The Feeding of Plaice and Sand-Eel Larvae in the Southern North Sea

Extract

Larvae of the sand-eel, Ammodytes marinus, appear in large numbers in the Southern Bight during March, some 2–4 weeks after the peak hatching period of plaice eggs. The sand-eel larvae outnumber those of the plaice about tenfold, but the factor may be 60 or more locally.

The principal food components in sand-eel guts in 1961 were copepod nauplii and appendicularians, while plaice were feeding almost exclusively on the latter. Larvae of both species show identical feeding patterns, with activity ceasing at night, and both aggregate during the day at depths of about 2–6 fm. Thus the sand-eel larvae are not separated from the plaice larvae by any pattern of behaviour, and are partially exploiting the same food reserve.

Consideration of prey size suggests that any direct competition from the larval sand-eels would first affect the smallest plaice larvae, which are in any case the most vulnerable to shortage of food. But any reduction in the crop of small appendicularians will later be reflected in the number of larger ones, and in this way older plaice larvae might be affected. While the food supply appears to have been adequate up to the time of the investigation, the reserve apparently represented only 3 days' rations. The significance of this observation is unfortunately obscured by our ignorance of appendicularian biology; but, should the food reserve become so diminished, competition between plaice and sand-eel larvae might be expected. The extent of its impact on survival is only likely to emerge from more comprehensive surveys.