Intertidal activity, breeding and the floating habit ofHydrobia ulvaein the Ythan estuary

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Abstract
INTRODUCTIONEcological studies of EiderSomateria mollisima, ShelduckTadorna tadorna, RedshankTringa totanus[Aves], and FlounderPlatichythys flesus[Osteichthys], in the estuary of the River Ythan, Aberdeenshire, have entailed investigations of their food requirements, and there is evidence to suggest thatHydrobia ulvae[Gastopoda], as a numerous food item common to all, may in some instances have influenced their breeding success and numbers in some years. In order to get an index of abundance ofHydrobia, the Ythan population has been sampled four times a year since May 1963. The full results of this sampling programme are being published separately. At the same time data on the annual cycle ofHydrobiawere collected from one area (transect number 3, Fig. 1), on the assumption that breeding onset and intensity would be the same in other parts of the estuary in any one year.