Digestibility ofCalanus finmarchicuswax esters in Atlantic salmon (Salmo salar) freshwater presmolts and seawater postsmolts maintained at constant water temperature

Abstract

Calanoid copepods are a rich source of marine lipid for potential use in aquafeeds. Copepod oil is primarily composed of wax esters (WE) and there are concerns over the efficiency of wax ester, versus triacylglycerol (TAG), digestion and utilization in fish. As smoltification represents a period of major physiological adaptation, the present study examined the digestibility of a high WE diet (Calanus oil; 48% WE, 26% TAG), compared with a TAG diet (fish oil; 58% TAG), in Atlantic salmon freshwater presmolts and seawater postsmolts, of similar age (9 months) and weight (112 g and 141 g initial, respectively), over a 98‐day period at constant temperature. Fish grew significantly better, and possessed lower feed conversion ratios (FCR), in seawater than freshwater. However, total lipid apparent digestibility coefficient (ADC) values were significantly lower in seawater fish, as were total fasted bile volumes. Dietary Calanus oil also had a significant effect, reducing growth and lipid ADC values in both freshwater and seawater groups. Postsmolts fed dietary Calanus oil had the poorest lipid ADC values and analysis of faecal lipid class composition revealed that 33% of the remaining lipid was WE and 32% fatty alcohols. Dietary prevalent 22:1n‐11 and 20:1n‐9 fatty alcohols were particularly poorly utilized. A decrease in primary bile acid, taurocholate, concentration was observed in the bile of dietary Calanus oil groups which could be related to the lower cholesterol content of the diet. The dietary WE : TAG ratio is discussed in relation to life stage and biliary intestinal adaptation to the seawater environment postsmoltification.

Marine wax ester digestion in salmonid fish: a review

“…Increased absorption with increased chain length and increased unsaturation (and decreased polarity) has been seen in trout enterocytes (Perez, Rodriguez & Henderson 1999; Oxley, Tocher, Torstensen & Olsen 2005). This was also observed in recent studies where absorption of 18:1n‐9 was higher than 16:0 using enterocyte assay (Bogevik, Tocher, Waagbo & Olsen 2008a), and for apparent digestibility calculated from the comparison between dietary and faecal lipid content in salmon (Bogevik et al 2009; Oxley et al 2009).…”

“…At the same time, molecular species of wax esters containing 14:0 fatty acid esterified to 20:1n‐9 and 22:1n‐11 alcohols would be poor substrates for the lipase and would have low absorption rates. In keeping with this, there was a substantial increase in the proportions of 14:0 fatty acid and 22:1n‐11 alcohol in the faeces lipid relative to that of the diet (Olsen et al 2004; Bogevik et al 2009; Oxley et al 2009).…”

“…However, Bogevik, Oxley and Olsen (2008) showed preferential hydrolysis of 16:0 compared with 20:1n‐9 in fish oil TAG hydrolysed by salmon lipases, indicating long‐chained monounsaturated fatty acids were poor substrates for the lipase. It follows that a preference of the salmon digestive lipase for ester linkages of unsaturated fatty acids would cause decreases in the proportions of 18:4n‐3 and 16:0 alcohols in faecal lipid as observed in Olsen et al (2004), Bogevik et al (2009) and Oxley et al (2009). At the same time, molecular species of wax esters containing 14:0 fatty acid esterified to 20:1n‐9 and 22:1n‐11 alcohols would be poor substrates for the lipase and would have low absorption rates.…”

“…Whether the slower hydrolysis of wax esters is due to the greater hydrophobicity of wax esters, associated with lower biliary emulsification or the specificity of the enzyme is still unclear. Feeding higher portions of wax esters would, however, give slower release of lipolytic products than from TAG, which is more or less completely hydrolysed to fatty acids in salmon (Oxley, Bogevik, Henderson, Waagbø, Tocher & Olsen 2009). However, there is a difference in the lipolytic activities between salmon and rainbow trout upon activation by bile.…”

“…Wax esters have a higher melting point than TAG and are intrinsically more hydrophobic, and therefore require proper emulsification for their hydrolysis by lipases. The high dietary wax ester:TAG ratio (4:1) in diets with 100% replacement of fish oil with Calanus oil (Bogevik et al 2009; Oxley et al 2009) could have limited the excess of the lipid for the lipases. Comparably, 100% replacement of fish oil with Calanus oil in Olsen et al (2004) resulted in a ratio of <2:1 between wax esters and TAG, due to a batch of C. finmarchicus with less wax esters (Table 3).…”