When enough is not enough: shorebirds and shellfishing

Abstract

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.

Assessment of Blue Mussel Mytilus edulis Fisheries and Waterbird Shellfish-predator Management in the Danish Wadden Sea

“…However, diet studies show that the proportion of mussels in the species' diet occasionally can be even higher than those used in our estimations (Hilgerloh 1997;Ens et al 1996a, b;Smit et al 1998;Dernedde 1994;Goss-Custard et al 1996;Hilgerloh 1999;Maagaard and Jensen 1994;Wilkens and Exo 1998;Kubetzki and Garthe 2003), and thus raising the total amount of blue mussels necessary for the species. In addition, the ecological factor used to multiply the physiological food amount to get the ecological food demand is uncertain for both eider (Goss-Custard et al 2004) and herring gull. For eiders, we have used the lowest factor in the range, due to our assumption that there is little intraspecific competition during feeding, but for oystercatcher we used the highest values.…”

“…The latter figure was also used for herring gulls, although the species has mixed feeding patterns: swallowing whole mussels, cracking larger shells by dropping them, and acting as a kleptoparasite. Goss-Custard et al (2004) estimated an ecological food requirement (EFR) for oystercatchers as 2.5-7.7 times more than the estimated food requirements needed to fulfil their food demand during winter, based on field data from five estuaries in Western Europe, and argued for the use of a similar factor for Eiders. The EFR takes into account that bird species can not remove all flesh from inside the shells, that parts of the mussels are stolen by gulls and that the mussels have flesh losses during winter due to respiration (and nearly no assimilation).…”

“…In this study, we use the low value (2.5) for eiders, since we think that the intra-specific competition during feeding is low and the species swallow the mussels whole, restricting losses. For oystercatcher we use the high value (7.7), as this was specifically found for oystercatchers feeding on blue mussels in other studies (Goss-Custard et al 2004). For the herring gull, which has different feeding strategies, the average value (5.46) of estimates given by Goss-Custard et al (2004) was used.…”

“…Recent studies demonstrate that such constraints are essential to take into account when considering the proportion of accessible prey in relation to energetic and food consumption issues in mussel-eating birds (e.g. Goss-Custard et al 2004, Nehls, G. 1995, de Leeuw et al 1999). …”

“…To test the efficacy of the measures, we used a before-after controlimpact (BACI) design (Bro et al 2004) and sought to demonstrate that (i) blue mussel biomass has increased since the collapse in 1988, (ii) blue mussel bed areas, mussel biomass and mussel-eating bird numbers had increased since the mussel management scheme was put into force in 1994, (iii) annual mussel landings had increased since 1994 and (iv) mussel stocks and musseleating bird numbers had increased in zones closed to mussel fishery. Second, we present new calculations for estimating the food demands of waterbirds feeding on mussels which are based on a combination of physiological, energetical, and ecological considerations relevant to mussel-eating birds, and using current best practice modelling based on energetic demands (Ens et al 2004;Goss-Custard et al 2004).…”