The diet and feeding behaviour of the flounder Platichthys flesus (L.) in the Ythan estuary, Aberdeenshire, Scotland

Abstract

The flounder Platichthys flesus was studied between October 1970 and August 1973 in the Ythan estuary, Aberdeenshire, Scotland. A general survey of its diet indicated that Corophium volutator was the main prey item on the mudflats of the upper estuary, whereas small polychaetes and oligochaetes were taken on the softer mudflats in the middle region. Nereis diversicolor and Carcinus maenas were eaten on the mussel beds, and shoals of clupeids were preyed upon by older fish at the mouth of the estuary. A detailed analysis of the diet was carried out on one mudflat (Sleek of Tarty) where it was found that variations in diet between day and night were relatively small, as were variations between age groups. Numerically, Cor. volutator was important for most of the year with Hydrobia ulvae secondarily important. In terms of calorific equivalents N. diversicolor was important in autumn and winter whilst Cor. volutator was most important in spring and summer.

Sampling of flounders throughout 24-h periods showed that their stomachs were filled during the high-tide period on the intertidal mudflats, irrespective of whether high tide occurred by day or night. Most fish had empty stomachs at low tide. Flounders remaining in the sublittoral river channel at high tide during the day had little food in their stomachs. From a combination of observations in aquaria and the interpretation of fin marks and feeding holes left on the intertidal mudflats it was found that flounders fed by arching the body above the substratum in order to obtain and angled bite of the sediment. The mouthful of mud and sand was sifted through the gill rakers to obtain the infauna, primarily Cor. volutator, the mud being expelled through both branchial apertures. This technique was used by day and night. Flounders also fed on organisms (including clupeids) in the water column where visual cues may have been important in detecting and securing prey.