Introduction
Aquaculture has today become the fastest growing food sector industry, moving forward rapidly in a profitable manner. The immense growth in modern aquaculture has been due to various factors which include increasing use of formulated aquafeeds and intensification of culture systems. Due to intensification, bacterial diseases have been noticed as a major cause of economic loss to producers. In order to better this situation, antibiotics have been used potentially to irradiate disease-causing bacteria but its excessive use leads to antibiotic resistant bacteria. Because of this, antibiotics in animal production have been banned by the European Union and are increasingly under public scrutiny and criticism in other parts of the world. This emphasizes the need to develop alternative products which can positively influence animal performance. Plant extracts, prebiotics, probiotics, and organic acids or their salts have been evaluated as alternatives to antibiotics but the results obtained are inconsistent.
The global market value of acidifiers in the animal feeds industry in 2015 was estimated to be about €1.0 billion (FEFANA 2014-2016). The most commonly used organic acid in animal feed is citric acid followed by lactic acids, formic acids and/or their salts. Till now various feed additives such as organic acids have been investigated for use in aquaculture feeds and are expected to receive increasing attention due to their strong antimicrobial and prophylactic properties against various pathogenic bacteria. Use of these acidifiers mainly consisting of organic acids and their salts or mixtures of these are receiving much more attention as a potential replacement for improving the performance and the health of fish. In animal feeds the use of organic acids was initially demonstrated in piglets to maintain a low gastric pH in the gut, which resulted in problems with digestion (Easter, 1988). Acidifiers are believed to be more specific to growth activity which can reduce harmful micro-organisms and promote beneficial microflora colonization in the gastrointestinal tract of fish (Cromwell, 1990).
This accelerated growth of emphasis on food safety and animal health makes Europe the largest market (41%) followed by other developed countries such as the United States (24%) and Japan (12%). By this prophylactic agent, the use of alternatives such as organic acids are expected to increase in the rest of the world.
Organic acids
The organic acids are basically organic compounds with one or more carboxyl groups in them. These include saturated straight-chain monocarboxylic acids (C1–C18) and their respective derivatives, such as unsaturated (cinnamic, sorbic), hydroxylic (citric, lactic), phenolic (benzoic, cinnamic, salicylic) and multi carboxylic (azelaic, citric, succinic) acids (Cherrington et al.,1991), with a general molecular structure of R-COOH, where R 
represents the monovalent functional group. These acids are commonly referred to as short-chain fatty acids, volatile fatty acids or weak carboxylic acids.
Citric acid and its salt
Citric acid/salts (CA) are the most investigated organic acid in aquaculture for the purpose of growth and immune. Numerous studies have reported that citric acid can improve growth, feed utilization and mineral availability, particularly phosphorus(P) in various fish species. Fishes supplemented with citric acid observed are a red drum, Sciaenopsocellatus tends to lower the pH of diets and stomach contents and rainbow trout, Oncorhynchus mykiss has been observed to improve weight gain, feed efficiency, protein efficiency ratio and enhance digestive enzyme activities (Castillo et al., 2014). Hernandez et al. (2012) fed CA to Huso huso observed an improved growth, SGR, FCR, phosphorus absorption/retention reduced P load to the environment. Addition of CA in feed resulted in improved weight gain, SGR, PER, and FCR and also apparent protein, Ca and P digestibility in yellowtail, Seriolaquinqu eradiate (Khajepour and Hosseini, 2012). Likewise, Oreochromis niloticus, Oreochromis aureus increased the activities of protease and amylase in the digestive tract and did not affect lipase activities on the addition of citric acid to the diet (Li et al., 2009).
Lactic acids or their salts.
Use of lactic acid (LA) as a feed additive to rainbow trout, Oncorhynchus mykiss resulted in increased in bone Zn. Red sea bream, Pagrus major improved in the absorption of P but not in weight gain and feed utilization (Hossain et al., 2007). Ca-lactate supplemented with LA improved in growth and survival (Pandey and Satoh, 2008). Castillo et al. (2014) fed LA for red drum, Sciaenopsocellatus resulted in lowering the pH of the stomach contents, depressed growth performance at 30 g /kg diet and had no effects on body indices and composition.
Na-lactate salts as a feed additive to Arctic charr, Salvelinus alpinus, and Atlantic salmon, Salmo salar improved the growth of Arctic char, but not in salmon. The study conducted by Gislason et al. (1996) resulted in double feed retention time in Arctic char digestive tract; Gislasonet al. (1994) fed Atlantic salmon, S. salar with LA. There were no effects on fish growth, mortality or gross chemical composition of the faeces. Whereas Ringo et al. (1994) supplemented LA to Arctic charr, S. alpines and reported an improvement in weight gain and feed efficiency but didn’t observe any effects on lipid/fatty acid composition of muscle tissue and body proximate composition.
