thereby preventing a build-up of hunger levels and ensuring that fish are fed to satiation. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Atlantic halibut; Aggression; Feeding; Behaviour
1. Introduction
The production of Atlantic salmon in both Scotland and Norway has grown steadily over the last 20 years, and their aquaculture has become a major industry and source of
employment for many outlying communities. However, salmon is no longer considered a luxury food item, and increased tonnages and intense competition among producers
have reduced prices over the past 2 years. As a consequence, there has been consider- able interest in diversifying the range of species for cold water marine aquaculture.
Atlantic halibut is a high-value fish, capable of commanding up to four times the market price of salmon. Native to the Arctic and Atlantic oceans, halibut grows well year-round
in the ambient waters of Scotland, Norway and Canada. For these reasons, much effort has recently been made in the intensive rearing of this species.
A consistent finding by researchers and producers is that there is a significant Ž
. incidence of injury early in the culture process, i.e. from weaning 0.3 g weight .
Weaning is defined as the transitional period when a live prey of enriched artemia and copepods is replaced by an inert manufactured crumb diet. Fish may lose weight at this
time while they learn to accept a new diet, and nipping and physical damage to some
Ž .
individuals is first observed at this stage Greaves, unpub. data . Injuries are sustained to the eyes, tails, and pectoral fins. In some cases, cannibalism has been reported. Badly
bitten fins and tails can invite secondary bacterial infection, and eye injuries in such young fish are especially concerning as fish become stressed, feeding is impaired and
Ž .
mortality may occur. It is now estimated that 3–5 of halibut juveniles 150 g weight have eye injuries, and up to 30 have tail andror pectoral fin damage. The nature of
these injuries suggests that the cause is unlikely to be handling or tank contact, but interactions between fish, possibly of an aggressive nature. Aggressive interaction, i.e.
Ž behaviour that inflicts non-accidental injury on other animals Huntingford and Turner,
. 1987 , is common among fish. This is well documented and occurs in a variety of
Ž contexts, but mainly where fish are contesting limited resources, such as food Magnu-
. son, 1962; McCarthy et al., 1993 . A common finding of aggression studies in farmed
fish is that individuals vary widely in the extent to which they use aggression to acquire Ž
. resources Shelbourne, 1964 . Marked aggression among juveniles has been documented
Ž .
Ž with halibut Ottesen and Strand, 1996 , and for other flatfish species: plaice Shelbourne,
. Ž
. Ž
. 1964 , greenback flounder Carter et al., 1996 and sole Howell, pers. comm. 1997 .
The level and intensity of aggression in any given species depends on a balance between the advantageous and disadvantageous consequences of this behaviour for the
Ž .
individuals concerned Krebs and Davies, 1987 . Aggression can be minimised in farmed fish by increasing the costs or reducing the benefits associated with this
Ž .
behaviour Christiansen and Jobling, 1990; Grand and Grant, 1994 . However, before adopting this approach, one must understand what fish are fighting over and why
aggression arises.
Few data are available on the activity or distribution of wild halibut, although it is Ž
known that individuals spend up to 4 years in coastal nursery grounds Haug, 1990; .
Trumble et al., 1993 . They are essentially solitary fish, and conditions prevalent in hatcheries and on-growing facilities are in stark contrast to their natural environment.
High stocking densities in tanks may increase stress and induce aggressive interactions. On the other hand, if fish are held at artificially high densities, injuries may be the result
of accidental collisions during feeding rather than of targeted aggression. Given that these interactions are not mutually exclusive, the purpose of this study was to examine
halibut behaviour in culture conditions and to identify any behavioural interactions that could cause the kinds of injuries seen in farmed fish. The specific aims were: to examine
social interactions in farmed halibut to see whether potentially injurious aggression occurs in the different size classes in which injuries have been reported; to understand
the context in which such behaviour happens; and to investigate individual variation in the level of aggressive behaviour.
2. Materials and methods
