Introduction
The European Seabass, Dicentrarchus labrax, is the second most important marine finfish species produced in the Mediterranean area with and annual fry production of over 500million . The majority of the fry are produced using the Green Water Technique (algae, rotifers and Artemia), but around 20% is produced without algae and rotifers, keeping the larvae in the dark until around 9dph and starting first exogenous feeding with Artemia instead of rotifers. This technique is normally resulting in a slower growth of the larvae and lower robustness compared to the green water technique .
In this study, new feeding protocols were developed to improve the performance of Seabass during larval rearing without the use of rotifers , introducing a rotifer substitution diet from the time of mouth opening. For the classic green water technique, a variant was studied introducing the new diet at 5dph and reducing the normal rotifer quantities with around 50-60%.
Materials and methods
Seabass experiment
Nearly hatched S eabass larvae, originating from the same pool of eggs, were stocked at the a density of around 80 larvae.l-1 in 1 ,000l larval rearing tanks. Four different feeding protocols were used:
In all treatments, Artemia has been fed from 10dph onwards. A photoperiod of 14L/10D was used and the treatments were done in triplicate.
During the larval rearing period , weekly biometrics were carried out to compare growth rates . A salinity stress test was done at the end of the trial to determine the stress resistance. At 56 dph, larval survival , produced biomass per tank and deformity levels were evaluated .
Results
The growth of the Seabass larvae during the period of 56days showed a superior growth up to 14dph for the Full Live Food Control. Later , the growth accelerated in the treatments where the rotifer substitution diet was used. A t 56dph, the treatment with 50% r otifer substitution had the lowest weight , even if the difference with the other treatments was not significant.
Average s urvival rate wa s highest for treatment 4, without rotifers, using the rotifer substitution diet and green water (65%). The lowest survival rate was obtained in the clear water treatment (45%). The survival rate of the improved LFC was slightly higher than the standard LFC (55 versus 52% ). Combining the data of survival and average weights, tank biomasses were compared and lowest values were obtained for the Clear water technique . The highest biomass was obtained in the Green water technique without rotifers , using the new diet at the start of exogeneous feeding. Tank biomasses for the other two treatments showed intermediate and similar values.
Regarding the stress resistance, treatments without rotifers or with low amounts of rotifers, but adding the dry diet from the early beginning , were performing like the LFC .
Deformity levels showed a higher percentage of head deformities in the Clear water treatment , where no algae were used. Th e treatment showing the highest percentage of fish without any deformity was the improved Live Food Control, using the new diet fed from mouth opening. In the treatments wh ere no rotifers were fed, the green water treatment showed significant less deformities compared to the clear water treatment.
Conclusions
This study shows that Seabass larvae can be reared obtaining a high survival rate and good fry quality when the rotifer substitution diet is used from the first days of exogeneous feeding. The treatments where no rotifers were used, but in which high- quality marine algae were added during the larval rearing water until 11days post hatch, resulted in the best treatment. The treatment with 50% of rotifer substitution perfo rmed less in terms of growth compared to the standard LFC, but the quality of the produced fry improved in terms of deformities . The lower growth could be explained by the fact that the Seabass larvae preferred rot ifers above the rotifer substitution diet when administered in the same period. As such, a lower amount of dry feed was ingested, resulting in a suboptimal growth.
The use of high-quality marine algae is fundamental to obtain optimal growth and quality in the larval rearing of Seabass. Even in the absence of rotifers, but using algae and the rotifer substitution diet, results are excellent. This allows the simplification of larval rearing protocols for Seabass without compromising the growth, neither the quality of the produced fry.