Oystercatcher, Haematopus ostralegus
A model was developed incorporating the oystercatcher and shellfish populations, and the climate. The numbers of oystercatchers were recorded from a national monitoring programme and the abundance and distribution of shellfish from routine shellfishery surveys.
Goss-Custard et al. 2004:
In a number of extensive coastal areas in northwest Europe, large numbers of long-lived migrant birds eat shellfish that are also commercially harvested. Competition between birds and people for this resource often leads to conflicts between commercial and conservation interests. One policy to prevent shellfishing from harming birds is to ensure that enough food remains after harvesting to meet most or all of their energy demands. Using simulations with behaviour-based models of five areas, we show here that even leaving enough shellfish to meet 100% of the birds’ demands may fail to ensure that birds survive in good condition. Up to almost eight times this amount is needed to protect them from being harmed by the shellfishery, even when the birds can consume other kinds of non-harvested prey.
Stillman et al. 2003 © 2003 British Ecological Society:
- The debate over the interaction between shellfishing and shorebirds is long-running. Behaviour-based models predict how animal populations are influenced by environmental change from the behavioural responses of individual animals to this change. These models are a potential tool for addressing shellfishery problems, but to be of value they must produce reliable predictions using data that are readily available or can be collected relatively quickly.
- We parameterized a behaviour-based model for the oystercatcher population of the Wash, UK, for 1990–99 using data from shellfishery (mussels and cockles), shorebird and climate monitoring schemes. During the 1990s the overwinter mortality rates of Wash oystercatchers varied widely. The model correctly identified the years in which the observed overwinter mortality was either low (1–2%) or high (10–26%) from annual variation in the food supply, oystercatcher population size and temperature.
- Many oystercatchers were observed and predicted to die when only a fraction of the available food was consumed. Within the model at least, this was because interference competition excluded the least dominant birds from part of the food supply and the least efficient foragers died before the food supply was fully depleted. A simplified model, which excluded interference and individual variation, incorrectly predicted that all birds survived in all years. Models that exclude these two components of behaviour will tend to underestimate the effect of mussel and cockle food shortage on oystercatchers. Shellfishery management based on such predictions may cause high oystercatcher mortality rates even though enough food would appear to be reserved for the birds.
Synthesis and applications. This study shows how a behaviour-based model can be parameterized and predict annual variation in oystercatcher mortality using data routinely collected from the Wash. The principle on which the model is based, that animals behave in order to maximize their chances of survival and production, applies to any system, and the shellfishery, bird and climate data used to parameterize the model are widely available. The model can be used to advise how to manage shellfisheries, by predicting the proportion of the stock that needs to remain unfished in order to maintain low oystercatcher mortality rates.
Oystercatcher, Haematopus ostralegus
Common scoter, Melanitta nigra
Eider, Somateria mollissima
Contains public sector information licensed under the Open Government Licence v3.0
This report presents the results of a research project carried out by the Centre for Ecology and Hydrology for Natural England in order to determine the capacity of The Wash shellfish stocks to support eider ducks Somateria mollissima.
Since 2003, mussel Mytilus edulis farmers on The Wash have reported high levels of mussel predation on their lays by eider ducks. In order to limit this predation some lay holders applied for consent from English Nature (now Natural England) to use scaring devices and some also applied to the Department of the Environment, Food and Rural Affairs (Defra) for a licence to shoot eiders. In advance of the public inquiry to address this conflict between eiders and the shellfish industry, Defra identified a need to develop knowledge and understanding of four key areas so that objective data would be available to facilitate decision making:
- The eider population size in The Wash.
- The number of eiders that the site can support when the mussel and cocklebeds are in favourable condition.
- The effectiveness of scaring devices in protecting the mussel stocks.
- The impact of the scaring devices on eiders and non-target species.
This project was commissioned by Natural England to address the second key area. The principal aims of this project were as follows.
- To conduct a literature review of the current understating of eider feeding behaviour
and scoping of modelling work.
- To develop and parameterise a model of eider ducks in The Wash.
- To validate this model against independent empirical data
- To conduct model simulations of a range of alternative shellfish stock/management scenarios in order to predict their effect on the level of eider duck mortality.
