Ecosystem impacts of intertidal invertebrate harvesting: from benthic habitats to bird predators.

Abstract

Intertidal harvesting of marine invertebrates has significant potential to come into conflict with the interests of nature conservation. This is particularly so for overwintering shorebirds that rely heavily on invertebrate prey to maintain body condition throughout the winter and to fuel migration towards breeding grounds. Harvesting activities in these areas therefore require careful management to achieve sustainability and to maintain healthy ecosystem functioning. This thesis investigates impacts of intertidal harvesting on benthic habitats and invertebrate communities as well as the potential impacts of harvesting on shorebird populations. Implications for management of inshore and intertidal fisheries are discussed. A meta-analysis investigated the response of key invertebrate prey groups to different gear types used in different intertidal habitats. Hand gathering most severely reduces prey abundance, which is likely to be due to the accuracy of harvesting with these gear types, while recovery trends vary between different combinations of gear and habitat and taxonomic groups. Results suggest that impacts may persist for longer in sandy habitats than in muddy habitats. In some cases fishermen may develop gears in response to local circumstance and the development of harvestable populations of new and introduced species. Extensive fieldwork was carried out to assess benthic impacts of ‘pump-scoop’ dredging in Poole Harbour, UK, a designated Special Protection Area under the European Union Birds Directive. The pump-scoop dredge is a novel gear type developed by local fishermen following the introduction of the manila clam Ruditapes philippinarum in the 1980s. The use of this gear type elicits significant changes to macrobenthic community structure and a loss of fine sediments, while reductions in abundance of the target species of up to 95% occur in some areas throughout the open season. Although population dynamics of R. philippinarum vary across a gradient of fishing pressure, determining cause and effect is prevented by a lack of environmental data that could help isolate fishing impacts more confidently. Data on fishing effort is often lacking, particularly in inshore fisheries where Vessel Monitoring Systems (VMS) data are not collected. The analysis of aerial imagery collected by an unmanned aerial system (UAS) was used as an alternative measure of fishing effort in intertidal areas. Results indicate that the physical scarring of the sediment (quantified through image classification methods and calculation of a measure of image texture) is a reliable proxy for the distribution and intensity of fishing effort in intertidal areas. Remote sensing techniques offer an alternative source of data, useful to inform management of inshore fisheries, where no log book program or VMS data exists. A combination of fieldwork and individual-based modelling (IBM) was used to investigate the effect of shellfish dredging on shorebird populations in Poole Harbour. Field surveys showed no significant effect of dredging on shorebird feeding or intake rates, nor species distribution across the site, although continued monitoring is recommended. IBM results indicate that increased shellfish landings in Poole Harbour elicit a behavioural response in the Eurasian oystercatcher Haematopus ostralegus population, characterised by an increase in the time spent feeding and the amount of marine worms consumed. These shifts in behaviour and diet represent compensatory measures in response to a loss of preferred shellfish prey. The work presented in this thesis can directly contribute to ecosystem-based management of inshore fisheries. Results from the meta-analysis will assist managers in predicting the effects of harvesting on benthic ecosystems and provide useful evidence of recovery patterns, while survey data provide information on the impacts of pump-scoop dredging in Poole Harbour, directly contributing to management. Other work provides demonstration of how tools such as remote sensing and IBMs can be applied to accurately quantify disturbance and predict the responses of shorebird populations to harvesting. The work presented will help ensure sustainable fishing, productive benthic habitats and healthy shorebird populations into the future.