USING OPTIMAL FORAGING THEORY TO DETERMINE THE DENSITY OF MUSSELS MYTILUS-EDULIS THAT CAN BE HARVESTED BY HAMMERING OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS

In a previous paper (Meire & Ervynck 1986) it was shown that Oystercat chers, opening Mussels by the hammering method, selected the most prof itable length classes of Mussels in terms of energy gain. It was assum ed in the model that thick-shelled Mussels could not be opened. In thi s paper the possibility is explored that opening is not impossible, bu t would take a disproportionate amount of time. To this end, the numbe rs of blows of an artificial Oystercatcher bill, necessary to open a M ussel, was measured experimentally and was found to increase supraprop ortionally with shell thickness. Based on this experiment and measurem ents in the field the profitability of Mussels as a function of shell thickness was calculated for different length classes. Profitability d ecreased sharply with shell thickness and differed, for each shell thi ckness class, between length classes. Based on this result it was pred icted that (1) per length class of Mussels, the thick-shelled Mussels should be dropped from the diet and (2) that the shell thickness of Mu ssels accepted should increase with mussel length. Both predictions we re supported by the data. As birds selected the thin-shelled Mussels i t was expected that the average shell thickness of the Mussels on the bed should increase in the course of the winter. This was not found, p robably because the fraction of the mussel population that is harvesta ble for hammering Oystercatchers was very small. However, the consumpt ion of the birds over the winter amounts to 70% of the production of t his harvestable fraction. It is concluded that a clear description and understanding of the foraging behaviour is crucial to understand the relation between the distribution of a predator and its prey.