comparisons possible, the catches have been converted into a standardized catch per unit effort (cpue), i.e. catch in numbers or kg per species per net per night of fishing. In 2020, 43 sites were sampled, of which 30 sites were in Lake IJsselmeer and 13 sites in Lake Markermeer. Based on numbers, catches in Lake IJsselmeer in 2020 were dominated by perch, ruffe (Gymnocephalus cernua) and smelt (Osmerus eperlanus). In contrast, the catches in Lake Markermeer consisted mainly of ruffe, roach and pikeperch. In 2020, the total catch in numbers of smelt across both lakes was the highest since the start of the gillnet monitoring, with smelt being caught almost exclusively on the IJsselmeer. For pikeperch, the total catch was the lowest in the time series from 2014. Houting (Coregonus oxyrinchus), flounder (Platichthys flesus), like smelt, were caught almost exclusively in Lake IJsselmeer. Three non-native species were found, namely round goby (Neogobius melanostomus), monkey goby (Neogobius fluviatilis) and the Chinese mitten crab (Eriocheir sinensis). The length-frequency distribution of bream seems to indicate that more large adult bream (> 40 cm) were caught in 2020 compared to previous years. Given the length frequency distribution of pikeperch, zero-year-old pikeperch were exceptionally large in 2020 and appear to have experienced a very good growing season. The majority of the zero-year-old pikeperch probably became piscivores in their first year of life. In 2020, 33 kg of fish per net per night of fishing were caught with the Dutch gillnet in Lake IJsselmeer, compared to 14 kg of fish in Lake Markermeer. Analysing only the data of the 101 mm mesh size panel, which is mainly used for commercial fishing on these lakes, almost 13 kg and 10 kg of fish per net per night of fishing were caught in 2020 for Lake IJsselmeer and Markermeer, respectively. The landings in Lake IJsselmeer with this 101 mm panel consisted of 4 kg of both perch and pikeperch (16% of the pikeperch were undersized (< 42 cm)), 2.5 kg of roach, almost 2 kg of houting and 1 kg of bream. For Lake Markermeer the landings consisted of almost 6 kg pikeperch, more than 1 kg of roach and bream and almost 1 kg of perch. The landings per net per night of fishing can in this context be compared with those of the commercial fishing activities on Lake IJsselmeer and Markermeer, but also with landings elsewhere in the Netherlands or Europe where gillnets are used.