Summary1. In order to assess the future impact of a proposed development or evaluate the cost eectiveness of proposed mitigating measures, ecologists must be able to provide accurate predictions under new environmental conditions. The diculty with predicting to new circumstances is that often there is no way of knowing whether the empirical relationships upon which models are based will hold under the new conditions, and so predictions are of uncertain accuracy. 2. We present a model, based on the optimality approach of behavioural ecology, that is designed to overcome this problem. The model's central assumption is that each individual within a population always behaves in order to maximize its ®tness. The model follows the optimal decisions of each individual within a population and predicts population mortality rate from the survival consequences of these decisions. Such behaviour-based models should provide a reliable means of predicting to new circumstances because, even if conditions change greatly, the basis of predictions ± ®tness maximization ± will not. 3. The model was parameterized and tested for a shorebird, the oystercatcher Haematopus ostralegus. Development aimed to minimize the dierence between predicted and observed overwinter starvation rates of juveniles, immatures and adults during the model calibration years of 1976±80. The model was tested by comparing its predicted starvation rates with the observed rates for another sample of years during 1980±91, when the oystercatcher population was larger than in the model calibration years. It predicted the observed density-dependent increase in mortality rate in these years, outside the conditions for which it was parameterized. 4. The predicted overwinter mortality rate was based on generally realistic behaviour of oystercatchers within the model population. The two submodels that predicted the interference-free intake rates and the numbers and densities of birds on the dierent mussel Mytilus edulis beds at low water did so with good precision. The model also predicted reasonably well (i) the stage of the winter at which the birds starved; (ii) the relative mass of birds using dierent feeding methods; (iii) the number of minutes birds spent feeding on mussels at low water during both the night and day; and (iv) the dates at which birds supplemented their low tide intake of mussels by also feeding on supplementary prey in ®elds while mussel beds were unavailable over the high water period. 5. A sensitivity analysis showed that the model's predictive ability depended on virtually all of its parameters. However, the importance of dierent parameters varied considerably. In particular, variation in gross energetic parameters had a greater in¯uence on predictions than variations in behavioural parameters. In accord with this, much of the model's predictive power was retained when a detailed foraging submodel was replaced with a simple functional response relating intake rate to Correspondence: R. A. Stillman. CEH Dorset, Winfrith Technology Centre, Winfr...

Summary 1.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. 2. 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. 3. 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. 4. 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.

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Population dynamics of the intertidal mussel population in the River Exe estuary, Devon (UK) over 7 yr (1976 to 1983) are described. There was no significant trend in adult numbers, and annual variations were small (1.5-fold, minimum to maximum). In contrast, the density of 'spat' mussels (0-year) varied uldely ( l ?~) , suggesting that a strong damping process acted during the first year. Considering just first-year mortality, failure to settle on the beds (k3) had the greatest influence on annual variations in total mortality (K). But losses in the first winter after settlement (k5) were strongly and positively denslty dependent. k3 was inversely density dependent, but only in 6 out of 7 yr. Other first-year losses were independent of density. After the large first-winter mortality (mean 68 %), there were only small gains in summer and small losses in winter for the next 4 yr. Thereafter, increasingly large losses occurred (mean 39 % ) each summer after spawning and each winter (mean 24 ?h), due mainly to bud predation. Some of the over-summer and over-winter changes in adult densities were density dependent, but their effect was small and only served as a 'fine tuning' mechanism keeping densities within very narrow limits. Though mortalities at various stages were correlated with the weather, the main feature of the population was its pronounced stability, due mainly to the powerful regulation acting during the first year on the mussel beds.

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