Causes of variation in prey profitability and its consequences for the intake rate of the Oystercatcher Haematopus ostralegus

Prey species have different morphological and behavioural adaptations to escape their predators. In this paper we review how these prey defenses affect prey profitability and intake rate for one predator, the Oystercatcher. Four rules govern profitability. First, within each species large prey are more profitable than small prey, because flesh content increases more steeply with prey size than handling time. Second, soft-bodied prey, such as worms and leatherjackets, which can be swallowed whole, are much more profitable than armoured prey, such as bivalves, which Oystercatchers have to open before the flesh can be extracted from the shell. Third, heavily armoured surface-dwelling prey, like Mussels and Cockles, are the least profitable prey of all, even if the armour is bypassed through stabbing the bill between the valves. Fourth, within the burying prey species, the profitability of prey decreases with depth. Hence burying bivalve species that bury in winter at larger depth than in summer, are in winter, if not our of reach of the bill, anyway less profitable. Despite the large differences between the profitabilities of the various prey species, the intake rates do not differ much when the prey species are com pared, presumably because prey with a low profitability are only exploited if the search time is relatively short, i.e. if the density of harvestable prey is high. On the other hand, within each species, the intake rate goes up if larger, more profitable prey are taken. Thus, if the birds have to feed on smaller prey specimens, they fail to fully compensate for the low profitability by an increase in the rate at which these prey are found. Although the profitability of prey differs seasonally due to the variation in the prey condition, only a small seasonal variation in the intake rate was found. Because burying bivalves and soft-bodied worms bury deeper and are less active in winter, Oystercatchers necessarily rely on bivalves living at, or just beneath, the surface at that time of year

Predicting shorebird mortality and population size under different regimes of shellfishery management

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 birdsor to investigate how possible alternative policies would affect their survival and numbers. 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 (Burryinlet, UK) fishery have on the survival and numbers of overwintering oystercatchers. It also explored how alternative regimes might affect the birds. 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. 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

Predicting shorebird mortality and population size under different regimes of shellfishery management

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 birdsor to investigate how possible alternative policies would affect their survival and numbers. 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 (Burryinlet, UK) fishery have on the survival and numbers of overwintering oystercatchers. It also explored how alternative regimes might affect the birds. 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. 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

Population consequences of winter habitat loss in a migratory shorebird: I. Estimating model parameters

1. In order to construct a model to predict the effect of winter habitat loss on the migratory population of the European subspecies of the oystercatcher, Haematopus ostralegus ostralegus, data on the reproductive and mortality rates collected throughout Europe over the last 60 years are reviewed. Within the Continental and Atlantic regions, inland-breeding and coastal-breeding subpopulations use the same coastal areas in winter. 2. Census and experimental data suggest pairs compete for territories and that an increasing proportion is excluded from breeding altogether, or nest in poor quality habitats, as the number of pairs attempting to breed increases. This provides a main source of density dependence in the basic model. 3. Mean clutch size, hatching success and fledging success were estimated for each subpopulation in each region. Data from one site suggested that the numbers fledged per breeding pair decreases as the total numbers of territories occupied increases, probably because of a reduction in chick survival. This additional source of density dependence was included in some versions of the model. 4. Most post-hedging mortality occurs in winter. Annual mortality was measured from the annual return rates of adults to the breeding areas and probably gives over-estimates. An additional 7-15% of adults die in severe winters once in 7 years in the Continental region but not in the milder Atlantic region. Oystercatchers in their first and second winter have a 20% higher winter mortality rate than adults. 5. The sometimes quite substantial annual fluctuations in the main production and mortality parameters were generally not correlated across sites within a subpopulation. This allowed the standard deviations of the annual variations in these parameters to be estimated for both subpopulations in each region so that realistic annual variations could also be included in the model