The Impact that Geomorphological Development of Managed Realignment Sites has on Fish Habitat

When coastal sites are breached and introduced to intertidal processes for flood defence and habitat creation purposes, the design primarily focuses on energy dissipation, the flora and the ‘air breathing fauna which are fluffy, feathered or rare’. Little attention is paid to the underwater habitat created which, for fish, can provide rich feeding grounds and refuge from larger predators, as well as acting as a nursery. This positive impact on the aquatic eco-system can, in turn, add to the local economy through improved commercial fishing and increased tourism (e.g. diving, recreational fishing, birdwatching) leading to improvements in human health and wellbeing. This paper presents a holistic overview of a large meso-tidal coastal managed realignment (MR) site, Medmerry MR, as it evolved during the first five years following site breaching. The data gained from bi-annual fish surveys is synthesised with the changes in geomorphology and hydrology to understand how MR design is impacting on fish colonisation in a changing habitat, providing lessons for future MR designs.

Limited design catering for fish was incorporated into the Medmerry site. The borrow pits, where material was excavated to create the flood bunds, were designed as lagoon habitats to attract fish primarily as bird feed. Saloon type tidal flaps were installed to allow for the migratory passage of eels. Following site inundation, the resulting variations in salinity levels and depth across the site attracted a large variety of fish species, both estuarine and marine, with thirty species of fish recorded. During the earlier stages of development, the borrow pits near the breach quickly became used by fish. However, geomorphological changes meant that one of these borrow pits filled rapidly with sediment, morphing into an intertidal sand/mud flat three years after breach. After four years, the others remained flooded at low tide, but blockage of the single feed channel resulted in reduced water exchange and higher low-tide water levels. This impacted on the ingress and egress of fish on the tide and may have impacted temperature and salinity levels which also affect fish colonisation.

This paper proposes features and actions that can be easily incorporated when designing new intertidal wetlands to help optimise fish habitat. Incorporation could potentially open up additional funding streams, given the benefits accruing to society.