This joint research proposal relies on an integrated investigation upon interindividual competition, including cannibalism in young-of-the-year fishes of cultured predatory species, using perch (Perca fluviatilis) and sea bass (Dicentrarchus labrax) as comparative freshwater and seawater models. The objectives are
i) to provide the biological bases allowing to minimise the heterogeneity of growth and subsequent potential impact of competition under culture conditions;
ii) to generate models aiming at optimising culture conditions for the early life stages of predatory fish with a comprehensive insight into the interrelationships between feeding physiology, behaviour and production dynamics (mortality-growth), and
iii) to test the model in commercial culture conditions.
State of progress
In general, all the tasks scheduled in the first year of the project have been carried out as expected. In perch, different independent variables belonging to the boxes population, feeding and environmental variables have been investigated or are currently under investigation. The effects of initial size heterogeneity on survival, growth, individual feed intake (by morphometric relationships), final size heterogeneity and cannibalism of larvae and post-larvae were investigated and fish samples were preserved for further physiological analyses (enzymatic activities). The same variable was investigated in juvenile perch after some methodological adaptations of the rearing, self-feeding and monitoring equipment and procedures. The effects of food composition (namely the supply of live Artemia nauplii) were investigated in perch post-larvae. Several experiments were conducted in larvae, post-larvae and juveniles in order to determine the effects of photoperiod on the physiological, behavioural and production variables previously described. Methodology of labelling newly--hatched larvae with alizarine was successfully tested and will be applied in 1998 in order to investigate the effects of hatching time on the dependent variables described above. In sea-bass, independent variables investigated in 1997 belonged to the population and feeding boxes. Effects of stocking density were studied in larvae and post-larvae, while effects of feeding level and feed reward were studied in juveniles. Dependent variables and procedures of feed intake measurement were similar to those reported for perch. In juveniles of both species analysis of individual feed intake by X-ray procedure and N-P retention and loss are still going on.
Due to the biological cycle of the species and time required for the detailed analysis of samples and results, data processing of most experiments is still on-going and conclusions can not be put forward at this stage. Moreover, variables investigated in perch and sea-bass in 1997 were not the same, due to facilities or technical constraints in the different partners. Hence, this comparison between these two predatory species cannot be done at this moment. Nevertheless, some interesting results are already available, allowing preliminary conclusions to be advanced in some cases. From the present results, interindividual competition and cannibalism appear highly dependent on the development stage of the fish and on the species. Cannibalism was observed very soon during the ontogeny of perch while this phenomenon was absent during the first weeks of sea bass larval rearing. In the latter species, mortality was mainly due to feeding adaptation to erogenous diet. At the post-larval stage, cannibalism occurred in both species but this phenomenon affected more deeply the global survival and structure of population in perch than in sea bass. Although growth heterogeneity (coefficient of weight variation) was higher in sea-bass than in perch, proportion of missing fish (as a cannibalism index) ranged between 1 and 5% in sea-bass post-larval experiment while it reached up to 20% in perch of the same stage. In both species, the population variables already investigated (initial size heterogeneity in perch and stocking density in sea-bass) did not induce marked effects on competition and cannibalism. Similar conclusions can be put forward on basis of results obtained from feeding and environmental variables, except for perch post-larvae reared at different photoperiods. At this stage, the hypothesis of a predominant role of endogenous factors in growth heterogeneity and competition between conspecifics could be proposed, but requires further analysis of data, mainly at the individual and physiological level (ingestion rate, digestive activity and nutrient utilisation, protein synthesis rate, etc.), and experiments. At the juvenile stage, mortality was very low during the experiments. Aggressivity and cannibalism appeared as a secondary cause of mortality in sea bass, but constituted the major factor of mortality in perch. Competition was mainly related to differences in feed intake and consequent growth heterogeneity among the fish population, although the relationships between individual feed intake and growth rate have not been demonstrated yet. The efficient use of self-feeder by perch and sea-bass juveniles constitutes an interesting result (particularly in perch since it was not demonstrated yet), which allows specific experiment of feeding behaviour to be designed. Most of experiments on juveniles have been completed a few weeks ago, or are still going on. Analysis of video recording and electronic monitoring of social interactions and feeding activity are in process. These results, combined to the individual measurement of feed intake and nutrient utilisation, should provide interesting and integrated information on the relationships between growth and interindividual competition in fish.
V. Future actions
The second year of the project will be devoted to the continuation of the experimental programme, with involvement of the different partners in the following experiments: population variables (effects of hatching time in perch and sea-bass, initial size heterogeneity in sea-bass, stocking density in perch and juvenile sea-bass), environmental variables (photoperiod in sea-bass and perch larvae, light intensity in perch and sea-bass) and feeding variables (feeding level and meal timing in perch). Moreover, physiological and behavioural analyses of experiments carried out in 1997 will be performed in 1998. A preliminary predictive model should be proposed in the second progress report, based on data obtained during the two first years of experiments.
Description of work
The research programme has been defined to study the relationships between some variables identified as having a major potential role in fish competition and cannibalism, and the main production variables usually recorded in Aquaculture (e.g. survival, growth, final size heterogeneity, feed efficiency). The recent advances in the intensive aquaculture of the species used as models have allowed the settings of experimental factors to be fixed precisely as population variables (i.e. hatching time, fish stocking density and size heterogeneity within the population), feeding variables (i.e. feeding levels and feeding rhythms, supplementation of live preys in weaned fish) and environmental variables (i.e. light intensity and photoperiod). The influence of these factors are assessed at two complementary levels (feeding physiology and behaviour) that are used for the understanding of typical production variables. Depending on life stages and species, the investigated physiological variables are the feed intake (calculated in larvae and post-larvae by morphometric relationships between fish body weight and corresponding head weight, and in juveniles by X-ray with balloting labelled feed), the gastric evacuation rate and feed assimilation, the proteolytic activities (trypsin, chymotrypsin and pepsin) in the digestive system and the nutrient (N and P) retention and loss. The feeding behaviour is studied by various and complementary approaches as the video monitoring and/or the use of transponders (passive integrated transponder tags) detected by immersed antennas placed inside the feeding area and connected to a data entry station. Investigated variables are the feeding rhythms, the social interactions and the space utilisation by fish in relation to their individual growth rate and feed intake. At the end of the project, some large scale experiments will be designed to evaluate to what extend the optimal rearing strategy identified from the results obtained during the experimental work will reduce the inter-individual competition in commercial scale facilities and conditions, and finally improve the net fish production.
Funding SchemeCSC - Cost-sharing contracts
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