Improved Rapeseed as Fish Feed in Aquaculture

Due to a growing aquaculture sector, stagnating prices for fishmeal and an increasing prioritization of a sustainable global food supply, needs for alternative protein sources for fish nutrition have never been more urgent. While protein products from rapeseed are considered to be valuable candidates for replacing fishmeal in diets for different species of fish, nutritive value of rapeseed protein products appears to be inferior to fishmeal due to the present of anti-nutritive factors. The present study investigated the potential of new purification methods to improve the effectiveness of rapeseed protein products as alternative protein sources for fishmeal in diets for carnivorous fish. In the first experiment, effects from supplemented glucosinolates and phytic acid were tested on growth performance and health status of juvenile rainbow trout. Triplicate groups of fish were fed eight identical basal diets which were supplemented with graded levels of either glucosinolates or phytic acid. A ninth basal diet served as control. After 38 days of feeding, results showed that up to 1.7 µmol/g of glucosinolates do not impair growth performance or health status of rainbow trout significantly. A significantly affected growth performance was observed after 56 days of feeding with 3% dietary phytic acid inclusion. The potential of a highly purified rapeseed protein isolate as an alternative protein source for fishmeal was tested in the second experiment. Nutrient digestibility of the protein was determined. Triplicate groups of juvenile rainbow trout were fed isonitrogenous and isoenergetic diets with increasing dietary levels of rapeseed protein isolate. Crude protein digestibility of the rapeseed protein was exceptionally high. Feeding rainbow trout to apparent satiation twice per day for 56 days showed that 66% of fishmeal can be replaced with the rapeseed protein isolate without negatively affecting growth performance or health status. When fishmeal was replaced by rapeseed protein isolate to 100%, DFI was significantly decreased and overall growth performance was subsequently impaired. In the third and last experiment, a different rapeseed protein isolate was tested as an alternative protein source for fishmeal in diets for rainbow trout. Nutrient digestibility of the protein was determined. Rainbow trout were fed seven isonitrogenous and isoenergetic diets in triplicate groups. In three of those diets, fish meal was exchanged to 0, 66 and 100% of digestible protein with rapeseed protein isolate. Copies of those diets were coated with stearic acid and fish oil. A portion of rapeseed protein isolate in the 100% exchange diet was replaced by a mixture of amino acids. The highest crude protein digestibility of a rapeseed protein product was measured. After 56 days of feeding both exchange levels of fish meal reduced feed intake as well as feed conversion significantly compared with control in both coated and non-coated diets. Supplementation with free amino acids could not increase feed intake significantly but increased protein utilization.