EFFECT OF DIETARY LIPID LEVEL ON THE PRESENCE AND ACTIVITY OF LIPASE IN THE GATROINTESTINAL TRACT OF RED DRUM, Scaienops ocellatus

Knowledge of the nutritional requirements of red drum, Sciaenops ocellatus, is considerable, since its commercial culture development started in the 1980s. Dietary lipid requirement for this species has been estimated to be around 10%, and lipase, the gastrointestinal (GI) tract enzyme digesting this nutrient, is apparently not stimulated by increasing dietary fat content, at least not in larvae, where a plateau in lipase activity is reached at around 12% dietary neutral lipid, suggesting this is the limit for maximal capacity in lipase synthesis at this developmental stage. It would be interesting to verify if the same holds true for other developmental stages. Thus, the present work further investigated the effect of dietary lipid level on the presence and the activity of lipase in the GI tract of S. ocellatus during the growout stage, as well as on its growth performance, and body composition.

An 8-week feeding trial was performed in an indoor clear-water recirculating culture system at the Texas A&M University Aquacultural Research and Teaching Facility, consisting of 110-L aquaria provided with biological and mechanical filtration, a UV light chamber, and water aeration. Juvenile fish were fed different diets with a fixed dietary crude protein (CP) level of 40% and increasing levels of dietary crude fat (CF), from 2 to 20% of dry diet. Each diet was randomly assigned to four replicates. Fish were fed to apparent satiation twice daily, with pre-weighed rations based on a percentage of total fish weight per aquarium (3-5% of total body weight) and visual feeding cues. Total ammonia nitrogen, nitrite, and pH were monitored weekly, while dissolved oxygen, salinity and temperature were monitored daily.

At the end of the feeding trial, the GI tract of fish subjected to the different dietary lipid treatments were dissected and homogenates in a 50 mM Tris-HCl buffer solution (pH 7.5) were obtained. The molecular mass of native lipase and its content in the fish GI tract were determined by SDS-PAGE using 10% polyacrylamide gels. Lipase activity was measured by using Na cholate hydrate, 50 mM Tris-HCl buffer and β-naphthyl-caprylate as substrates, followed by incubation and addition of 100 mM Fast Blue BB salt, with the reaction stopped after 5 min. by using 0.72 M trichloroacetic acid and ethanol-ethyl acetate (1:1, v/v). The samples were read at 540 nm against DI water as a blank. Body proximate composition and growth performance indexes such as final weight, weight gain, percent weight gain, daily weight gain, thermal growth coefficient, and specific growth rate were also determined for fish in each dietary treatment.