Aquaculture, 89 (1990) 225- 232 Elsevier Science Publishers B.V., Amsterdam 225 The effect of fish density and feeding regimes on individual growth rate and mortality in rainbow trout zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC (Oncorhynchus mykiss) zyxwvutsrqponmlkjihgfe Jens Chr. Helm”, Terje Refstieb and Sigbjorn B&* aAustevoll Aquaculture Research Station, Institute ofMarine Research, N-5392 Storebe (Norway) bThe Agricultural Research Council ofNorway, Institute ofAquaculture Research, N- 6600 Sunndalsera (Norw ay ) ‘Department of Fisheries Biology, University of Bergen, P. 0. Box 1839 Nordnes, N-5024 Bergen (Norway) (Accepted 3 January 1990) ABSTRACT Holm, J.C., Refstie, T. and Bra, S., 1990. The effect of fish density and feeding regimes on individual growth rate and mortality in rainbow trout (Oncorhynchus mykiss). Aquaculture, 89: 225-232. Feeding frequency and fish density were varied in a 32 matrix rearing experiment with rainbow trout of initial size between 130 and 250 mm fork length. All fish groups were given similar daily rations. Initial densities were 107-219 kg mm3 and final densities 240-450 kg rne3. The parameters of daily fork length increase, specific growth rate and mortality were recorded. Mean individual growth rate, irrespective of feeding regime, was highest in the lowest density and lowest in the highest density. Growth rate increased with increased feed availability. High feed availability (high frequency of feed- ing) was especially important when densities were extremely high. INTRODUCTION Competition for food (e.g. Magnusson, 1962) is an important limiting fac- tor for growth of young fish. The competitive ability of an individual often determines its rank in a dominance hierarchy, with high ranking individuals having preferential access to food resources (Morse, 1980). Dominant indi- viduals maintain higher growth rates than subordinates (Yamagishi, 1962; Fenderson et al., 1968; Li and Brocksen, 1977; Fausch, 1984). Competition and aggressive behaviour increase under situations of food shortage (M agnusson, 1962; Symons, 1968,197 1). Earlier experiments have shown that growth rate and behaviour in salmo- *Present address: Skretting a.s., P.O. Box 3 19, N-400 1 Stavanger (Norway) 0044-8486/90/$03.50 0 1990 - Elsevier Science Publishers B.V.