Effect of lipid inclusion levels in aquafeed on carcass composition, quality change during storage and nutrient excretion in dusky kob (Argyrosomus japonicus)

Dusky kob, Argyrosomus japonicus, is an aquaculture species in South Africa that is in pilot commercial production. While the major nutrient requirements of the species are known, the advantages of incorporating formulated feeds into the diet of the species has yet to be fully explored. Research on formulated feed composition is required to: minimise input costs; improve the organoleptic properties and meat quality; and minimise nutrient loss, which contributes to environmental pollution. This study sought to test the impact of different lipid levels in aquafeeds fed to dusky kob juveniles by determining:

1. growth performance, feeding efficiency, proximate and fatty acid composition;
2. chemical changes and shelf-life of refrigerated fish fillets; and
3. metabolic rates and nitrogen excretion of juvenile dusky kob.

Dusky kob juveniles of 35.12 ± 10.3 g (P = 0.718) and 122.90 ± 11.9 mm (P = 0.062) were sourced from Oceanwise (Pty) Ltd, East London for the growth trial which was run a semi-recirculation system for 106 days. The dusky kob were fed isoenergetic, isonitrogenous diets with lipid levels of 8 %, 12 %, 16 % and 20 % hereafter referred to as 8F, 12F, 16F and 20F respectively. Specific growth rate (SGR) was highest for the 8F treatment (P = 0.001) and decreased with increasing dietary lipid levels. Feed conversion ratios (FCR) increased with increasing dietary lipid levels from 1.24 at the 8F treatment to 1.39 at the 20F treatment (P = 0.034). The deposition of lipids in tissue closely reflected the dietary lipid levels. Lipid levels did not significantly affect the levels of saturated fatty acids in fillets (P = 0.674). Monounsaturated fatty acids were significantly higher in the 8F lipid treatment (P < 0.001), while polyunsaturated fatty acids were significantly higher in the 20F lipid treatment (P = 0.047). Omega 3 fatty acids (n-3) were significantly lower in the 8F treatment (P < 0.001) and increased with increasing dietary lipid levels, while omega 6 fatty acids (n-6) were significantly higher in the 8F treatment (P < 0.001) and negatively correlated (r = −0.96, P = 0.044) with dietary lipid levels.

The second experiment investigated the effect of different dietary lipid levels on the biochemical changes in dusky kob fillets refrigerated (6.6 ± 1.5 °C) for 18 days. The total volatile basic nitrogen (TVBN) level, an indicator of total nitrogenous compounds that develop during muscle tissue deterioration, differed significantly across all treatments (Factorial ANOVA: F (_{12, 140}) = 4.502, P < 0.001), with the 16F and 20F treatments producing the highest concentrations of volatile bases. Free fatty acids, an indicator of lipid hydrolysis, increased significantly in all treatments with duration of refrigeration (Factorial ANOVA: F (_{12, 20}) = 924.97, P < 0.001). The thiobarbituric acid reactive substances (TBARS), indicator of lipid oxidation, were significantly different among treatments (P = 0.005) and were elevated at higher dietary lipid levels. Our results show that the meat quality of dusky kob fillets fed comparatively low levels of dietary lipids (8F and 12F treatments) was acceptable up to 12 days in refrigerated storage. After this time, the quality of fillet meat became unfit for human consumption. For the higher levels of dietary lipids (16F and 20F treatments), the maximum refrigeration storage time before reaching levels that are unfit for human consumption decreased to 8 days.

The third experiment measured the effect of dietary lipid levels on metabolic rates and nitrogenous excretion rates, and was tested in a closed respirometer run for 25-30 hours. Postprandial metabolic rates (respiration) were not affected by dietary treatments (P = 0.521), but rose by between 47 and 81 % of the basal metabolic rate after 3 hours. Metabolic rates were not correlated to dietary lipid levels. The total excreted NH₄-N was higher in the 8F and 12F diets and decreased with increasing lipid levels (P < 0.001). The 8F treatment registered the highest urea excretion while the other treatments were relatively similar during the first 12-hour interval (P = 0.033). The peak excretion rates of NH₄-N and urea-N were reached between 3 and 5 hours after feeding. In all treatments, a rapid change in pH was observed during feeding, with the 8F and 12F treatments recording the highest rate of change in pH. Ammonium-nitrogen contributed the major share (69 - 86 %) of total excreted nitrogen with urea making up a smaller (14 - 31 %) but significant portion. The results show metabolic rates in dusky kob peaks 3 hours after feeding in all dietary treatments. Higher dietary lipid levels (16F and 20F treatments) results in low NH₄-N and urea excretion rates of dusky kob.

The experiments showed that low dietary lipid levels increase growth rate and improve feed conversion ratio. An increase in dietary lipid levels significantly increased the concentration of tissue polyunsaturated fatty acids, and lowered NH₄-N and urea excretion rates. However, higher dietary lipid levels tended to slow growth rates and reduced refrigeration shelf-life. The three experiments provided preliminary data on the effect of dietary lipid on growth performance, assimilation efficiency, carcass composition, meat quality deterioration and nutrient excretion. The demonstrated methodologies employed during this study may prove useful tools for laying a foundation for future studies