Although shellfish stocks in the coastal zone are monitored since 1994, causes for observed strong increases and declines in population sizes are unknown. Being able to explain sudden shifts and long-term changes is important for Wageningen Marine Research as part of its knowledge base on shellfish ecology and stock assessments, and important for LNV and other government parties involved in fisheries and nature management in evaluations of policy measures.
Shellfish stock assessments are carried out in the Dutch coastal zone since 1994. Apart from the target species, Spisula subtruncata and Ensis directus, all shellfish species are registered. In 24 years time dramatic shifts in population sizes have been observed, the most notable being the collapse of S. subtruncata around 2001, the strong increase of E. directus since 2002, and the extreme increase in stock size of S. subtruncata in 2017: from 39 million kg fresh weight in 2016 to 1283 in 2017. At the same time we see strong increases in stock size of several species, such as Lutraria lutraria, Chamelea striatula and Donax vittatus. Overall the total biomass of bivalve filter feeders appears to increase, which may have consequences for the carrying capacity for bivalves in not only the coastal zone but also the Wadden Sea and Ooster- and Westerschelde estuaries.
The problem is that we do not know how to explain the observed changes, e.g. whether it is related to changes in food composition and/or abundance, or changes in abiotic conditions in the coastal zone. More insight in causes of shifts in shellfish population sizes is a necessary knowledge base for researchers working on stock assessments and population studies of shellfish. Such insight is crucial in the management of marine resources in the coastal zone and adjacent marine and brackish waters, in the evaluation of management decisions and in impact assessment studies.
We will analyse correlations between shellfish data from the WOT survey and data on environmental (a)biotic parameters to identify which driving factors caused the observed changes in abundance of S. subtruncata, E. directus, L. lutraria, C. striatula and D. vittatus.