| Abstract |
The aim of the present study was the investigation upon interindividual competition, with emphasis on cannibalism, in the early developmental stages of sea bass (Dicentrarchus labrax). Three experiments have been carried out with the aim to determine the effects of stocking densities and size heterogeneity on survival, growth and behaviour of sea bass larvae and post-larvae under controlled culture conditions. Experimental populations were reared in cylindroconical tanks of 50-l in a closed water system for the larvae and in a semi-closed system for the post larvae. Larvae were fed with live Brachionus plicatilis replaced successively by Artemia. Post-larvae were fed with live Artemia replaced successively by an adequate industrial dry food. Total length and weight was measured weekly on 30 individuals/tank. Dead fish were counted daily, classified into cannibalised and non-cannibalised fish and photos were taken. Cannibalism was estimated by the number of cannibalised dead fish and the number of missing fish at the end of the experiment. All experimental conditions were tested in triplicate. Results indicated that stocking density (50, 100, 150, 200 larvae/l) did not effect survival and larval growth performance. Most deaths were recorded before d10 and mainly related to problems of exogenous feed ingestion and lipid drop absorption. At this developmental stage no cannibalism was observed. In contrast, initial stocking density and size heterogeneity effect the survival and the extent of cannibalism of reared post-larvae. Survival was significantly higher in the lower densities (5 and 10 post-larvae./l) than in the higher densities (15 and 20 post-larvae./l) than in the higher densities. Also, the percentage of missing fish and thus cannibalism was significantly higher in the highest density. Three different size heterogeneity of post-larvae were tested : 1. Natural heterogeneity, 2. Minimum heterogeneity, and 3. Maintenance of the homogeneity. Survival was significantly higher in the populations with the modified homogeneity compared to all others. A positive correlation between coefficient of variation of weight and cannibalism was observed. In post-larvae experiments, cannibalism was detected on all dead fish in the form of heavy damage to dorsal, anal and mainly to the caudal fin and was observed on all occasions even after food was given. Cannibals attacked their prey from the back catching it by the caudal fin, swallowing it slowly or releasing it.
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Μελέτη του κανιβαλισμού των νεαρών σταδίων του λαβρακίου Dicentrarchus labrax(L.1578) σε συνθήκες εκτροφής. Επίδραση της πυκνότητας του πληθυσμού και της ετερογένειας του μεγέθους
