Mussel Growth Diverges in the Netherlands in 2026: Eastern Scheldt 13% Below Expectation, Wadden Sea Up 8% in Weight, Omdraai Shows About 35% Meat Yield for the First Time

Dutch mussel farmers have long operated in the Wadden Sea, the Eastern Scheldt and some offshore waters, where farming conditions in these natural waters vary greatly from year to year. On 20 October 2026, the Dutch fisheries media outlet Visserijnieuws published a report by farming observer Nico Laros, summarizing monitoring data from researchers at Wageningen Marine Research (WMR) and on-site feedback from several Zeeland farmers.

The controversy began with an article published in Visserijnieuws on 18 September 2026. In that article, Laros and WMR researcher Jeroen Wijsman, PhD, sought the reasons for lagging mussel growth. Wijsman's hypothesis was that mussels have higher energy requirements in summer, while warm weather reduces the available feed in the water. This hypothesis was subsequently rebutted by farmer André de Ronde (operating number BRU 27). De Ronde argued that the mussels currently refusing to grow belong to a weak year class: mussels seeded in 2024 were behind from the start. He gave an example: last year, on the same No. 7 farming plot at Inschot, and even when half-grown mussels from the same section were relayed to Zeeland, a bucket held 150; but in 2026, when relaying half-grown mussels from 2025 seed collection, a bucket held 85, with individuals almost twice as large. On the same plot and relayed to Zeeland at the same time, he attributed the difference to a weak year class, while saying that after nearly 50 years in mussel farming he still could not determine the cause.

In response, Wijsman called De Ronde's view another hypothesis for this year's lagging mussel growth. He said the key is to have sufficient and suitable feed in the water during the peak growing season, and that growth shortfalls early in the season are not easily made up; the two hypotheses may both hold, so that their effects add up.

WMR researcher Jacob Capelle, PhD, has studied mussel growth in the Eastern Scheldt and the Wadden Sea for many years. He carries out comparative monitoring at 24 fixed sites in the two waters: in spring, mussel-filled net baskets are suspended at each site, three baskets per site, each basket divided into two compartments with 40 mussels per compartment, i.e. 240 mussels per site; at the start in April, seed size is uniformly about 3 cm. After that, 40 mussels per site are taken out each month for measurement and weighing. The baskets have contained Atlantic cod, red eel, shrimp, crab, starfish, a small northern brown crab and cuttlefish eggs.

Capelle's preliminary conclusion for 2026 is: the Eastern Scheldt in 2026 was a poor growth year, with growth 13% below expectation and, after correction for season and starting length, the second-worst year since 2017, with only 2023 worse; the shortfall in the Eastern Scheldt occurred mainly from August to September, and meat yield from July to August was unusually low at about 26% to 28%, compared with the usual 31% to 37%. Wadden Sea growth in 2026 was average, with length growth close to expectation and weight growth 8% above expectation, but this comparison is still preliminary because the data only run to the end of August. Across the years covered by the study, differences between sites were large: Oosterom 1B, Oosterom 2 and Meep 12 in the Wadden Sea consistently grew faster than average, while Slenk 14 and Vlieter 94 were clearly slower. The Eastern Scheldt appears to have a recurring problem of weak late-season growth, with weak growth in both September 2024 and September 2025. The 2026 data are still preliminary, especially for the Wadden Sea, and conclusions may change once the final rounds of measurements are available. In summary, mussel growth in the Eastern Scheldt in 2026 was clearly poor, the Wadden Sea performed roughly normally, and the main negative change is weak late-season growth in the Eastern Scheldt.

Conditions on the farming front corroborate the above data. On the morning of Monday 5 October, Laros saw the vessel "De vier gebroeders", operating number ZZ 10, unloading large bags of consumption mussels in Harlingen. Skipper Joachim Schot said the batch was 160 to 180 mussels per kg with a meat yield of 30%, from Kabelgat (i.e. Meep 4D). Mussels from Kabelgat have always come into good condition later than those from Oosterom, but this batch was of very good quality.

On the afternoon of Wednesday 7 October, Laros saw the vessel "Cornelia", operating number YE 157, passing fully loaded through the Boontjes channel from the Harlingen dike. Skipper Flip Sinke said the cargo came from Omdraai, caught close to shore, which requires a long detour at low tide; the distance from Omdraai to Wolfshoek is not long. The seed was still on the small side, with 500 per bucket. Sinke speculated that green growth on the shore at Omdraai suppressed growth, but the 1,400-metric-ton batch was still landed; the Scheurrak 2 plot was also good this year and has been transferred to Oosterom 1.

Lambrecht Nieuwenhuize, skipper of the vessel "Eben Haëzer", operating number YE 57, reported that seed sown at Balgen 60 will probably have to be taken up and relayed, because it grew poorly, with very clear water and little feed. A video he sent showed mussels standing on the bottom, with water clear as gin.

The report concludes by mentioning that for the first time in years, consumption mussels with a meat yield of about 35% have come to market from Omdraai.