INFLUENCE of DENSITY ON THE WELFARE OF RAIMBOW TROUT (ONCORHYNCHUS MYKISS) IN ORGANIC AQUACULTURE
Introduction
In 2009 the European Commission approved the Regulation n° 710/2009 on organic aquaculture. Healthy, fresh and local alternative are the key traits of the near future aquaculture development (EU initiative http://ec.europa.eu/fisheries/inseparable/) and the issue of farmed fish welfare is pivotal for both a sustainable aquaculture and compliance with the organic principles. Welfare conditions can support increased immune competence, increased ability to react to stress conditions, best performance in terms of growth, and flesh quality in farmed fish (Lembo et al., 2011). In the European Regulation, stocking density is considered an appropriate fish welfare indicator. This is viewed in a holistic perspective including also aspects and indicators related to good environmental conditions, water and feeding quality, level of domestication, etc. It is still quite controversial what is the threshold limit of density to preserve the actual conditions of welfare on farms.
The purpose of this study was to assess the welfare conditions of rainbow trout (Oncorhynchus mykiss) reared at two stocking densities and fed with organic food.
Material and methods
Two rearing densities (L=12kg m-3 and H=17kg m-3) in triplicate (6 concrete tank of 3m3) were tested., 65 trout (average weight: 401.1 ± 9.3g) and 95 trouts (average weight:374.4 g ± 9.02g) were respectively reared in each tank at the lower and higher densities. The trial lasted 4 months. Fish were fed 1% of the whole biomass using organic commercial feed (Emerald Fingerling & Trout SkrettingTM). The other environmental parameters (temperature, photoperiod, oxygen concentration and water renewal) were maintained constant. Two samplings (10 fish per tank) for hematological and serological parameters (hematocrit, erythrocytes concentration, cortisol, glucose, lactate, lysozyme) and morphometric measures (total length; total, liver and gutted weight) were taken at beginning and at the end of the trial. At the end of the trial, samples of liver and intestine were taken from 6 fish from each tank and fixed in buffered formalin (5%). Moreover to remotely monitor the fish red muscle activity, 6 specimens per tank were surgically implanted with accelerometer V9A (VEMCOTM, Canada) acoustic transmitters at the beginning of the trial. The acceleration signal from the fish is measured as a vector quantity that is a result of measuring acceleration on 3 axes. This acceleration value can be used as a measure of muscle activity of a free ranging animal. The muscle activity was also correlated with the rate of oxygen consumption by critical swimming speed test (Ucrit) in a Blažka style swimming chamber (Carbonara et al., 2014). During these tests fish were induced to swim at increasing speed steps (0.1m s-1 every 10 min) until fatigue (Ucrit) (Lembo et al. 2007). In this way, the monitoring of acceleration gives an estimation of the relative cost of living for fish in an environment. A GLM model was also applied to estimate the swimming behavior using the hours of day and the experimental days as explanatory variables.
Results
The hematological and serological data for each density are reported in table 1. The symbols denote the statistical differences (Kruskall-Wallis test; p<0.05). From GLM it is evident that most of the swimming activity of trout is recorded during daylight hours, and in particular, the activity levels recorded in the experimental group kept at high density are more elevated than those of fish reared at low density.
The average levels of activity of the trout in the two experimental densities were respectively 56.62 ± 0.083 and 65.30 ± 0.098, respectively for the low and high density. These results are significantly different in the two densities (Kruskal-Wallis test, p<0.05) and are related to oxygen consumption levels equal to 334.3 and 392.2mgO2 kg-1 h-1 respectively. Such levels of energy demand are below the critical threshold limit, identified with a value of the Scope for Activity (SFA) equal to 549.032mgO2 kg-1 h-1. In the table 2 the performance parameters and the somatic indices are reported.
The gut histological analysis didn't reveal significant differences between the two density groups. These results confirm that the two stocking densities here tested did not affected the enterocyte functionality, responsible for the absorption of nutrients, thus confirming the data related to the growth parameters recorded during the trial.
Discussion and conclusion
Significant differences were found between the experimental groups in terms of oxygen demand and muscle activity, although the used high density did not generate a critical condition for the welfare and survival. It is, indeed, granted a wide margin of energy for the rest of the vital functions: reproduction, growth, defense against various types of stress. The analysis of hematological and serological parameters, together with the gut histology showed how the increase in density, stimulated in the two groups a full adaptation response without the deterioration of well-being condition (Skov et al., 2011)
References
Carbonara, P., Scolamacchia, M., Spedicato, M. T., Zupa, W., McKinley, R. S., Lembo, G., 2014. Muscle activity as a key indicator of welfare in farmed European sea bass (Dicentrarchus labrax, L. 1758). Aquaculture Research, DOI: 10.1111/are.12369.
Lembo, G., Carbonara, P., Spedicato, M.T., Scolamacchia, M. & McKinley, R.S. 2007. Use of muscle activity indices as a relative measure of well-being in cultured sea bass Dicentrarchus labrax (Linnaeus, 1758). Hydrobiologia, 582: 271-280.
Lembo, G., Zupa, W., 2011. Il benessere dei pesci in allevamento. Misurare la qualità in Acquicoltura. Un approccio scientifico a servizio delle aziende e dei consumatori. I Georgofili, Quaderni 2010-IV: 51-66.
Skov, P. V., Larsen, B. K., Frisk, M. et al. (2011). Effects of rearing density and water current on the respiratory physiology and haematology in rainbow trout, Oncorhynchus mykiss at high temperature. Aquaculture 319, 446-452.
