Intertidal areas, such as estuaries, can be both important fisheries and key habitats for wildlife. During winter months, large flocks of wading birds (Order: Charadriiformes) gather to feed within estuaries.
Key species in the UK include curlew, (Numenius arquata), oystercatchers (Haematopus ostralegus), knot (Calidris canutus), dunlin (Calidris alpina), black-tailed godwits (Limosa limosa), bar-tailed godwits (Limosa lapponica) and redshank (Tringa totanus). Shellfish - such as mussels (Mytilus edulis) and cockles (Cerastoderma edule) - are major prey items for oystercatcher and knot, but are also important species for shellfishing.
These shared shellfish resources have sometimes led to conflicts between fisheries and conservation interests, in particular where shellfish stocks are too low to satisfy both groups (Atkinson et al., 2010). Bird species are key components of the biodiversity of UK coasts and are consequently protected under the European Union Wild Birds Directive (2009/147/EEC), which legally obligates the UK to safeguard the birds and their habitats to maintain healthy populations.
To help achieve this, many estuaries have been designated as Special Protection Areas which are managed to sustain overwintering wading bird populations. Often, these estuaries are also shellfisheries. To ensure that enough shellfish are reserved for the birds and for shellfishery sustainability, fishery managers limit shellfish harvests by setting a Total Allowable Catch (TAC) of shellfish for the fishery. In years of very low shellfish stocks the entire fishery may be closed.
Setting the correct levels
Setting the correct level of TAC is of critical importance for the birds, the fishery, and for future shellfish reproduction. If the TAC is set too high, birds may starve and subsequent shellfish reproduction may be low. Conversely, if the TAC is set too low, the economic potential of the fishery will not be realised.
Currently, some fishery managers are attempting to balance conservation and economic interests using the ‘rule of thirds’. This is when one third of the total annual shellfish stock is designated for fisherman, one third reserved for birds, and one third left for subsequent shellfish recruitment. However, arbitrary rules-of-thumb, such as this ‘rule of thirds’, are flawed because the value of one third for each interest is not based on any evidence.
An issue with this is that the value of one third is independent of the bird’s food requirements, shellfishery economic viability, and shellfish reproduction. In any given fishery or year, some of these three interests may need more than one third of the shellfish stock, whereas others may need very much less. Because the value of one third is known to be arbitrary, fishery managers struggle to maintain the support of fishermen and conservationists. Consequently both groups lobby the fishery managers to increase or decrease the TAC respectively.
Developing an alternative method
In recent years an alternative method of informing TACs has been developed, by considering how much food the birds actually require relative to the amount available within estuaries. In the Dutch Wadden Sea, for example, fishery managers began to reserve enough shellfish to meet 70% of the bird population’s energy requirement for the overwinter period; it was assumed that the birds could acquire the remaining 30% from non-harvested species within intertidal and nearby terrestrial feeding areas (Goss-Custard et al., 2004).
However, research that examined bird survival relative to food availability showed that wading birds required more food than would be predicted from their energy needs alone - as not all available food can be found and individual birds are often excluded from food through competition with others (Goss-Custard et al., 2004). Depending on the shellfish species and the site, between 2.5 and 7.7 times more food will be required (these values are called Ecological Multipliers below). Typically the value is higher on mussel-dominated, rather than cockle-dominated, estuaries because mussels are more aggregated and so more birds can be excluded from the food supply by competition.
Individual-based model method
These values were derived from individual-based models (IBMs). These account for the bird’s ability to find food and competition between birds when predicting the quantity of shellfish that a bird population needs to survive through the winter. The story behind this model and examples of its application to shellfisheries is told in Goss-Custard (2014). Such models have been used in the process of managing a growing number of shellfisheries within the UK, including the Solway Firth, Morecambe Bay, Dee Estuary, Menai Straights, Burry Inlet, Three Rivers, Exe Estuary and the Wash (Stillman & Goss-Custard 2010; Stillman et al., 2010).
But there are drawbacks associated with the use of IBMs. In particular, specialist computational knowledge is required to run the models and they have typically been applied on a site-by-site basis. IBMs also have high requirements for data. As such models require information on many aspects of the biology of both wading birds and shellfish. This information may not be routinely available for many estuaries. Despite some recent attempts to try to make IBMs more user-friendly (eg West et al., 2011), a simplified approach to shellfishery management would be the ideal solution.
Developing a simpler model
Recently, we have developed a simpler model (the Ecological Requirement Model) to calculate the food requirements of bird populations. This can be used by people without specialist modelling experience and using the type of data typically available from shellfisheries (Stillman and Wood, 2013).
The model is based on a range of biological parameters, including bird population sizes, the energy requirements of the birds, the energy content of shellfish, and the minimum size of cockles and mussels consumed by the birds. Many of these parameters are routinely collected and so do not need to be measured for each new application of the model. This helps reduce the data requirement for each new site.
The model calculates the amount of shellfish food that will be consumed by an overwintering bird population (this is termed the physiological requirement). It then calculates the amount of food that needs to be reserved in the environmental to allow the birds to survive the winter in good condition (this is termed the ecological requirement). The ecological requirement is found by multiplying the physiological requirement by a factor called the ecological multiplier.
Ecological requirement = Ecological multiplier x Physiological requirement
The value of the Ecological Multiplier depends on whether the estuary is dominated by cockles or mussels (see above). To date the Ecological Requirement Model has been used to inform shellfishery management in Morecambe Bay, Burry Inlet, Solway Firth, and Exe Estuary. The model provides a straightforward way of quantifying the amount of food required by a bird population and hence offers fishery managers an evidence-based means of managing shellfisheries.
If you are interested in discussing or using our models, please contact Richard Stillman at Bournemouth University. This article was written with Richard A. West & John D. Goss-Custard; Bournemouth University.
For updates please sign up to email alerts from this blog, email me or you can follow us on Twitter @CefasGovUK.