Evaluating impacts of shellfish and baitworm digging on bird populations: short-term negative effects on the availability of the mudsnail Hydrobia ulvae to shorebirds

Abstract

Many shorebird populations show evidence of declines. To identify the causes is a key issue in developing comprehensive shorebird conservation plans. In coastal areas, shorebirds are vulnerable to effects of shellfish and baitworm digging, including reduction of the food supply. The mudsnail Hydrobia ulvae is often reported to be the dominant inhabitant of intertidal mudflats, and is common in the diet of migrating and wintering shorebirds. This prosobranch mollusc lives at or just below the surface of intertidal mudflats, so it might be directly damaged and killed or buried within the mud by hand diggers. We studied the short-term effects of digging by hand on the availability of mudsnail to shorebirds. Twenty centimetres deep core samples were collected from undisturbed and recently disturbed intertidal mud. The total mudsnail density and biomass per core sample was similar in disturbed and undisturbed mud. However, mudsnail density and biomass were significantly lower in disturbed mud than in undisturbed mud when only the upper five centimetres of the mud were compared. If only the mudsnails found in this surface layer are potentially available for shorebirds, the available mudsnail density and biomass fraction for shorebirds had decreased by 62.6 ± 11.4% and 75.7 ± 7.2% in disturbed mud, respectively. The potential impact of this decreasing mudsnail fraction on shorebirds is addressed.