Interactions between dietary docosahexaenoic acid and other long-chain polyunsaturated fatty acids on performance and fatty acid retention in post-smolt Atlantic salmon (Salmo salar)

A study with varying dietary inclusion levels (1, 5, 10, 15 and 20 g kg-1) of docosahexaenoic acid (DHA; 22:6n-3) was conducted with post-smolt (111 ± 2.6 g; mean ± S.) Atlantic salmon (Salmo salar) over a 9-week period. In addition to the series of DHA inclusion levels, the study included further diets that had DHA at 10 g kg-1 in combination with either eicosapentaenoic acid (EPA; 20:5n-3) or arachidonic acid (ARA; 20:4n-6), both also included at 10 g kg-1. An additional treatment with both EPA and DHA included at 5 g kg-1 (total of 10 g kg-1 long-chain polyunsaturated fatty acids, LC-PUFA) was also included. After a 9-week feeding period, fish were weighed, and carcass, blood and tissue samples collected. A minor improvement in growth was seen with increasing inclusion of DHA. However, the addition of EPA further improved growth response while addition of ARA had no effect on growth. As with most lipid studies, the fatty acid composition of the whole body lipids generally reflected that of the diets. However, there were notable exceptions to this, and these implicate some interactions among the different LC-PUFA in terms of the fatty acid bio- chemistry in this species. At very low inclusion levels, DHA retention was substantially higher (*250 %) than that at all other inclusion levels (31–58 %). The inclusion of EPA in the diet also had a positive effect on the retention efficiency of DHA. However, EPA retention was highly variable and at low DHA inclusion levels there was a net loss of EPA as this fatty acid was most likely elongated to produce DHA, consistent with increased DHA retention with addi- tional EPA in the diet. Retention of DPA (22:5n-3) was high at low levels of DHA, but diminished with increasing DHA inclusion, similar to that seen with DHA retention. The addition of EPA to the diet resulted in a substantial increase in the efficiency of DPA retention; the inclusion of ARA had the opposite effect. Retention of ARA was unaffected by DHA inclusion, but the addition of either EPA or ARA to the diet resulted in a substantial reduction in the efficiency of ARA retention. No effects of dietary treatment were noted on the retention of either linolenic (18:3n-3) or linoleic (18:2n-6) acids. When the total n-3 LC-PUFA content of the diet was the same but consisted of either DHA alone or as a combination of EPA plus DHA, the performance effects were similar

Dietary methionine spares the requirement for taurine in juvenile Yellowtail Kingfish (Seriola lalandi)

Taurine, a β-sulphonic amino acid, is a growth and health promoting dietary supplement in commercial finfish aquaculture. Reported recommendations for taurine supplementation in Seriola spp. feeds broadly range from 2.6 to 10.2 g kg−1 diet. Methionine is an essential amino acid and substrate for various metabolic compounds and acts as a methyl and sulfur donor, potentially sparing taurine. Dietary methionine requirements are currently unknown for Yellowtail Kingfish (Seriola lalandi); however, recommendations for the closely related Japanese Yellowtail (Seriola quinqueradiata) indicate that 11.1 g kg−1 diet is adequate. The taurine requirement and sparing effect of methionine of juvenile Yellowtail Kingfish was quantified by conducting a feeding experiment and applying a factorial, orthogonal dose-response design. Fourteen isonitrogenous and isoenergetic diets were prepared using practical raw ingredients with either one of two levels of methionine (10.9 ± 0.2 g kg−1 or 17.2 ± 0.6 g kg−1) and either one of seven levels of taurine, increasing from 1.6 to 20.4 g kg−1, respectively. Triplicate groups of 14 fish (53.3 ± 0.4 g fish−1) were fed one of the 14 diets over seven weeks. Based on growth and feeding results, juvenile Yellowtail Kingfish do not require dietary taurine supplementations when the basal taurine diets content is at least 1.6 g taurine kg−1 at a dietary methionine content of 17.2 ± 0.6 g kg−1 diet. This demonstrates that dietary methionine has a sparing effect on taurine supplementation. Yellowtail Kingfish fed dietary methionine exceeding the current minimum industry standard (~11.1 g kg−1), grew more rapidly than those fed high dietary taurine contents at dietary methionine levels approximating that of current industry practice, indicating the indispensability of adequate methionine supply. Breakpoint analysis on the specific growth rate in Yellowtail Kingfish fed a methionine level of current industry practice, estimated a digestible taurine requirement of 1.98 g kgBW−1 d−1 at an average digestible methionine intake of 3.4 g kgBW−1 d−1 This equates to a dietary taurine content of 7.7 g kg−1 diet at a dietary methionine content of 10.9 g kg−1 diet. Our results indicate that in juvenile Yellowtail Kingfish: adequate dietary methionine spares dietary taurine supplementation; insufficient dietary methionine provokes a taurine requirement; and current industry specifications for dietary methionine for Yellowtail Kingfish aquafeed require reassessment