Positive feedback fishery: Population consequences of ‘crab-tiling’ on the green crab Carcinus maenas E.V. Sheehan ⁎, R.C. Thompson, R.A. Coleman 1 , M.J. Attrill Marine Biology and Ecology Research Centre, School of Biological Sciences, Marine Institute, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK abstract article info Article history: Received 15 March 2008 Received in revised form 26 August 2008 Accepted 2 September 2008 Available online 13 September 2008 Keywords: Crab fishery Habitat provision Population structure Estuary Commercial species Bait collection Collection of marine invertebrates for use as fishing bait is a substantial activity in many parts of the world, often with unknown ecological consequences. As new fisheries develop, it is critical for environmental managers to have high quality ecological information regarding the potential impacts, in order to develop sound management strategies. Crab-tiling is a largely unregulated and un-researched fishery, which operates commercially in the south-west UK. The target species is the green crab Carcinus maenas. Those crabs which are pre-ecdysis and have a carapace width greater than 40 mm are collected to be sold to recreational anglers as bait. Collection involves laying artificial structures on intertidal sandflats and mudflats in estuaries. Crabs use these structures as refugia and are collected during low tide. However, the effect that this fishery has on populations of C. maenas is not known. The impact of crab-tiling on C. maenas population structure was determined by sampling crabs from tiled estuaries and non-tiled estuaries using baited drop-nets. A spatially and temporarily replicated, balanced design was used to compare crab abundance, sizes and sex ratios between estuaries. Typically, fisheries are associated with a reduction in the abundance of the target species. Crab-tiling, however, significantly increased C. maenas abundance. This was thought to be a result of the extra habitat in tiled estuaries, which probably provides protection from natural predators, such as birds and fish. Although crabs were more abundant in tiled estuaries than non-tiled estuaries, the overall percentage of reproductively active crabs in non-tiled estuaries was greater than in tiled estuaries. As with most exploited fisheries stocks, crabs in exploited (tiled) estuaries tended to be smaller, with a modal carapace width of 20–29 mm rather than 30–39 mm in non-tiled estuaries. The sex ratio of crabs however; was not significantly different between tiled and non-tiled estuaries. These results illustrate the potential to manage fished populations using habitat provision to mitigate the effects of fishing pressure. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Collecting invertebrates for use as fishing bait is widespread worldwide (Fairweather, 1991; Wynberg and Branch, 1991; Olive, 1993), and affects target organisms and other ecosystem components (Underwood, 1993; Thompson et al., 2002). When evaluating the sustainability of recreational fishing, Skilleter et al. (2005) highlighted the need to consider the impacts of bait collection as well as the sustainability of fish stocks, as this component is often ignored (McPhee and Skilleter, 2002). The addition of man-made structures to the marine environment is also of increasing concern, due to the rise in the number of coastal defences (Glasby and Connell, 1999), renewable energy related structures such as wind turbines (Wilhelmsson et al., 2006a), artificial reefs, and semi-permanent fishing equipment e.g. oyster trestles in estuaries (Nugues, 1996; Rilov and Benayahu, 2000; Hilgerloh et al., 2001). These structures add habitat complexity to often, soft sediment environments thereby altering species assemblages (Moschella et al., 2005; Wilhelmsson et al., 2006b). The present study examines a crab fishery ‘crab-tiling’, which involves not only collecting crustacea for bait, but also, installing semi-permanent man-made structures into estuarine mudflats. Over 1 million green crabs Carcinus maenas (L.) (Decapoda, Portunidae), are removed annually from estuaries in south-west UK to be sold as bait (Fowler, 1998; Black, 2004). To collect C. maenas, bait collectors known as ‘crab-tilers’, lay hard, stable structures, such as roof tiles, pieces of half round guttering, and car tyres on estuarine intertidal mudflats or sandflats to provide shelter that crabs will burrow beneath (Fig. 1). Crab-tilers only collect and sell those crabs which are (i) over 40 mm carapace width (CW), (ii) not brooding eggs (‘berried females’) and (iii) in a stage of pre-ecdysis. Crabs which are in this moulting stage are known within the angling community as ‘peeler crab’ and make excellent bait for bass Dicentrachus labrax (L.) and other species of fish. Qualifying crabs usually represent about 10 % of the crabs found under ‘crab-tiles’. The number of crab-tiles in Journal of Sea Research 60 (2008) 303–309 ⁎ Corresponding author. Tel.: +44 1752 232906; fax: +44 1752 232970. E-mail address: emma.sheehan@plymouth.ac.uk (E.V. Sheehan). 1 Present Address: Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories (A11), The University of Sydney, NSW 2006, Australia. 1385-1101/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2008.09.002 Contents lists available at ScienceDirect Journal of Sea Research journal homepage: www.elsevier.com/locate/seares