Effects of Various Levels of Silica Ash in the Diet of Juvenile Red Drum,Sciaenops ocellatus

The demand for alternative feedstuffs to fish meal is anticipated to continue increasing due to fish meal's limited supply and escalating price. One group of alternative feedstuffs includes lipid‐extracted algae (LEA) by‐products from algae production for biofuels. Most LEA by‐products are known to contain relatively high levels of ash ranging from 20 to 30% of dry weight. Thus, inclusion of LEA by‐products in aquafeeds may contribute a substantial amount of ash, which potentially could have negative effects on utilization of other nutrients. To study the ash component of LEA by‐products, diatomaceous earth (DE) was used as a homogeneous source of silica ash. Therefore, two feeding trials were conducted with red drum, Sciaenops ocellatus, in a closed, recirculating system to evaluate the effects of graded levels of dietary silica ash. In Experiment 1, semi‐purified diets were formulated to contain 35% crude protein, with DE included at 0, 5, 10, 20, or 30% of dry weight in place of cellulose to provide diets with analyzed ash levels of 8.6, 12.4, 16.7, 25.5, and 33.8% of dry weight in a regression design. Similar diets were fed in Experiment 2 but DE and cellulose were restricted to no more than 20% of diet. Each diet was fed to triplicate groups of juvenile fish (initial average weight of 1.4 ± 0.2 and 2.3 ± 0.1 g) for 8 wk. In both experiments it was apparent that red drum did not respond negatively to even the highest dietary ash levels based on weight gain, feed efficiency, survival, or body composition. No apparent histological changes in the gastrointestinal tract of fish fed the graded levels of ash were observed. Therefore, inclusion of algae by‐products in diets of red drum will not be limited due to their contribution of ash.