Ecological impact of the Carcinus maenas (L.) fishery 'crab-tiling' on estuarine fauna

Abstract

The ecological effects of the green shore crab Carcinus maenas (L.) fishery was investigated in estuaries on the south west coast of the UK. The fishery known as 'crab-tiling' involves 'crab-tilers' laying artificial refugia such as car tyres, guttering or roof tiles, intertidally in estuaries. 'Crab-tiles' are specifically laid on mudflats and sandflats to attract moutling crabs seeking refuge, these crabs are then collected to use or sell as bait for recreational angling. Many estuaries that are crab-tiled are subject to a range of International, European and National legislation as they are of great conservation importance. For example, they provide feeding grounds for migratory wading shorebirds, and nursery and feeding areas for commercially and ecologically important fishes. Little is known regarding the impacts of the crab-tiling fishery on the-estuarine ecosystem. The aim of this study was to identify the direct effects that crab-tiling has on estuarine fauna, allowing a better understanding of the implications of crab-tiling for ecosystem functioning, subsequently allowing conservation groups to manage crab-tiling activity based on ecologically informed decisions. Efforts to establish the main impacts of crab-tiling focused on infauna that live within sediments where crab-tiles are laid, on estuarine epifauna (such as crabs, birds and benthic fishes) which forage around crab-tiled habitat, and aquatic fauna which utilise the intertidal environment during high tide. To quantify the impact of crab-tiling on populations of the target organism, C. maenas, a mensurative experiment was conducted over six estuaries in south-west England, including three commercially tiled estuaries and three relatively not-tiled estuaries. Estuaries where commercial crab-tiling took place were found to support substantially larger population sizes of C. maenas than not-tiled estuaries although the C. maenas modal size class was smaller, suggesting that, in terms of C. maenas abundance, the effect of crab removal by fisheries was outweighed by the increase in habitat availability due to the presence of crab tiles. The diversity and abundance of wading birds feeding over mud flats was not affected by the presence of crab-tiles, though birds aggregated around the tiles when feeding; conversely, crab-tiles generally had an opposite effect on aquatic fauna, which tended to be more abundant in control sites than in crab-tiled sites. A possible explanation for these observations was associated with the concentration of crabs around the tiles: these localised crab populations could provide prey for wading shore birds, which were seen collecting C. maenas from crab-tiles, but could simultaneously predate on aquatic fauna; during high tide, tiled sites had the highest abundance of crabs, yet control sites had the highest abundance of all the other aquatic fauna. The crab-tiling fishery has been found to cause ecological impacts to a range of estuarine fauna. Changes in any one species abundance could have implications for that species' predator and prey; additionally the abundance mediated change in the ecological functions which they perform, will have consequences for ecological processes within the estuary as a whole. The indirect impacts occurring as a result of the direct effects measured here also need to be quantified so that the full effect that crab-tiling has for ecosystem functioning in estuaries can be understood and effectively managed.