Determination of bioavailability and bioactivity of enzymatically hydrolysed fish protein and phosphates in the African catfish Clarias gariepinus

[摘要] ENGLISH SUMMARY: There is an ever-increasing demand for high-quality aquaculture feed ingredients by this growing food-production sector. Wild fish stocks are dwindling, and aquaculture is a potentially sustainable method for producing the food required to feed an increasing world population. The global population is increasing at 1% per annum, and current projections indicate that food production must be increased to sustain the global population. Fish plays an important role in the food sector globally, and fish consumption increased by over 100% from 1990 to 2014. During fish processing operations, by-products (wastes) are produced, which can be utilised to produce high-quality aquaculture feed ingredients. In this way, valuable natural resources are utilised optimally and nutrients are recycled back into the food-production sector, thereby decreasing pressure on natural resources.The aim of this work was to conduct aquaculture feeding trials to evaluate two potential aquafeed ingredients derived from fish processing by-products. This evaluation indicated the suitability (bioactivity and bioavailability), performance and potential economic viability of the ingredients as aquaculture feed ingredients. The two feed ingredients were fish protein hydrolysates (FPH), and fish bone dicalcium phosphate (FB-DCP), both produced from monkfish (Lophius vomerinus) heads, which are by-products of the South African trawling industry.The study utilised a standard aquaculture approach for the evaluation of the feed ingredients, by having a control feed formulation (recipe) and different experimental diets to incorporate (include) the experimental feed ingredients. The evaluation compared the performance of the experimental diets and the control diet. The formulation included seven experimental diets and one control diet. FPH replaced a portion of the fish meal (FM) in the diet, and FB-DCP replaced conventional rock dicalcium phosphate (R-DCP) as dietary phosphate source. The FPH replaced a fraction of the total FM (5%, 10%, 15% and 20% of FM) in diets named FPH-1, FPH-2, FPH-3, FPH-4 and distinguished as Trial 1. Two diets replaced R-DCP with either FB (untreated) or FB-DCP, and a final diet employed a 1:1 ratio of R-DCP and FB-DCP diets – named FB-5, DCP-6 and DCP-7 respectively and distinguished as Trial 2. Each diet was evaluated using six replicate tanks of 75 l each, which were initially stocked with 30 fish of 1 g mean weight each, in a recirculating aquaculture system. A feeding trial of 10 weeks was performed and production performance parameters, such as growth, nutrient utilisation, non-specific immunity and production quality, were evaluated. In addition, a two-week digestibility trial was conducted on all FPH treatments to determine the influence of different levels of dietary FPH on the dry matter digestibility of the experimental diets.In Trial 1, there were indications that FPH inclusion enhanced a component of the non-specific immune function of the experimental animals significantly, and that this action was dependent on the inclusion level of the FPH. Significantly higher immunoglobulin levels were found in diets (10% and 15% of FM replacement). There were no significant differences in production parameters (weight, length and protein utilisation), mortality or dry matter apparent digestibility between any treatments in Trial 1. FPH inclusion at levels 5% and 20% of FM replacement did, however, affect overall body composition of the experimental animals, as there were, compared to the control diet, significantly increased lipid deposits in diets FPH-1 and FPH-4.In Trial 2 significant differences were measured for whole-body crude lipids when the phosphate diets were compared to the control, with increased lipids in diets FB-5, DCP-6 and DCP-7. In Trial 2 the production parameters, mortality, non-specific immunity and mineral composition of vertebrae illustrated no statistically significant diet-dependent differences.The control diet's results were utilised to investigate the length-weight relationship of the African catfish (Clarias gariepinus), and a Condition Factor = Weight/Lengthb relationship, b = 3.02 was found. This coincides with the b = 3 assumption during Trial 1 and Trial 2, as well as previous studies reported in open literature.From the results, it can be concluded that both FPH and FB-DCP can be considered as high-quality feed ingredients for aquaculture, as their inclusion had no negative impacts on animal production performance, immune status or final body composition. However, the FPH influence has inclusion level dependencies – the product is optimally effective at the correct dietary inclusion level of FPH. The results indicate that FPH and FB-DCP can be successfully included into aquaculture feeds for African catfish (Clarias gariepinus), thereby reducing waste of high-value aquaculture feed ingredient products and promoting sustainable aquaculture.

[发布机构] Stellenbosch University