Dee Estuary

53.292, -3.157

March 2008

Birds

Oystercatcher, Haematopus ostralegus

Modelling

In the past, cockle populations in each SPA have been sampled only for fisheries purposes. The surveys provide a good estimate of cockle stocks in the fished areas, but do not estimate the total stock throughout the site. We designed grid-based sampling schemes to estimate the abundance and location of cockles and mussels throughout each site and all three estuaries were surveyed. The data were used to develop individual-based models for each site.

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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. Finally, the implications of this study’s findings for modelling the oystercatcher Population of the Dee estuary are discussed.

 

Stillman et al. 2013:

In UK estuaries conflicts have routinely occurred between economic and conservation interests regarding shellfish such as cockles Cerastoderma eduleand mussels Mytilus edulis. The harvest of these species is economically important, but shellfish also constitute the main overwinter food supply of the oystercatcher Haematopus ostralegus. In this report we use a simplified spreadsheet model to predict the overwinter food requirements of oystercatchers in the Dee Estuary and compare the predictions of this model with those of an individual-based model which has been used to advise the setting of Total Allowable Catch in the Dee Estuary over recent years.

The models are based on the energy requirements of the birds and the energy value of their shellfish food. The spreadsheet model predicts the amount of shellfish required to maintain high survival rates within the oystercatcher population. The individual-based model predicts how the survival rate within the oystercatcher population is related to the amount of shellfish food and the amount removed by shellfishing. Although more complicated, the individual-based model represents the system in a more realistic way and can simulate specific shellfishing scenarios.

The models produced relatively similar predictions, especially when it was assumed that birds fed on upshore and terrestrial food in addition to cockles. As the biomass of cockles has declined since 2008, the models predicted that the amount required by the birds became close to the total available in 2012. The cockle biomass during 2013 was lower than that during 2012 and the spreadsheet model predicted that the birds required virtually all of the cockle stocks available.

Funding and Collaboration

Countryside Council for Wales, Environment Agency 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.

Stillman, R.A. and Wood, K.A., 2013. Predicting oystercatcher food requirements on the Dee Estuary. A report to Natural Resources Wales, Bournemouth University, Poole, Dorset.