Abstract:
Substitution of fish oil with economical and environmental friendly terrestrial
alternatives in aquafeed has drawn significant attention on a global scale.
Nevertheless, alternative oils rich in C18 PUFA are known to modulate fatty acid
metabolism in fish. The present study evaluated the effects of C18 polyunsaturated
fatty acids (ALA+LA) on fatty acid metabolism and final fillet long chain PUFA
concentrations in cold water rainbow trout and warm water Murray cod. Two
separate sets of six isoproteic and isolipidic experimental diets using three different
dietary lipid sources (sunflower oil, linseed oil, and beef tallow) were formulated
with varying concentrations of total C18 PUFA (Murray cod: 7.3, 18.8, 29.8, 41.7,
51.7, 63.8 w/w%; rainbow trout: 9.1, 15.4, 29.0, 41.3, 54.0, 66.2 w/w%)
maintaining a constant ratio (1:1) of ALA/LA, whereas fish oil was used for the
control diet for each feeding trial for Murray cod and rainbow trout separately. Fish
were fed twice daily at 8.00 a.m. and 4.00 p.m. to apparent satiation for a period of
133 days (M cod) and 91 days (trout). In Murray cod, fatty acid metabolism
estimations demonstrated an increase in delta-6 desaturase activity acting on ALA
over LA as the substrate availability increased, while the efficiency of delta-6
activity in rainbow trout was negatively affected by an increasing C18 PUFA
content. However, total desaturase activity was directly proportional to total C18
PUFA content in rainbow trout. With the reduction of C18 PUFA, a shift in
substrate preference of delta-6 activity was noted in Murray cod. Delta-6 activity on
ALA was higher across all concentrations of C18 PUFA in rainbow trout. An
increasing trend of delta-5 desaturase activity was noted in rainbow trout fed
elevated C18 PUFA while no delta-5 activity was observed in Murray cod. Murray
cod exhibited maximal delta-6 enzyme activity at an average C18 PUFA level,
denoting that excessive C18 PUFA concentrations are counterproductive. This
information provides a valuable insight into the formulation of eco-friendly,
sustainable fish oil free aquafeed for different species.
