The design of a reef tile for the restoration of flat oyster reefs in the Dutch North Sea

Abstract

Because of human interventions in our seas, like commercial (over)fishing and the introduction of diseases, the presence and biodiversity of natural reefs is declining severely. Reefs provide a habitat for a variety of marine species and are of great value to marine ecosystems. Therefore, it is of importance to recover affected reefs.

Also in the Dutch North Sea, a majority of the reef has disappeared, including reefs of the European flat oyster (Ostrea edulis). Oysters in particular play a key role in reef ecosystems and are therefore the ideal starting point for the recovery of reefs in the Dutch North Sea.

There is no possibility that these oysters will naturally recover: they are not numerous enough to naturally breed and spread. Therefore, there is a need for active restoration. Several methods for restoration exist already, however, these are not proven to be effective and have several drawbacks.

Recently, marine biologists suggested a new method for the restoration of flat oyster reefs, where oyster larvae settle on a small product, which then gets sown in the ocean and naturally form oyster beds. However, this method is theoretical; no such product is developed yet. The goal of this graduation project is to develop this product: from conceptualization to embodiment to preliminary validation in laboratory settings. By the end of the project, the product is ready for field tests and will ultimately enable the restoration of reefs.

Literature review showed no reference model or design guide, as no such products exist yet. Therefore, ideation started with the exploration of basic shapes and their behaviour underwater. These basic shapes were altered in such a way to fit the purpose, whereafter several iterative steps concluded in five final concepts. A practical approach enabled quick iterations.

Simultaneously, exploration of bio-based materials suitable for an application underwater resulted in a material that meets requirements. Two concepts were chosen and made into operational prototypes to test in laboratory settings. These tests, executed in a sediment flume, indicated the performance of the products in water flow. One of the two concepts was more successful in this simulated environment; this product is elaborated.

The final product promotes larvae settlement on the product and allows people to touch the product without touching settled larvae. The product sinks to the sea bottom and stays in place as much as possible, which entails resisting water flow up to 1.9 m/s. The design protects larvae against larger predators and makes sure they stay above the sediment. Finally, the product is large enough for a grown oyster and persists between six months and two years on the seabed, after which the naturally occurring material degrades into biologically safe components.

In conclusion, a product has been designed that can enable the restoration of oyster reefs. With this product, large areas of reef can be restored in an effective and minimally invasive manner. As the indication of performance is only performed in laboratory settings, the advice is to validate in a relevant natural environment to improve knowledge about this product and method. These tests should clarify the performance of the product and the material.