COPEPOD LIPIDS IN AQUACULTURE
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On a global scale marine oil is a limited resource and if growth of the aquaculture industry is going to continue, alternatives to fish oils in the feeds are required. As many of the north Atlantic Calanus species accumulate high amounts of depot lipids during the short spring and summer algal blooms, they are appealing as a source for marine lipids. Moreover, the aquaculture industry seems in need for two kinds of marine lipid sources, one to feed the masses during the on-growing phase and one that contain the proper phospholipid composition to suit the early developmental stages of marine fish larvae. The effect of laboratory produced copepod nauplii (Acartia tonsa) and enriched rotifers was examined in first feeding of cod (Gadus morhua) larvae, before the diet was changed to lipid rich Artemia or early weaning with a formulated diet. The A. tonsa nauplii provided a phospholipid fatty acid composition comparable to the natural copepods, while the rotifers and Artemia contained a different fatty acid composition in their phospholipids. The cod larvae obtained significant higher dry weight when their diets included A. tonsa nauplii during the first period of feeding, compared to when the larvae were initially fed on only rotifers (P<0.05). We demonstrated that genes involved in appetite regulation and digestion showed differential expression profiles after different feeding protocols, and the transcriptional level of mRNA for phospholipase A2 in the cod larvae was substantially up-regulated between 16-38 days post hatching, which may suggest limited capability to remodel fatty acid in sn2 position during the earliest period of feeding. The work towards controlled mass production of marine copepods will be important in order to provide for copepod nauplii containing the desired phospholipid fatty acid composition. This tool may assist researchers to gain increased knowledge on the nutrient impact on functional development of marine fish larvae, this knowledge is required in order to prepare a functional compound diet to replace the use of live feed. The copepod Calanus finmarchicus has a high standing biomass in the north Atlantic and if C. finmarchicus is to be harvested in large quantities as raw material for aqua feed, a better understanding of the biochemical content and composition, and how it changes during harvest and post harvest treatment is required. The wax ester deposits from all harvests of C. finmarchicus contained high level of 18:4n-3 and 20:5n-3 when the animals were collected from coastal surface waters during spring and early summer. On a dry weight basis, the total content of 18:4n-3 increased with increasing wax ester and lipid content during spring and early summer, and less was found during wintering (P< 0.05). Based on the increasing content of wax esters and their content of n-3 poly unsaturated fatty acids, the best period for harvesting period for feed ingredients seems to be during late spring and summer. Post mortem storage or heating of C. finmarchicus material appeared to have no influence on the quantity of wax esters. A higher total content of phospholipids was found during late spring and early summer and less content was found in the wintering stages of C. finmarchicus (P<0.05). As these lipids contained stable and high share of 22:6n-3 and 20:5n-3, their phospholipids should be suitably composed for use in early weaning diets for marine fish larvae. The optimal harvesting period in order to obtain high yield of phospholipids seems to be during late spring and early summer. Both the phospholipid fraction and the proteins in C. finmarchicus were susceptible for post mortem autolytic degradation during storage of the harvests. But, it seems like lowering of the temperature to 0 °C is sufficient to restrain substantial degradation, which is a manageable method for most fishermen that bring the harvest to shore on a daily basis. However, stabilization is required in order to maintain these constituents in the harvests. During heating, the highest loss of phospholipids was found when C. finmarchicus was incubated at 40 and 50 °C, incubation at 70 °C also significantly reduced the phospholipids content (P<0.05), but the degradation process ceased within 30 minutes at this temperature. Higher total amount of peptides and free amino acids was also formed at 40 and 50 °C than at 70 °C (P<0.05). Rapid inactivation of the proteolytic enzymes required a temperature at or above 70 °C, but the range from 40-60 °C should be passed quickly in order to prevent extensive formation of peptides and free amino acids and loss of phospholipids.