Supplementation of Organic Acids and Algae Extracts in Aqua Feeds: Immunological Impacts

Two organic acids, polyhydroxybutyrate (PHB) and potassium diformate (KDF) have been researched to only a limited extent with aquatic species but have been shown to have various positive effects on terrestrial animals. Two algae extracts, carrageenan and alginic acid, also have been shown to elicit immunostimulation in some fish. Therefore, the present study was conducted with red drum (Sciaenops ocellatus) as a model marine species to study the effects of organic acids and algae extracts as feed supplements by evaluating several humoral immune responses.

Two feeding trials, one of 7-week duration and the other of 3-week were conducted with disease-free juvenile red drum (average initial wt. 2.6?0.2 g and 78.2 ?0.2 g, respectively). Semipurified diets were formulated to be isocaloric and contain 40% crude protein. Experimental diets were produced by supplementing the basal diet with KDF at 0.6%, PHB at 2%, alginic acid at 1% or carrageenan at 0.5% by weight in place of cellulose. Fish were stocked into 110-L aquaria operated as a recirculating system with each diet assigned to three replicate aquaria containing either 15 fish (7-week trial) or 9 fish per aquarium (3-week trial). All fish were fed their respective diets at the same fixed percentage of body weight (initially 6% and gradually reduced to 4% as the fish grew). Body weight was monitored by collectively weighing fish from each aquarium every week.

At the end of each feeding trial, weight gain and feed efficiency were significantly (P<0.0001) reduced in fish feed PHB compared to the basal diet and both algae extracts. There were no significant differences in condition indices such as hepatosomatic index (HSI) and intraperitoneal fat (IPF) ratio among fish fed the various diets. Lysozyme activity was significantly higher in fish fed alginic acid. The greatest phagocytic activity was found in fish fed the diet containing PHB. Total immunoglobulin level was higher in fish fed the diet supplemented with carrageenan. Goblet cell proliferation was greatest in the posterior end of the gastrointestinal tract but not different among dietary treatments. Organic acids and algae extracts evaluated in this study produced variable immunological responses in red drum with carrageenan showing the greatest potential as an immunostimulant.