THE AVAILABILITY AND QUALITY OF THE MUSSEL PREY (MYTILUS-EDULIS) OF OYSTERCATCHERS (HAEMATOPUS-OSTRALEGUS)

Spatial variations in the availability and quality of the mussel Mytil us edulis food supply of Oystercatchers Haematopus ostralegus on the E xe estuary, England, are described. Oystercatchers open mussels by sta bbing into gaping mussels (or prising open closed ones) or by hammerin g a hole in either the dorsal or ventral shells. Spatial variations in the food supply are considered at four scales. In decreasing order of size, these are (i) whole mussel beds, (ii) zones within a mussel bed , (iii) different places within one zone, and (iv) different places wi thin one clump of mussels. The first two scales are clearly related to exposure time. Both between and within the 12 main mussel beds of the estuary, most upshore mussels are up to 10% less likely than downshor e mussels to be hidden under mud. However, upshore mussels of a given length contain less flesh, have thicker ventral shells and, except on high-level beds subject to wave erosion, have thicker dorsal shells th an downshore mussels. Mussels at the top of the shore also contain the highest infestations of the helminth parasite of Oystercatchers, Psil ostomum brevicolle. At a particular shore level, mussels of a given le ngth have less flesh and thicker shells, though only on the dorsal sid e, in areas of high mussel density. Within one clump, mussels of a giv en length with thick dorsal shells have more flesh than those with thi n shells. In contrast, flesh content is slightly higher in mussels tha t are thin on the ventral side. Flesh content and shell thickness on b oth sides are unaffected by whether a mussel is visible at the edge of a clump or hidden inside. Simulations with a model of foraging Oyster catchers suggested that variations in mussel availability (visibility and shell thickness) and flesh content at all four scales could someti mes have an important influence on intake rate. Most published values of intake rates of Oystercatchers eating surface-dwelling mussels may be biassed (and often considerably over-estimated) by the erroneous as sumption that prey of average flesh content are taken.