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. Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208
1. Introduction
The success of an animal in obtaining food depends on its efficiency in locating resources under various environmental conditions Bell, 1990. The probability of
locating, encountering and capturing prey may vary through time and space, and thus lead to disproportionate use of time and space. Several studies have demonstrated diel
¨ ¨ rhythms in feeding activity in fish e.g. Thorpe, 1978; Fraser et al., 1993; Alanara and
¨ ¨
Brannas, 1997, and such rhythmicities have often been explained by behavioural adaptations to temporal variations in the probability of detecting and capturing prey
¨ Løkkeborg, 1998; Løkkeborg and Ferno, 1999. Diel variations in the utilization of
space have been observed in several field studies on fishes Hawkins et al., 1974;
˚ Hawkins et al., 1980; Bradbury et al., 1995; Engas et al., 1996. Disproportionate use of
space may be a reflection of temporal variations in activity or an adaptation to spatial variations in food availability. To understand the feeding behaviour of animals,
therefore, it may be essential to study spatio–temporal variations in activity patterns.
The sensory modalities involved in food search are an important aspect of feeding behaviour. Vision and chemoreception are vital senses in food search, but the senses by
which prey are most effectively located and captured may vary with time and space, as well as depending on prey type Atema, 1980; Løkkeborg, 1998. Their relative
importance is affected by environmental conditions such as light level, turbidity, current speed and turbulence, and by prey characteristics and behaviour. Chemoreception and
rheotaxis are probably the most important factors in locating food at some distance, due to the limited visibility in water Atema, 1980.
In this field experiment we studied several aspects of the foraging behaviour of ling Molva molva L., such as activity rhythm, use of space and the role of the senses. Ling
is a commercially important species Bjordal and Løkkeborg, 1996. It is solitary and benthic or semi-benthic and inhabits rocky and sandy substrates at depths of 60 to 1000
˚ m in the east and north-west Atlantic Pethon, 1989; Sunnana, 1992. Adult ling are
regarded as top predators Bergstad, 1991, and their behaviour can therefore be related mainly to feeding. Ling have a piscivorous diet supplemented by cephalopods,
crustaceans and echinoderms Rae and Shalton, 1982; Svetovidov, 1986. Since they are predators in relatively deep water where light is limited, it is reasonable to assume that
they use both chemoreception and vision for locating food. To the best of our knowledge the activity pattern and food search behaviour of ling have not been studied previously.
Behavioural observations in the field using underwater TV cameras have contributed to our understanding of the feeding behaviour of animals Wilson and Smith, 1984;
¨ Ferno et al., 1986; Zimmer-Faust et al., 1995. However, this method is not capable of
demonstrating the patterns of movement of marine animals. In this study we used a stationary positioning system with a fixed hydrophone array to study the movement
patterns of ling under natural conditions and in response to sources of food odours. Similar tracking experiments have been carried out to study the feeding behaviour of cod
¨ Gadus morhua L. Løkkeborg, 1998; Løkkeborg and Ferno, 1999. We discuss
similarities and differences in the food-search behaviour of ling and cod in relation to the behavioural adaptations of these gadoid species.
S . Løkkeborg et al. J. Exp. Mar. Biol. Ecol. 247 2000 195 –208
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2. Materials and methods
