POTENTIAL BENEFITS OF A LOW FAT FINISHING DIET FOR WHITE STURGEON Acipenser transmontanus CAVIAR PRODUCTION  

White sturgeon were reared in six-outdoor circular tanks (15m diameter), at stocking densities 30-40 kg/m³, using well water mixed with pond water (annual range; 10-28°C). The fish were fed a commercial diet with crude protein not less than 42%, and crude fat not less than 18%, until age 5 ½ years, when three tanks were then shifted to a finishing diet of 47% protein and 8% fat. The average sturgeon weight for all six tanks was 16 ± 4 kg at the initiation of the diet shift. At age 7 and age 8, the mature females from both diets were processed for caviar. Body size, condition factor, ovary weight, gonadosomatic index (GSI), caviar weight, caviar yield expressed as a percent body weight and ovary weight, and amount of ovarian fat were compared between diets. Data was pooled for the three tanks fed each diet, at each harvest, and a student t-test used to determine differences between means (P<0.05). Although sturgeon fed the low fat finishing diet were  significantly smaller and leaner, with a similar or lower weight of caviar per individual, when yield was expressed as a percent of body weight and ovary weight, the females shifted to the finishing diet had higher yields (Table 1).

A subsample of ninety ovaries per diet, after egg screening, were analyzed for quantity of fat lobes, and the females fed the low fat finishing diet had significantly smaller fat lobes, compared to those remaining on the high fat diet. Females were categorized with "low fat ovaries" when their ovaries produced over 65% caviar, while "high fat ovaries" produced less than 50% caviar. The tanks fed the finishing diet had a significantly lower proportion of females with high fat ovaries, and a higher proportion of females with low fat ovaries. Considering the complex nature of culturing sturgeon for caviar, a cost-benefit analysis needs to determine if a finishing diet makes economic sense for individual farms; to balance trade-offs between body size, caviar yield, number of fish to handle/process, speed of processing, feed cost, and other factors.