Colonisation of a managed realignment site by fishes

Increasing pressures for coastal land reclamation intended for the purpose of settlement, flood defence and industrial development have resulted in a loss of intertidal habitats on a global scale. The result of this can be seen in the south-east of England on the Thames estuary where intertidal habitats such as mudflats and salt marshes are in decline. These environments are important for many fish species from varying life stages for protection from predators and strong currents, and also for foraging and spawning. Intertidal habitats also act as natural coastal buffers by absorbing kinetic wave energy and reducing coastal erosion. Mitigation measures such as managed realignment schemes have started to be put into practice to compensate for any loss of habitat, and as an alternative to traditional hard defences such as sea walls. Implementing a managed realignment involves breaching sea walls to allow the sea to reclaim land, usually agricultural.The aim of this study was to assess fish utilisation of a new managed realignment site breached in 2010 in the Thames estuary, south-east England, intended to compensate for the loss of an adjacent intertidal mudflat habitat through the construction of an international port. Seine nets, fyke nets and trawling were used to gather data over an 18-month period from October 2010 to April 2012. By comparing fish species composition, density/catch-per-unit-effort, size structure and diet composition in the realignment and adjacent estuary, community structure was analysed, allowing an assessment of the success of the realignment.Comparisons between a natural mudflat found on the Thames estuary site and the realignment site demonstrated that species composition was similar in both sites but density was higher in the realignment 50% of the time. Species richness was also similar between the two sites, with the realignment having a slightly higher richness compared to the estuary as a result of habitat heterogeneity. A wide diversity of species inhabited both sites site on both the flood and ebb tides, including bass (Dicentrarchus labrax), common goby (Pomatoschistus microps), smelt (Osmerus eperlanus), thick-lipped grey mullet (Chelon labrosus), three-spined stickleback (Gasterosteus aculeatus), thin-lipped grey mullet (Liza ramada), flounder (Platichthys flesus), plaice (Pleuronectes platessa) and eel (Anguilla Anguilla). Specific to the realignment site were herring (Clupea harengus), sprat (Sprattus sprattus), sand smelt (Atherina presbyter) and sand goby (Pomatoschistus minutus), and specific to the Thames estuary were sole (Solea solea) and whiting (Merlangius merlangus).Bass was the dominant species in both the Thames estuary and the realignment with one exception when common goby dominated; common goby is a species that is well documented to be present in abundance in estuaries. Densities of fish caught by seine nets were higher in the realignment compared with the estuary during October and November 2010 and August 2011, whereas in June 2011 and April 2012, the Thames estuary had greater fish densities, showing seasonal differences in fish utilization. April 2011 had under ten fish caught by seine in both sites therefore density was minute. Over half of the fyke surveys found the realignment to have a higher catch-per-unit-effort than the Thames estuary. Although common goby and bass were the only species where data allowed a comparison between the sites, diet analysis revealed similar taxonomic richness in these species at both sites.The similarities found in this study suggest that the realignment is operating in a similar manner to the adjacent estuary, and that managed realignments can compensate for habitat loss to some extent. Future management plans should include a detailed study over a longer time period, and take in to account the wider dynamics of the estuary, including invertebrate samples, sediment samples and also the impact managed realignments could have on bird communities