Effect of temperature, stocking density, feeding conditions and experimental day on the horizontal and vertical distribution of sea bass fry Dicentrarchus labrax (Linnaeus, 1758)

The working hypothesis of the present study is to verify whether, the spatial distribution of sea bass fry remains invariable under the pressure of certain rearing conditions, due to its extended plasticity or, certain situations lead to specific distributions. For this reason, we investigated and compared horizontal and vertical distribution of sea bass reared in aquaria, at three different stocking densities (5 ind. l(-1), 10 ind. l(-1) and 20 ind. l(-1)), two different temperatures (16 and 23 degrees C) and two different feeding conditions (feeding fish and starving fish). The analysis of the video records and General Linear Models revealed a strong effect of stocking density, feeding condition, and day of experimentation on fish behavior interpreted by the vertical and horizontal distribution of fish. Temperature also affected distribution when it was combined with stocking density and feeding condition. However, this effect seemed to be lessened by the effect of the other variables (i.e. stocking density, feeding condition and day of experimentation). The exposure time (chronic stress) to the stocking density, temperature and feeding condition as well as to their additive cumulative effect (variables interactions) was the cause of the development of stereotypic behaviors that increased in occurrence frequency and over an experimental time lapse and gradually led to the weakening and death of the fry (bad welfare). Thus, it is assumed that stocking density, feeding condition, day of experimentation and temperature can have negative effects on sea bass "well being" and in order to maintain its welfare during rearing it is important to take all necessary precautions of handling when dealing with these variables in the fish environment.