J. Ecobiol. 6(1) 001-004 (1994) Pa1ani Paramount Publications-Printed in India COMPETITION EFFECT ON THE SURVIVAL OF SEABASS FRY WITHOUT FUNCTIONAL SWIM BLADDER M L BIANCHINI*, S CASADEI#,E INGLE" AND F LOMBARDI# * PF RAISA, NATIONAL RESEARCH COUNCIL, VIA TIBURTINA 770, 00159 ROMA, ITALY # COlPA SRL VIALE G. MAZZINI 55, 00195 ROMA ,ITALY "ICRAM, VIA L RESPIGHI 5, 00197 ROMA, ITALY (Received 10.7.93; revised 31.8.93; accepted 27.9.93) An experiment has been carried out to determine the effects of relative density on survival of seabass, Dicentrarchus labrax L. without deve10ped swim bladder. Two groupS of tanks were stoeked at the same density with mixed quality fry, using a proportion ofhealthy anima1s around 5% or 50% ofthe total population. Nonna1 fry outperfonned the another fish but, whi1e the overal1 survival was the same in each tank, the rate of survival ofunbladdered animals was more than double in the tanks where the nonnal animals were scarce. These results suggest that while the survival of norma! fry might be an intrinsic phenomenon, for the anomalous juveni1es the survival rate depends on the interactions with healthy competitors. One of the major problems when dealing with larval rearing of seabass, Dicentrarchus labrax is the correct development of the swim bladder (Giavenni & Doimi 1983 ; Chatain 1986). In fact, the functional development of the swim bladder requires that the fry gulps down an initial air microbubble (Foscarini 1988 ; Pousao Ferreira & Soares 1991). Only recently some of the agents that could negatively intluence that step have been identified: excessive water turbulence (Re et al 1985) ;too much light intensity (Ronzani Cerqueria & Chatain 1991; Weppe & Jossard 1986); presence of oily film over the water surface (Chapman et al 1988) ,etc. In mariculture farrns, it is essential to know the survival difference between "normal" fry and fry without intlated swim bladder ;trom the productive point of view, the latter are considered oflower quality or even OfbUll value as they grow slower, often present other morphological anomalities, show an incorrect hydrostatic trim (Chatain 1987) . In this paper , the survival trom 55 to 100 days ofage is compared for seabass fry with and without functional swim bladder in two different crowding situations.