Menai Straits

53.238738, -4.116786

Birds

Oystercatcher, Haematopus ostralegus

Modelling

Non-MORPH

Abstract

West and McGrorty (2003): © CNC/NRW 2003

  1. This study of oystercatcher and cockle populations on the Dee Estuary, Traeth Lafan and the Burry Inlet had two main objectives. The first was to assess the power of existing surveys to detect changes in the cockle population at each site and to recommend possible improvements. The second objective was to determine how each site was used by the population of oystercatchers that roosted there.

 

  1. Cockle densities at all three sites were highly variable, indicating that cockle Distributions are aggregated. The amount of variation differed between sites and between years within a site. The amount of variance measured at a particular site in any given year was not related to the size of the surveyor the mean cockle density measured.

 

  1. Between 200 and 400 0.1 m2 samples appeared to provide a reasonable balance between accuracy and efficient use of time and resources. This level of sampling would allowing on average the detection of a 25-40% decrease or a 30-65% increase in cockle density, depending on the site. Increasing sample numbers above 400 provided a relatively small return in terms of improved accuracy for the extra effort involved.

 

  1. A survey scheme is presented which aims to provide a balance between the information needed for fisheries and conservation management purposes. Methods for surveying other common sources of food for oystercatchers are also described.

 

  1. WetlandBird Survey (WeBS) low-tide counts show a substantial proportion of the Oystercatcher population feeding outside the areas covered by fisheries cockle stock surveys on all three sites. On the Burry Inlet, these birds are known to be feeding both on cockle patches occurring outside the fisheries survey area and on other sources of food, particularly mussel beds.

 

  1. WeBS high-tide counts at each site show substantialvariation,50% or more at every site, in the numbers of oystercatchers roosting at each site from month to month over winter. The total numbers across all sites as a whole also varied considerably.

 

  1. Comparison of high and low-tide WeBS counts showed no significant difference between the numbers of oystercatchers roosting on the Dee and feeding there at low tide. More birds appear to feed on Traeth Lafan than the numbers roosting there. On the Burry Inlet, counts show increased bird numbers at low tide during early winter and decreased Numbers at low tide in later months.

 

  1. The energy requirements of oystercatchers, combined with physiological constraints on their intake, mean that birds roosting on the Dee could not fly further than the Ribble to feed at low tide. In mid winter when temperatures are lower, they would only profitably be able to fly as far as the Mersey or Alt estuaries.

 

  1. High- and low-tide counts on the Dee and surrounding estuaries show that most Oystercatchers that roost on the Alt probably feed in the Mersey. A small proportion of the Dee population may also feed in the Mersey.

 

  1. Finally, the implications of this study’s findings for modelling the oystercatcher Population of the Dee estuary are discussed.

 

Caldow et al. (2004) © 2004 by the Ecological Society of America:

Bottom cultivation of mussels on intertidal flats is practiced throughout the world. This often generates conflicts between commercial interests and competing birds such as oystercatchers. At the Menai Strait, United Kingdom, the overwinter consumption of 242 tonnes (1 metric tonne = 1000 kg) of commercially harvestable mussels (>40 mm) by oystercatchers in 1999-2000 was worth ?133 000 ($226 000 U.S. dollars). This represents 19% of the value of the landings. We used a behavior-based simulation model to predict the extent to which such losses can be reduced by novel commercial management practices, and to explore the consequences for the oystercatcher population.

Simulations of novel lay management practices indicated that the losses of commercially harvestable mussels to oystercatchers can be considerably reduced by altering the shore level and/or extent of the commercial lays. We propose a novel management strategy for the bottom cultivation of mussels in intertidal areas. Seed mussels (15-20 mm) should be laid relatively far upshore, where losses to oystercatchers will be minimal. As the mussels grow over the next 2-3 years, they should be moved progressively further downshore such that the largest mussels spend their last season prior to harvest in a relatively small area, lower on the shore than all mussels earlier in the cultivation cycle. Support for the effectiveness of this proposed management strategy can be found in the reports of commercial operators who have incorporated this management strategy in new management practices in the last few years. They report an increase in the ratio of the live mass of harvested to seeded mussels from the previous norm of 1:1 to 4:1. By accepting greater losses of mussels earlier in the cultivation cycle, rather than later, the feeding conditions for oystercatchers might even be improved under this system. With appropriate management, the interest of shellfish growers and competing shorebirds need not conflict.

Funding and Collaboration

Environment Research Council LINK Aquaculture award ENVI1. Countryside Council for Wales

Related Paper:

West, A.D. and McGrorty, S., 2003. Marine monitoring project: Modelling osytercatchers and their food on the Dee Estuary, Traeth Lafan and Burry Inlet Spa to inform target setting and site management, Centre of Ecology and Hydrology, Dorchester, Dorset.

Caldow, R.W.G., Beadman, H.A., McGrorty, S., Stillman, R.A., Goss-Custard, J.D., le V. dit Durell, S.E.A., West, A.D., Kaiser, M.J., Mould, K. and Wilson, A., 2004. A Behavior-Based Modeling Approach to Reducing Shorebird-Shellfish Conflicts. Ecological Applications, 14(5): 1411-1427.