Over 100 published scientific papers and reports principally concerned with the ecology of diving dicks in general and eider in particular were collated and reviewed. The key points of relevance to the development of a behaviour-based model of the system were added to an existing Excel database arsing from a previous project on common scoters Melanitta nigra. This combined database is presented as a series of appendices to this report. This literature review provided information with which to parameterise the model and other independent data against which to validate it.
An existing behaviour-based model developed previously by CEH was parameterised to create a model of the populations of eider ducks and oystercatchers Haematopus ostralegus within The Wash and of the principle populations of shellfish that they exploit within it ie mussels, cockles Cerastoderma edule and American jack-knife clam Ensis directus. Parameterisation was based on information gleaned from the literature review and on the results of recent surveys of the shellfish stocks of The Wash conducted by Eastern Sea Fisheries Joint Committee, Centre for Environmental, Fisheries and Aquaculture Science and Ecomaris Ltd.
The output generated by the model was validated against independent data concerning: the proportion of time that birds spend feeding, their daily consumption of food, daily energy expenditure, body mass, distribution and over-winter mortality. In all cases, the output of the model, when parameterised to mimic current day conditions in The Wash, fell within the likely range of expected values.
One series of model simulations was conducted to explore the consequences for the existing over-wintering populations of eiders and oystercatchers of changes to the total quantity of mussels available to them on the commercially cultivated lays against a number of alternative backgrounds in which the other shellfish stocks were varied in the light of the historical variation that they have shown. These simulations served to explore the impact on the
existing eider and oystercatcher populations of a reduction in the stock of lay mussels and whether this impact varied in relation to the abundance of other shellfish stocks available to the birds.
A second series of model simulations was conducted to explore the consequences for the existing over-wintering populations of eiders and oystercatchers of changes to the distribution of the current total stock of mussels available to them on the commercially cultivated lays (c 10,000 tonnes) against a number of alternative backgrounds in which the other shellfish stocks were varied in the light of the historical variation that they have shown. These simulations served to explore the impact on the existing eider and oystercatcher populations of changes to the management of the commercially cultivated mussel lays and whether this impact varied in relation to the abundance of other shellfish stocks available to the birds. In both of these series of simulations, the model was also used to predict the tonnage of shellfish removed by both eider ducks and oystercatchers from each of the shellfish stocks.
A third series of simulations was conducted in which the size of the peak population of eider ducks was varied and the stocks of the two ‘un-natural’ shellfish resources in The Wash i.e. commercially cultivated lay mussels and Ensis directus were set to either large or small values. These simulations were conducted to establish the extent to which the maximum size of the eider population that could be supported varied in response to variation in these two shellfish resources which appear to be currently of overwhelming importance to the eider population.
The key conclusions that can be drawn from all of these simulations are as follows:
- The recent aggregation of eiders on the Roger and Toft lays can be explained purely in terms of the high density of high-quality shellfish of a suitable size present there.
- The proportion of the oystercatcher population that exploits the commercially cultivated mussel lays is far smaller than that of eiders.
- Over-winter losses of mussels from lays to eiders and oystercatchers are estimated to be around 600 tonnes and 100 tonnes respectively. In the case of eiders this is entirely from the Roger and Toft lays.
- The percentage of the peak eider population of 3,000 birds that cannot be supported under current circumstances is predicted to be around 4 per cent. This is in close agreement with independent estimates of the typical over winter mortality rate of eiders.
- The percentage of the peak oystercatcher population of 15,000 birds that cannot be supported under current circumstances is predicted to be zero.
- Provided that the stock of Ensis directus remains healthy, the stock of lay mussels could be reduced by up to 50 per cent without any significant effect upon the percentage of the eider population ‘at risk’.
- If the stock of lay mussels is reduced below around 50 per cent of its current value, the percentage of the eider population that could be supported is predicted to decline significantly, even in the presence of a healthy Ensis stock.
- As the abundance of mussels on the best lays is reduced, eider predation on the remaining lays will increase and they will switch to alternative shellfish resources. This will not be sufficient to maintain the percentage of the population that can be supported at the current low value.
