Do stocking density and feed reward level affect growth and feeding of self-fed juvenile European sea bass?

Abstract

Survival, growth, nitrogen (N) and phosphorus (P) budgets and feeding behaviour were studied in groups of sea bass, Dicentrarchus labrax L., that fed using self-feeders with different feed reward and maintained at different densities (up to 35 kg m −3 or 500 fish tank −1). The experiment was conducted in 500-l indoor tanks under ambient conditions of light and temperature during two successive years. In the first year (experiment I), fish (5.5±0.1 g) were separated into five groups according to stocking density (50, 200 or 500 fish tank −1) and reward ( R=0.6 g trigger −1) level: 50: R, 200: R, 200:4 R, 500: R and 500:10 R. Both increases in stocking density and in feed reward affected negatively all the studied parameters concerning growth and feed efficiency (FE). In the second year (experiment II), the experiment was repeated with fish of similar weight (5.3±0.0 g), which were held under the following four feeding protocols: 50:0.5 R, 50: R, 500:0.5 R and 500: R. The lowest feed reward fish groups presented better tendencies in growth and feed efficiency independent of stocking density. Survival was high in all feeding regimes in both experiments, but at high density (500 fish tank −1), some mortalities (<4%) were observed. Also in both experiments, feeding rhythm was affected by stocking density, but not by reward levels. Thus, although fish presented a typical diurnal activity, they restricted their feed demands at the first and late hours of daylight (08:00–09:00 and 19:00 h) when held in the lowest stocking density, while in high density they did not. It is concluded that a stocking density of up to 35 kg m −3 (or 500 fish tank −1) does not affect self-fed sea bass kept in tanks, provided that feed reward is set to a low value (0.3 g).