Burry Inlet / Three Rivers
Oystercatcher, Haematopus ostralegus
Knot, Calidris canutus
Stillman et al. (2008) © CNC/NRW 2009
The Burry Inlet Special Protection Area (SPA) supports nationally and internationally important populations of wildfowl and waders which feed on the mudflats and sandflats over the winter period. Oystercatcher feed predominantly on cockles, and knot feed on a range of prey including cockle spat.
CCW have commissioned a series of studies to determine the food requirements of oystercatcher and knot, to develop conservation objectives, monitoring targets and to assess the implications of cockle and mussel fishing scenarios on the bird populations. These studies were initiated with the Centre for Ecology and Hydrology (Dorset) and have now been taken on by Bournemouth University. They use an individual behaviour-based model to predict the food requirements of a bird population. The model requires information on bird foraging behaviour together with site-specific data on bird numbers, and the size and distribution of cockle and mussel stocks. Other reports therefore describe surveys of the cockles and mussel in the Burry Inlet (e.g. Moore, 2009) and the distribution and numbers of wading birds (e.g. Banks et al., 2007). The analyses of these combined projects are the subject of the present report. In the model simulations here it is shown that birds move between the Burry Inlet and the nearby Three Rivers Estuary. This shows that the conservation objectives and management of these two sites should not be considered in isolation.
The present project also shows that, for the Burry Inlet and Three Rivers Estuary, oystercatchers need twice as much food available than they actually consume in order to maintain high survival rates. This needs to be accounted for in management decisions concerning cockle and mussel fisheries.
Although generally not the case during 2004 to 2007, cockle or mussel fishing are capable of decreasing knot and oystercatcher survival rates and the common strategy of harvesting a fixed proportion of the shellfish stock could adversely effect potential fishing yield in years of high stock size and adversely effect birds in years of low stock size. This finding warrants further consideration in the future management of the site.
The present study also shows that in 2007 the Burry Inlet could no longer support the population size of oystercatcher for which it was designated and was therefore in an unfavourable conservation condition. Against a back-drop of declining cockle stocks and cockle mass mortalities, the future conservation prospects of the site do not appear to be favourable either. The management of the estuary therefore needs to be addressed and possibly also that of the wider catchment.
Stillman et al. (2010) Reproduced with permission from Elservier:
We use an individual-based model to assess the conservation objectives for knot Calidris canutus L. and oystercatcher Haematopus ostralegus L. on the Burry Inlet Special Protection Area (SPA), UK. Population monitoring has identified a decline in oystercatcher numbers, but cannot determine whether this is due to a decline in site quality. Long term data on cockle stocks show that the biomass of the large-sized cockles consumed by oystercatcher declined after 2004, whereas a similar decline was not observed in the smaller cockles consumed by knot. The model postdicts that during the winters of 2005/2006 to 2008/2009 the site was unable to support the number of oystercatcher present at the time it was designated (i.e. the SPA population). Large cockle biomass remained low during 2009/2010, but increases in mussel biomass meant that the model postdicted that the site could support the SPA population of oystercatcher. Knot food supplies remained high during most years, except 008/2009 during which the model postdicted that the SPA population could not be supported. The model postdicted that the stock reserved for oystercatchers after shellfishing needed to be 2–4 times the amount consumed by the birds in order to support the bird population. We recommend that where necessary, the conservation objectives of waterbirds should be assessed using a combination of thorough population size and behaviour monitoring to identify sites with population declines, and individual-based modelling on these sites to determine whether reduction in site quality may contribute to the site-specific population decline
Funding and Collaboration
Countryside Council for Wales
Oystercatcher, Haematopus ostralegus
West et al. 2003:
The Burry inlet, South Wales, supports a licensed cockle Cerastoderma edule fishery and occasional mussel Mytilus edulis fishery. It is also an important overwintering ground for oystercatchers Haematopus ostralegus. In recent years mussels have settled over parts of some cockle beds, preventing cockle fishery there and leading to a request by shellfishers to remove this ’mussel crumble’. Conservation managers, however, were concerned that the mussel crumble might be providing a high-quality food source for the oystercatchers, making its removal detrimental to the birds. A behaviour-based model of oystercatcher feeding on cockles and mussels was parameterised for the inlet and its predictions tested against the distribution of birds across the shellfish beds and the amount of time they spent feeding. The model was then used to explore whether the birds were currently food-limited and what would be the effects on their mortality rate and body condition if the mussel crumble were to be removed, thereby re-exposing underlying cockle beds. The model predicted successfully the proportion of birds feeding on the different types of food and the number of hours birds spent feeding on neap tides. It was predicted that, at current bird population sizes, there would have to be a 50% reduction in shellfish stocks and the areas of shellfish beds from 2000–01 levels to cause noticeable extra emigration or mortality. A given area of mussel bed was predicted to be able to support more birds than the same area of cockle bed, but the greater area of the cockle beds meant that they were more important than mussels in determining the number of birds supported by the inlet. The simulated removal of mussel crumble to expose underlying cockles had no effect on predicted bird mortality and body condition at 2000–01 shellfish stock levels. However, there were circumstances under which the mussel crumble was predicted to increase the inlet’s capacity to support birds, particularly when the area of existing cockle and mussel beds was substantially reduced.
