Relating a DEB model for mussels (Mytilus edulis) to growth data from the Oosterschelde : trace-back analysis of food conditions

In this study, the food conditions for mussels are estimated at different locations within the Oosterschelde. In 2014, 2015 and 2016, mussels with a uniform size were placed in baskets at the borders of commercial culture plots distributed over the Oosterschelde. Each month, a subsample was taken from each basket to measure growth (shell length and individual weight) of the mussels. The results show a variation in growth performance, both in shell length as in flesh weight, between the different locations. A model approach was used to translate the spatial differences in growth to spatial differences in food conditions. A Dynamic Energy Budget (DEB) model was fitted to the data in order to trace-back the food conditions. During this fitting, the food correction factor (ψψ) was optimized. ψψ can be interpreted as an indication of the food conditions (algae concentration, quality, current velocity) at that specific location in comparison to the average food conditions in the whole Oosterschelde. The results show that there is a spatial, but also year-to-year variation in food conditions within the Oosterschelde. Locations with the best food conditions were Neeltje Jans N in 2015 and Hammen 9, Dortsman and Krabbenkreek in 2016. Growth of the mussels in the baskets in 2014 was lower than in 2015 and 2016. This is probably caused by the larger size of the mussels that were used in 2014 and the fact that the growth of mussels reduces with size. In contravention to the expectations, there was no clear pattern in growth conditions from the western part of the Oosterschelde to the eastern and northern part. For example, the growth of the mussels at the two locations in the northern part of the Oosterschelde (Krabbenkreek and Viane) where ralatively good compared to the other locations. In practice, however, mussel farmers use the culture plots in the northern part mainly for storage of seed and halfgrown mussels. Possibly the mussels in the baskets perform better in this area than on the bottom culture plots. The DEB model is a good tool to trace-back the food conditions from the measured growth data. The parameters for blue mussel, that is used for the DEB model should be updated. The parameters are presently based on historical data, whereas new data are available