Average shell width with SD (n = 30) of hatchery produced O. edulis from two broodstock origins (pre-screened Bonamia free = RvY and Bonamia naïve = NIOZ) in 2020 (A) and (2021 (B). From February until August 2020 the two groups were kept in the harbour of Yerseke (Oosterschelde). From August onwards they were placed in Lake Grevelingen. * = significant difference between the two groups, P < 0.05.