Food-search strategy in ling (Molva molva L.): crepuscular activity and use of space

The activity rhythm and use of space in the food-searching behaviour of ling (Molva molva L.) were studied by means of a stationary positioning system. The system consisted of a fixed array of three hydrophones, which monitored the position of each individual being tracked once every three minutes. Five ling were tagged in situ by allowing them to ingest a transmitter wrapped in bait and were tracked continuously for 6 to 11 days. Ling showed a crepuscular activity rhythm with higher levels of swimming activity at dawn and dusk. The fish occupied a home range throughout the study, and during inactive periods (about 65% of the time) they remained within a small core area. Most of the time spent outside the core area was during the period of high activity at dawn. Food-search behaviour was studied by setting mackerel-baited fishing gear in the experimental area. When baits were present, ling moved more slowly and within a more limited area, indicating area-restricted searching in the presence of a food odour. Ling responded to and located baits in both their active and inactive periods during the day, but were not observed to react at night. Comparing these findings with those from similar tracking experiments on cod (Gadus morhua L.) suggested that ling are less active at night and show a more restricted use of space by occupying a core area. This may reflect different feeding strategies between these gadoid species with ling taking a higher proportion of mobile prey and being more dependent on visual stimuli.