Mixes of plant oils as fish oil substitutes for Nile tilapia at optimal and cold suboptimal temperature

Nile tilapia production worldwide is concentrated in the subtropics, where lower winter temperatures is a challenge for the high productivity of tropical fish species. Adequate fatty acid nutrition of fish is a way to offset the lower temperature, as this nutrient is responsible for the structure, fluidity, and functionality of all cell membranes. However, there are still many gaps in Nile tilapia fatty acid nutrition, particularly at lower temperatures. In this study, three plant oil mixes and fish oil (FO) were fed to juvenile Nile tilapia at optimal (28 °C) or suboptimal (22 °C) temperatures for 9 or 12 weeks, respectively. The mixes of plant oils (coconut, olive, sunflower and linseed oils) were formulated to mimic fatty acid groups in FO, only varying the proportions between n-3 and n-6 polyunsaturated fatty acid (PUFA). They were named MIX-S, MIX-SL, and MIX-L since they contained an increasing amount of linseed oil (L) and n-3 PUFA as well as a decreasing amount of sunflower oil (S) and n-6 PUFA. Tilapia fed plant oil mixes exhibited similar growth at each rearing temperature. However, plant oil mixes led to lower daily weight gain than FO, except for MIX-S, which was comparable to FO when tilapia were fed at 22 °C. Feed efficiency and protein utilization were similar at 28 °C, but at 22 °C, the highest values were obtained in fish fed the FO diet. As expected, apparent net utilization of n-3 and n-6 PUFAs was higher when these groups of fatty acids were scarce in the diets. The body fatty acid profile of tilapia was influenced by dietary oil sources, but among the fish fed plant oils, total PUFA and total LC-PUFA were similar within each rearing temperature. Additionally, PUFA retention and conversion to higher chain length homologues in Nile tilapia seemed to be a strategy of adapting to lower temperatures. Our main finding is that changes in the n-3/n-6 PUFA ratio in diets with plant oil mixes do not alter Nile tilapia growth or feed efficiency at either 28 °C or 22 °C rearing temperatures. However, when compared to the diet with fish oil that was rich in LC-PUFA, plant oil diets resulted in lower growth. Plant oil mixes led to similar feed efficiencies as fish oil at 28 °C but not at 22 °C.