Behavioural studies on the lesser sandeel Ammodytes marinus (Raitt) I. The effect of food availability on activity and the role of olfaction in food detection

The behaviour of the lesser sandeel, Ammodytes marinus (Raitt), has been investigated using a photographic method of recording activity. Three main states of activity were recognized: (1) swimming, (2) partially emerged from the sand, or (3) completely buried in the sand. When food was present there was a basic diurnal rhythm of activity; during the dark period most of the fish remained buried in the sand and in the light period they emerged to swim and feed. In the absence of food the level of swimming activity during the light period was extremely low, while the level of partial emergence activity increased. The fish appear to be visual feeders and those which were buried in the sand did not appear to be able to detect the presence of food in the water by means of olfaction. There was usually a low level of partial emergence activity during the dark period, which tended to increase in the few hours before the lights were switched on. The timing of this increase was thought to be related to an internal clock. The basic diurnal rhythm of activity shown in the laboratory experiments is discussed in relation to the marked diurnal variation in the catch of sandeels in the fishery. It is concluded that the failure to catch sandeels at night is due to their remaining buried in the sand.