Egg and larval quality can be influenced by broodstock diet even when larvae are reared under uniform conditions and fed a high-quality diet. A study was conducted to examine diet-egg dynamics in Southern Flounder (Paralichthys lethostigma), particularly with respect to fatty acids. Questions addressed included: (1) Which fatty acids in eggs can be altered by manipulating broodstock diet? (2) Can dietary manipulations of egg fatty acid composition affect egg quality? (3) What is the temporal window for egg composition to equilibrate with broodstock diet changes?
Two populations of Southern flounder broodstock were fed a common diet of shrimp, squid, and mackerel (2:1:1 by wet weight). One population was given a year-round nutritional supplement (Mazuri Gel); the other received the supplement 1 month before, and throughout spawning. Two other populations of flounder were fed a common diet of shrimp and sardines (1:1 by wet weight) and then switched to either shrimp only or sardines only 16 weeks before spawning started. After hormone injection, adults from each diet treatment were strip-spawned, eggs were incubated under uniform conditions, and egg quality (fecundity, viability, energy density, hatching rate, hatching length) was assessed. Fatty acid composition (% total fatty acids) of broodstock diets and eggs were analyzed by gas chromatography.
Almost all of the 27 fatty acids measured varied between eggs from one diet and another; only 2 (20:2n-6 and 20:3n-6) did not differ significantly among all four broodstock diet treatments. Four fatty acids in eggs (16:1n-7, 16:2n-4, 16:3n-4, 20:4n-6) were especially sensitive to differences in these broodstock diets, as they were significantly different for all diet treatments. Among the five egg quality measurements, only length at hatching exhibited significant differences among diet treatments. Larvae from eggs produced by adults fed shrimp only (2.71 mm + 0.27 mm) were significantly larger than all other diet groups, and larvae from the diet of shrimp, squid, and mackerel with year-round nutrient supplementation (2.30 mm + 0.19 mm) were significantly smaller than all other diet groups.
Comparing eggs from the two broodstock populations that received the nutritional supplement, nine fatty acids did not differ despite the difference in timing of diet supplementation. All of these were minor constituents of the supplement (< 4% of total fatty acids). Proportions of 12 fatty acids (16:0, 16:1n-7, 17:0, 16:3 n-4, 18:1 n-7, 18:2 n-6, 18:3 n-6, 18:3 n-4, 18:3n-3, 20:4n-6, 20:5n-3, 22:5n-3) were significantly higher in eggs produced on the year-round supplemented diet, indicating that equilibration of eggs to broodstock diet takes more than 1 month. Combining results of the diet change to shrimp only or sardines only with data from a prior study that used the same diets but different timing of the diet change (0-8 weeks before spawning), showed that equilibration of egg fatty acid composition with southern flounder broodstock diet takes 10 to 16 weeks.