Preliminary assessment of the use of intertidal grazers to create ‘self-cleaning’ boat slipways

Abstract

Boat slipways present a unique management challenge to local authorities as they need to be maintained in an algal free state, so they are safe and accessible for recreational and commercial use. Current slipway management techniques are ineffective, short-lived, and environmentally harmful. There is, therefore, a global demand for the development of a new approach to slipway management. In this thesis, a new approach is proposed using limpets to maintain slipways in an algal free state through grazing. A major impediment to using grazers such as limpets to clean slipways is that slipways lack refugia and thus grazers are at high risk of being trampled by foot and vehicle traffic. Substrate refuges (e.g., grooves) may provide protection from such stressors and therefore allow for greater grazer control. To investigate the association between limpets and algal cover, a quadrat survey was conducted on existing unused slipways in Pembrokeshire, Wales (UK). To investigate how alternative substrate designs influence refuge use of common limpets, Patella vulgata, on slipways, an experiment was carried out in which limpets were added to caged tiles with various refugia that were secured to a slipway in Pembrokeshire. Limpets were associated with lower cover of an opportunistic green alga (Ulva spp.) on existing slipways, suggesting they are controlling their abundance. In the experiment, limpet refuge use was higher than expected based in the proportional availability of refuge space during the day, and much lower and more in line with expectations at night. This shows that limpets rest in trample-safe refugia during periods of high foot traffic on slipways (daytime tidal emersion) and then move more freely throughout the surface of the slipway to graze at nighttime tidal emersion when the slipway is not in use.My findings indicate that, if slipway surfaces are modified to include refugia, limpets may be effective in cleaning the surface of slipways, without being trampled by foot/vehicle traffic. This would reduce the use of bleach and pressure washers to clean slipways, which are environmentally harmful to species on the slipway and adjacent ecosystems