Gilthead seabream (Sparus aurata) is the most important finfish species cultured in the Mediterranean region. Vegetable protein and oil sources are valid ingredients in seabream feeds. But recently, the use of processed animal by-products has been object of renewed interest. By June 2013, the European Commission will adopt a new regulation allowing nonruminant processed animal proteins (PAPs) exclusively for use for fish feeding and feed companies will be able to use a much wider variety of products from pork and poultry for fish feed, as long as they meet the category 3 criteria (safe for human consumption.)
In this general context, a performance trial was undertaken to evaluate the effects of replacing high levels of fishmeal and fish oil in a practical diet for grow-out gilthead seabream with a mixture of non-ruminant processed animal proteins (PAPs) and poultry fat, in terms of growth, feed utilization and apparent digestibility of nutrients.
Material and methods
Two isoproteic (crude protein, 44% DM), isolipidic (17% DM) and isoenergetic (23 kJ/g DM) experimental diets were formulated to fulfill the nutritional requirements of gilthead seabream. In the control diet (CTRL), a commercial seabream formulation, the total marine-derived ingredients accounting as protein sources represented 30% of the formula, while fish oil was used as the main fat source. The other experimental diet (PAP BREAM) was formulated in order to replace 60% of marine-derived proteins by alternative non-ruminant processed animal protein sources (hemoglobin powder and hydrolyzed feather meal) and 60% of fish oil by poultry fat. All other ingredients were kept relatively constant. Both feeds were manufactured under industrial extrusion conditions.
Six homogenous groups of 35 seabream each, with a mean initial body weight of 101.7 ± 17.5 g were stocked in circular plastic tanks (volume: 1000 L; water-flow rate: 5.5 L·min-1), supplied with flow-through seawater (temperature: 25 ± 4°C; salinity: 33-34 g·L-1, dissolved oxygen above 5 mg·L-1). Each dietary treatment was tested in triplicate tanks over 84 days.
Fish were fed to apparent satiety, by hand, three times a day. Feed intake was recorded and utmost care was taken to avoid feed losses. Fish were individually weighed at the beginning, bulk weighed every three weeks and at the end of the trial, following one day of feed deprivation. Fish from the initial stock and from each tank at the end of the trial, were sampled, and stored at -20ºC until subsequent analysis of whole-body composition and somatic indexes. Separately, but with fish of a similar size range (mean body weight: 60 g), the apparent digestibility coefficients (ADC) of the dietary components, was determined by the indirect method using 1% chromic oxide as inert tracer. Faecal samples were collected in a solids settling decantation system over 2 weeks and frozen at -20°C. Pooled faeces from each group of fish were freeze-dried prior to analysis.
Results and Discussion
After 84 days of experimental feeding ad in comparison to a commercial seabream feed (CTRL), overall performance of seabream, expressed as daily growth index (DGI), feed efficiency (FE) and feed intake (FI) has not been significantly affected (P>0.05) by the simultaneous replacement of 60% fishmeal by a mixture of non-ruminant processed animal proteins (PAPs) and 60% fish oil by poultry fat. Fish reached a final body weight of 250 g and DGI for the CTRL treatment was 1.95 ± 0.09.
Feed efficiency ranged from 0.69 in the CTRL diet and 0.64 in the PAP BREAM diet. Whole-body composition of fish was not affected by dietary treatments. No statistical differences were found in the apparent digestibility coefficients (ADC) of dry matter, organic matter, protein, fat and energy. Protein and phosphorus retention (% of intake) was little affected by dietary treatments, while retention of lipids and energy was significantly reduced in fish fed the PAP BREAM diet.
As it could be expected, fillets of seabream fed with the PAP BREAM diet showed a significant reduction of the n-3/n-6 fatty acids ratio. The EPA and DHA represented 22.6 % of total fatty acids in fillets of fish fed the CTRL diet, while 14.3% in those fed the PAP BREAM diet. Sensorial testing by trained panellist found no differences (P>0.05) on criteria such as typical odour, typical white colour, texture and typical taste.
Conclusions
The overall growth performance of gilthead seabream during the fattening phase can be sustained by a practical dietary formulation containing non-ruminant processed animal proteins (PAPs) and poultry fat. Despite without detrimental effects on the main sensorial properties, data clearly show that fish fed poultry fat require an effective finishing strategy to restore n-3 HUFA levels in the fillets and hence guarantee a high nutritional value of gilthead seabream.
October 2013
