Lipid and fatty acid dynamics in Atlantic cod, Gadus morhua, tissues: Influence of dietary lipid concentrations and feed oil sources

Summary

The influence of dietary fish and plant oils on fatty acid compositions of Atlantic cod tissues was examined to test whether modulation of fatty acid profile following a dietary change differs between lipid-poor (fillet) and lipid-rich (liver) tissues. A dilution model [Robin, J.H., Regost, C., Arzel, J., Kaushik, S.J., 2003. Fatty acid profile of fish following a change in dietary fatty acid source: model of fatty acid composition with a dilution hypothesis. Aquaculture 225, 283-293.] was used for the test. The experiment was divided into a build-up and a restoration phase. Fish weight increased from an initial 30 g to 100 g at the end of build-up and to 300 g at termination. During build-up, triplicate tanks of fish were fed one of four feeds; LF (fish oil, 13% lipid), HF (fish oil, 18% lipid), PO (40 palm oil:20 linseed oil:40 fish oil, 13% lipid) or RO (40 rapeseed oil:20 linseed oil:40 fish oil, 13% lipid). During restoration the cod were fed either LF or HF; four groups experienced a feed change (ROLF, ROHF, POLF, and POHF), and two groups were fed either LF or HF during both build-up and restoration. There were triplicate tanks of fish for each restoration phase treatment. Samples were taken at the start, at the end of build-up, mid-way through restoration and at termination, with four fish being taken from each tank each time; tissue indices were calculated, and analyses of lipid and fatty acids were undertaken, with a focus on 18C fatty acids (18:1 isomers, 18:2n-6 and 18:3n-3) and two n-3 HUFAs 20:5n-3 and 22:6n-3. During build-up, the cod fed HF and LF accumulated relatively more liver lipid than those fed PO and RO, implying that there were differences in the ways in which the lipids in the fish oil and plant oils were digested, absorbed and metabolised. At the end of build-up the cod fed PO and RO had tissues with higher 18C fatty acid, and lower n-3 HUFA, concentrations than the cod fed LF and HF; tissue fatty acid compositions reflected those of the feed oils. The fatty acid profiles of ROLF, ROHF, POLF, and POHF cod were modified during restoration. The changes in concentrations of 18C fatty acids were more rapid than predicted by the dilution model, implying preferential metabolism of 18:1 isomers, 18:2n-6 and 18:3n-3. Concomitant with the decrease in 18C fatty acid percentages there were increases in both 20:5n-3 and 22:6n-3. At termination, fillet percentages of n-3 HUFAs were statistically indistinguishable across treatments. but feed effects on liver lipids 20:5n-3 and 22:6n-3 were still discernible: cod fed PO and RO during build-up had liver lipids with lower percentages of n-3 HUFAs than those of cod fed LF and HF throughout the experiment. The fatty acid compositions of cod tissues are modified following a change in feed oils, but there are some tissue-specific differences in the responses shown. The results indicate that it should be possible to manipulate the fatty acid compositions of cod tissues in desired directions by introducing mixtures of different feed oils at various points in the production cycle.