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Optimized aquaculture product quality through better feed quality and feed management


The results obtained within the project show that the quality criteria usually applied to wild fish is adequate to farmed fish. Therefore the following criteria should be applied: Freshness criteria -Sensory evaluation of raw fish using the Quality Index Method (QIM). This scheme is based mainly on skin appearance (brightness), abdomen colour, shape/colour of eyes and colour/odour of gills. A maximum of 3 demerit points (d.p.) score each parameter: 0 represents high freshness and higher score indicated low fish quality. The scores of all characteristics are summed to give an overall score. Lower total d.p. corresponding to a higher freshness/quality of fish. In the case of sea bream the QIM scheme has a total of 20 d.p. and the following scale could be used to assess its freshness/quality: 0 - 6 d.p. : very fresh fish 7 – 12 d.p. : fish with good quality 12 – 16 d.p. : fish with acceptable quality Above 17 d.p. : fish in the borderline of acceptability -K1 value (based on the ATP products degradation) as chemical indicator. Values below 10% indicate high freshness Nutritional criteria Mainly based on the fatty acids composition of fish flesh: -High w3/w6 ratio -Good levels of EPA+DHA (minimum of 1g/100g muscle) The applicability of sensory evaluation Sensory evaluation of food is always affected by high variability since it consists in the subjective evaluation of the appearance and/or organoleptic properties of food. This aspect is very important in the case of acceptance/preference tests in which untrained people are used and the feeling of each person is important for the final result. Therefore, a high number of people should be used in this kind of tests. Contrary, when a trained panel is consulting low number of panellists could be used. In both cases sensory tests are a useful and important tool to determine the affective status of a food product and for quality changes evaluation. A small trained panel (three to 5 people) is generally used to assess the quality/freshness of fish and this kind of sensory test could be widely applicable in a fish farm since few farm workers could be trained in the sensory attributes, in particular the ones related with the appearance (form and colour of eyes, skin pigmentation, among others), which reflects well the freshness and quality of fish.
The finishing feeds produced for this project aimed at a total concept including nutritional effects on visual appearance (external colour through natural pigment sources), slaughter quality (reduction in visceral fat deposits through appropriate dietary fat level and composition), shelf life (through antioxidant nutrients like vitamin E and C), cooking quality, sensorial and nutritional qualities (through choice of raw materials and appropriate formulation. All the feeds produced have behaved well concerning stability and skinking properties in marine water. The analyses have followed the formulation specifications. Farms have use the finishing feeds for the last period of the grow-out phase and the results have shown to be more variable than in the laboratory, but even then, the colour and general welfare improvement of the fish in the ponds yielded good marketing results.
During the project a wide list of quality parameters have been evaluated, most of them related with the external appearance of the fish, since this is one of the most relevant quality indicator (skin appearance, abdomen colour, shape and colour of the eyes, colour and brightness of the skin....The fish were harvested, packed in ice and transported to the laboratory in order to perform all the measurements with sofisticated and appropiate laboratory equipment. The transport conditions were set to be standard for all the shipment, although not always possible, the fish deteriorate their aspect during transport (24-48 h) and in some cases, due to slaughtering methodology and/or transport misshandeling, the fish arrived in very poor conditions, sometimes not even suitable to go under QIM evaluation due to the external deterioration. The development of the standard, simple, robust equipment to be used in situ in the fish farm, just upon fish harvest, could give a much better, realistic and standardized method to measure fish skin colour and skin brightness, which are the main parameters to be evaluated by the consumer when the farmed fish in the market. In all the trials done during this project, the transport of the fish has been the main handicap for skin colour studies, playing a main role the distance of the fish farm from the analytical laboratory.
During the project several lab trials have been performed. In one of the last trials, two groups of sebream with different nutritional history and different growth rates (one group fed high energy diet, HE, and the other one a low energy diet,LE) were fed with the finishing diet. It was observed that the LE group had reached the same final average body weight as the HE group, with a higher specific growth rate and better food conversion. In the farm trials, fish fed finishing feed had equal or better growth than the fish fed farm feed; it has to be remarked that althouhg the amoung of crude protein in the feed was the same in both diets, the crude fat was 7% lower in the finishing diet.
In the laboratory trials performed during the project, it has been demostrated that a reduction of 30% in the daily feed ration did not yield a better growth. In the trial, two groups of fish were compared: one fed 100% of the feed ration, calculated on the bases of feeding tables for the species, water temperature and weight, and the other one fed 70% of the ration calculated in the same manner as described. After 12 weeks of feeding, the growth significantly decreased by restricting the feeding to 70% of the normal feeding ration. However, food conversion rate and protein efficiency ration were better in the case of reduced ration, indicating that this group of fish has used the feed much more efficiently. Thus, feeding tables advised by feed producers should be followed by the fish farmes with the specifications of the feed management applied to their farm.