- In the absence of a healthy stock of Ensis, the percentage of the eider population that can be supported is predicted to be far more vulnerable to any loss of access to lay mussel resources.
- Variation in the abundance of cockles or mussels on the regulated beds was not predicted to significantly alter the effect on eider ducks of changes to the abundance of lay mussels.
- Given most likely future shellfish stock scenarios, the population of oystercatchers, unlike that of eiders, is not predicted to be vulnerable to changes to the abundance of the lay mussel resource.
- Increasing the extent to which the current stock of lay mussels is concentrated is predicted to increase the relative profitability of the already best areas to eider ducks and to result in increased losses from them and hence from lays as a whole (> 650 tonnes v c 600 tonnes currently).
- Decreasing the extent to which the current stock of lay mussels is concentrated is predicted to decrease the relative profitability of the best areas to eiders, reduce predation pressure on the best lays and result in the losses of lay mussels decreasing (c 400 tonnes v c 600 tonnes).
- Evening out variations in the numerical density of mussels between the lays is predicted to even out eider predation pressure between them.
- Removing the best ‘hot spots’ of high mussel density on the lays is predicted to reduce the percentage of the eider population that can be supported.
- The percentage of the oystercatcher population that is supported is predicted to be constant irrespective of changes to the way in which the lay mussel stock is distributed.
- In contrast to eiders, increasing the extent to which the current stock of lay mussels is concentrated is predicted to reduce the profitability of the best lays to oystercatchers and to reduce losses from them. In contrast, spreading the mussels out more thinly increases the profitability of the best lays to oystercatchers and results in increased losses from them.
- Because of the relatively minor predation pressure exerted on the lays by oystercatchers in comparison to eiders, changes to the overall losses from the lays mediated by changing the distribution of mussels are driven more by the responses of eider ducks than oystercatchers.
- The current stocks of wild mussels on regulated beds and cockle stocks are, on their own, sufficient to support the current peak population of 15,000 oystercatchers.
- The current peak population of 15,000 oystercatchers could not be maintained if the stocks of mussels on the regulated beds and the stocks of cockles returned to the low values seen in the early 1990s.
- Current (or greater) stocks of wild mussels on regulated beds and cockle stocks are, on their own, insufficient to support the current peak population of 3,000 eiders. This reflects the relatively poor quality of these natural shellfish resources.
- A healthy stock of Ensis (in combination with current or greater stocks of mussels on the regulated beds and healthy cockle stocks) cannot, in the absence of commercially cultivated mussel stocks, maintain the present peak population of eiders.
- In the absence of a healthy stock of Ensis, the maximum peak population of eiders that the current lay mussel stock can support, is less than the current peak population of 3,000 birds.
- The recent high peak population of eiders in The Wash probably reflects the coincidence of an unprecedented abundant stock of lay mussel resources and a peak population of the non-native Ensis directus. Together, these resources have the capacity to support a peak population of between 10,000 and 12,000 eiders.
- The ability of The Wash to support the current peak population of eiders is determined by the abundance of the ‘un-natural’ shellfish resources ie commercially cultivated lay mussels and non-native Ensis directus rather than the stocks of the wild, native shellfish.
In summary, the recently observed concentration of the bulk of a large population of overwintering eider ducks in The Wash on the Roger and Toft lays is replicated by the model. This is a foraging model, and as such the only reason for it to generate an aggregation of birds in these two locations is the presence there of a large stock of suitably sized, high quality mussels growing at a high density. The model predicts that the eiders consume c 600 tonnes of mussels from these two lays. It also predicts that, as observed, the eiders switch in late winter to feed on Ensis directus. The model predicts that by exploiting these two resources alone, the current peak population of c 3,000 eider ducks can be supported with only around 4 per cent being at risk of not being supported. This state of affairs depends upon the continued availability of ‘hot-spots’ of high densities of high-quality mussels such as those available on the Roger and Toft lays, and upon a continued presence of a healthy stock of Ensis directus. The current stock and distribution of lay mussels, in combination with a healthy stock of Ensis directus, has the capacity to support a far larger peak population of eider ducks than has ever been observed in The Wash. However, in the absence of either of these two resources, there is predicted to be a significant increase in the percentage of the current peak population that will be at risk of not being supported. The health of the stocks of mussels on the regulated beds and cockle beds appear to be relatively unimportant in determining the wellbeing of the eider population in The Wash. The ability of the shellfish resources in The Wash to sustain the current peak population of c 3,00 birds is primarily a result of the coincidence of a large stock of highly aggregated, high quality lay mussels and a healthy stock of Ensis directus ie the stocks of the shellfish which are not a ‘natural’ part of the Wash ecosystem.