Stillman et al. (2001) © 2001 British Ecological Society:
- Human interests often conflict with those of wildlife. In the coastal zone humans often exploit shellfish populations that would otherwise provide food for populations of shorebirds (Charadrii). There has been considerable debate on the consequences of shellfishing for the survival of shorebirds, and conversely the effects of shorebird predation on the shellfish stocks remaining for human exploitation. Until now, it has been difficult to determine the impact of current shellfishery practices on birds or to investigate how possible alternative policies would affect their survival and numbers.
- One long‐running contentious issue has been how to manage mussel Mytilus edulis and cockle Cerastoderma edule shellfisheries in a way that has least effect on a co‐dependent shorebird, the oystercatcher Haematopus ostralegus, which also consumes these shellfish. This study used a behaviour‐based model to explore the effects that the present‐day management regimes of a mussel (Exe estuary, UK) and a cockle (Burry inlet, UK) fishery have on the survival and numbers of overwintering oystercatchers. It also explored how alternative regimes might affect the birds.
- The model includes depletion and disturbance as two possibly detrimental effects of shellfishing and some of the longer‐term effects on shellfish stocks. Importantly, model birds respond to shellfishing in the same ways as real birds. They increase the time spent feeding at low tide and feed in fields and upshore areas at other times. When shellfishing removes the larger prey, birds eat more smaller prey.
- The results suggest that, currently, neither shellfishery causes oystercatcher mortality to be higher than it would otherwise be in the absence of shellfishing; at present intensities, shellfishing does not significantly affect the birds. However, they also show that changes in management practices, such as increasing fishing effort, reducing the minimum size of shellfish collected or increasing the daily quota, can greatly affect oystercatcher mortality and population size, and that the detrimental effect of shellfishing can be greatly increased by periods of cold weather or when prey are unusually scarce. By providing quantitative predictions of bird survival and numbers of a range of alternative shellfishery management regimes, the model can guide management policy in these and other estuaries
Funding and Collaboration
Countryside Council for Wales, Commission of the European Communities, Directorate-General for Fisheries and the Natural Environment Research Council
Stillman, R.A., 2008. Predicting the effect of shellfish stocks on the oystercatcher and knot populations of the Burry Inlet and Three Rivers. Countryside Council for Wales Marine Monitoring Report No. 65, Bournemouth University for the Countryside Council for Wales.
Stillman, R.A., Moore, J.J., Woolmer, A.P., Murphy, M.D., Walkere, P., Vanstaen, K.R., Palmer, D. and Sanderson, W.G., 2010. Assessing waterbird conservation objectives: An example for the Burry Inlet, UK. Biological Conservation, 143(11): 2617-2630.
The 2010 paper supersedes the 2008 report.
West, A.D., Goss-Custard, J.D., McGrorty, S., Stillman, R.A., le V. dit Durell, S.E.A., Stewart, B., Walker, P., Palmer, D.W. and Coates, P.J., 2003. The Burry shellfishery and oystercatchers using a behaviour-based model to advise on shellfishery management policy. Marine Ecology Progress Series, 248: 279-292.
Stillman, R.A., Goss-Custard, J.D., West, A.D., Durell, S.E.A.L.V.D., McGrorty, S., Caldow, R.W.G., Norris, K.J., Johnstone, I.G., Ens, B.J., Van Der Meer, J. and Triplet, P., 2001. Predicting shorebird mortality and population size under different regimes of shellfishery management. Journal of Applied Ecology, 38(4): 857-868.