Red drum reared from captive-spawned eggs were used in a 2 × 4 factorial experiment comparing eight diets with soybean oil (SBO) or menhaden oil (MHO) added to a basal diet (about 3.4% lipid from practical ingredients) at 1.5, 5.2, 9.0, or 12.7%. Triplicate groups of 20 early juveniles (mean 0.4 g) were stocked in 1000 litre fiberglass tanks supplied with filtered estuarine water (17–26 °C, mean 22 °C; 15–35‰) and hand fed twice a day to apparent satiation for 56 days. The fish (range 0.3–9.4 g) appeared to utilize up to 1.5–5.2% dietary SBO (in 5.4–8.7% total lipid). With 0.67% EPA + DHA (eicosapentaenoic and docosahexaenoic acids) in the basal diet, no EFA-deficiency signs were observed. Dietary MHO was utilized up to 12.7%, but fish fed diets with more than 1.5% SBO had lower growth rates and increased feed conversion ratios (especially with 9.0 and 12.7% SBO). Diets containing 1.5% SBO or MHO produced the leanest fish (70.5–71.6% protein, 13.0–13.1% lipid, of whole body). Diets containing 9.0 and 12.7% MHO produced the fattest fish (64.5–66.0% protein and 19.1–19.9% lipid).
Percent liver lipid (wet) was highest for fish fed diets containing 5.2% SBO (41.4%), 9.0% MHO (36.5%), and 12.7% MHO (35.4%), and lowest with 1.5% MHO (24.0%). Considering the good survival, absence of disease, and excellent general health, this fatty liver condition did not appear to have short-term adverse effects. All fish in the study had fatty livers to some degree, and their liver histological features differed from those of wild fish. Liver lipid reserves might confer some survival advantage to fish released for stock enhancement if the reserves are used during their transition from cultured to wild status. However, if the hepatocyte changes we observed are not reversible, long-term detrimental effects on liver function are possible.