Centre of Ecology and Hydrology (internal), Natural England
Grey plover, Pluvialis squatarola
Oystercatcher, Haematopus ostralegus
Curlew, Numenius arquata
Bar-tailed godwit, Limosa lapponica
Black-tailed godwit, Limosa limosa
Dunlin, Calidris alpine
Knot, Calidris canutus
Redshank, Tringa tetanus
Reproduced with permission from Elservier:
Conservation managers responsible for estuaries are often required to monitor their site to ensure that the conservation status of any bird species for which the site is considered important is not affected by deterioration of their habitat or by disturbance of the birds themselves. Here, we use an individuals-based model to predict the quality of the Wash embayment, UK, defined in this case as overwinter survival rate, for eight shorebird species. We use the model to predict how site quality would be affected by changes in the types of prey available, prey density, mudflat area and the rate at which birds are disturbed. The results suggested that Macoma, Hydrobia and Corophium had relatively little influence on site
quality for any species modelled except black-tailed godwit, despite being the preferred prey for some bird species. Arenicola marina, other annelids and Cerastoderma edule were found to be important influences on site quality. Birds began to starve, when autumn, estuary-wide food biomass density was below about 5 g AFDMm−2 and survival rates fell below 90% at 4 g AFDMm−2. One possible conservation objective for the Wash estuary would be to monitor
whether the 99% confidence limit of biomass density falls below one of these limits, to determine whether site quality is being maintained. The system as a whole was predicted to be relatively insensitive to habitat loss. Black-tailed godwits were the most sensitive species, but their survival was not affected until 40% of the feeding grounds were removed. The survival of all species in the model remained high at fewer than 20 disturbances/hour. Although disturbance rates on the Wash were not measured during this study it is unlikely that present-day rates of disturbance on the Wash represent a threat to the survival of the bird species modelled. Our results show how an individuals-based model can assess present-day site quality and how it may change in the future. The model predicted prey biomasses below which survival rate decreased, which shorebird species were most vulnerable to changes in site quality, and that prey density was a more important factor in shorebird survival than habitat area on the Wash. They also show such models can be used to set maximum disturbance rates for each species by predicting how disturbance rates influence shorebird survival.
Centre of Ecology and Hydrology (internal), Natural England
Goss-Custard, J.D., Stillman, R.A., West, A.D., Caldow, R.W.G., Triplet, P., le V dit Durell, S.E.A. and McGrorty, S., 2004. When enough is not enough: shorebirds and shellfishing. Proceedings. Biological Sciences, 271(1536): 233-237.
Stillman, R. A., West, A. D., Goss-Custard, J. D., Caldow, R. W. G., McGrorty, S., Durell, S. E. A. le V. dit, Yates, M., G., Atkinson, P. W., Clark, N. A., Bell, M. C., Dare, P. J. & Mander, M. (2003) An individual behaviour-based model can predict shorebird mortality using routinely collected shellfishery data. Journal of Applied Ecology, 40, 1090-1101.
Caldow, R.W.G., Stillman, R.A. and West, A., 2003. Modelling study to determine the capacity of The Wash shellfish stocks to support eider Somateria mollissima, Centre of Ecology and Hydrology, Dorset. http://nora.nerc.ac.uk/id/eprint/1589/
West, A. D., Yates, M. G., McGrorty, S. & Stillman, R. A. (2007) Predicting site quality for shorebird communities: a case study on the Wash embayment, UK. Ecological Modelling, 202, 527-539.