J. Appl. Ichthyol. 11 (1995), 25-36 zyxwvuts 0 1995 Blackwell Wissenschafts-Verlag, Berlin ISSN0175-8659 zyxwvutsrq Received: June 6, 1994 Accepted: February 9, 199fi zy Observations on agonistic behaviour of CZarias zyx gariepinus larvae and juveniles under different densities and feeding frequencies in a controlled environment By H. KAISER, 0. WEYL and T. HECHT Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown, South Africa 6140 Summary Ten-day-old African catfish larvae, Clarias gariepinus, were stocked in aquaria at 5, 13, 22, and 30 larvae per liter with fish at each density being fed three and six times per da over 35 days. Fish were fed with an artificial diet in excess of their satiation requirements at each geding. Every week total length was determined for samples of 25 fish per tank and all fish were counted at weekly intervals. Behaviour patterns were categorized into timed behaviour (i.e. resting, swimming and browsing), and fre uency events e.g. number of contacts, aggressive contacts and number of head or body attacks. Fish at 8e lowest density grew faster than fish at the other densities. Feeding frequency had no effect on growth. With increasing densities fish less frequently changed timed behaviour patterns while spending a greater percentage of the time swimming. Aggressive behaviour was hi hest at the lowest density but remained uninfluenced at densities greater than five fish per litre. The hi$ feeding fre uency provoked a higher rate of aggressive contacts than the low feeding frequency. A suggested expyanation was that these fish were more often in anticipation of food and consequently more active between feedings. In summary, it was shown that density had an effect on duration behaviour patterns such as resting and swimming, while feeding frequency significantly influenced frequency events, in particular the rate of aggressive behaviour. The study contributes to an understanding of factors influencing zyx C. guriepinus juvenile behaviour and suggestions for follow-up experiments are discussed. Introduction Intraspecific aggression and cannibalism has been reported for more than 36 families and 106 species of teleosts (SMITH and KEAY 1991) and poses a considerable problem to the profitable culture of many aquaculture species, particularly during the early juvenile stages (HECHT and PIENAAR 1993). In addition to the immediate and direct loss of fish by way of cannibalism, aggressive behaviour results in fin damage and skin lesions. This in turn increases their susceptibility to disease which weakens the fish, making them more prone to cannibalism or death as a consequence of their wounds (PIENAAR 1990). While the majority of studies in the past have dealt with the quantification of the rate of cannibalism in feral populations (see reviews by SMITH and REAY (1991), FITZGERALD and WHORISKEY (1992), and SARGENT (1992)), only a few studies report on agonistic behaviour of fish under culture conditions (KNIGHT 1987; HECHT and APPELBAUM 1988; VAN DAMME et al. 1989; PIENAAR 1990; BRITZ and PIENAAR 1992; KERDCHUEN and LEGENDRE 1992). Moreover, cannibalism-induced mortality has mostly been quantified as the final result of agonistic intraspecific aggression with little insight into the actual behaviour of the fish. Several factors have been identified which influence the rate of coeval sibling cannibalism and agonistic behaviour in Clarias gariepinus larvae and juveniles. Amongst others, density, food availability and shelter have been tested as environmental parameters influencing the behaviour and the rate of cannibalism in larval and juvenile African catfish (HECHT and APPELBAUM 1987, 1988; PIENAAR 1990). Shelter was least important, while the rate of U. S. Copyright Clearance Center Code Statement: 0175-8659/95/1102-0025 $1 1.00/0