Use of butyric acids or their salts in aquafeeds
Na-butyrate supplemented with Sea bream, Sparus aurata enhanced growth, but there were no effects on SGR feed intake and feed utilization. It also altered the metabolic pattern at the intestinal level (Robles et al., 2013). Supplementing butyric acid as a feed additive in the omnivorous tropical Catfish, Clarias gariepinuswas observed to show no effects on weight gain, specific growth rate, protein efficiency ratio and food conversion ratio which tend to alter the bacterial community in the hindgut of catfish (Owen et al., 2006).
The fish diet supplemented with butyric acids can act as a diet attractant which increases the diet attractiveness and feed intake in Pacific white shrimp, L. vannamei resulting in no effect on phosphorus digestibility (Silva et al., 2013). In addition, Silva et al. (2016) noted improved growth performance and feed efficiency with no effect on phosphorus retention. However, it altered intestinal microbiota and increased serum agglutination titer value.
Sodium butyrate as a feed additive for omnivorous tropical catfish (Clariasgariepinus) was implemented with 2kg/ton in both fish meal-based diet and in defatted soya concentrates diet. The results observed show with no significant differences in supplemented sodium butyrate when compared with a control group. However, particularly in the catfish fed fish meal, the SGR was slightly higher and body weight gain observed was increased in sodium butyrate group than control group respectively, coupled with a concomitant reduction in the FCR of supplemented group fish. Apparently, sodium butyrate supplementation raised the proportion of gram-positive bacteria in the hindgut of catfish. In a similar way, the beneficial application of organic acid salts was also proven by addition of 5kg/ton sodium citrate with inactivated Lactobacilli to boost the growth of the Kuruma shrimp Masurpenaeus japonicas (Tung et al., 2006). A recent report suggests that the survival rate of brackish water shrimp grown in Taiwan was enhanced by supplementing with a dosage of 2.5kg/tonne calcium formate (Luckstadt, unpublished data).
Formic acid
Formic acid as a fish feed additive. Only a few studies are been carried out for the addition of potassium-diformate (a double-salt of formic acid) in herbivorous, carnivores, filter feeders’ fish and shellfish which show an apparent increase in weight gain, feed conversion ratio and the ability to counteract bacterial infections in fish. Ramli et al. (2005) tested potassium diformate (potassium salt of formic acid) as a growth promoter in tilapia grow-out and reported an increase with body weight. Similarly, commercial white shrimp feed added with Na-formate did not show any improvement in growth response, feed utilization and survival rate (Silva et al., 2015). In another experiment, Chuchird et al. (2015) provided evidence that Pacific white shrimp fed formic acid supplemented diets showed enhanced resistance to Vibrio parahaemolyticus challenge. When formic acid supplemented to shellfish not much result in positive evidence but in fish, it proved to be better.
Acetic acid
The dietary supplementation of 50 g kg1 acetic acid in trout diets tended to decrease gastric and caecal pH but did not profoundly reduce the pH of gastrointestinal tract contents, indicating that trouts are efficient in regulating endogenous gastric acid secretions to maintain their normal gut pH. Nevertheless, it was observed that the addition of acetic acid to trout diets significantly improved phosphorus availability in fish. Not much work has been carried out in using acetic acid as supplementation in fish and shellfish feed (Sugiura et al., 2006).
Propionic acid
Propionic acid contains one to five times more energy than wheat (Diebold and Eidelsburger 2006). If the energy content of organic acids should be completely used in metabolism it must be included in the energy content of feed rations. The effect of supplementation of sodium salts of lactic and propionic acid (10 kg/tone of feed) was tested in Arctic charr in brackish water at 8°C (Ringø, 1991). Use of formic and propionic acids and their salts on a sequential release medium has been successful in the grow-out of Turkish rainbow trout (Karl Sacherer, personal communication, 2006). The shrimps which were fed diets supplemented with propionate and butyrate resulted in an increase in final weight, feed efficiency, nitrogen retention, protein efficiency rate, survival, and yield. Specific shrimps fed diet containing 2% butyrate were noted higher in comparison with control treatment (Silva et al., 2013).
Fumaric acid
The inclusion of 0.5 g kg-1 of fumaric acid improved fish hematological parameters. Overall, incorporation of fumaric acid in C. gariepinus diets improved fish survival after Aeromonas sobria challenge (Omosowone et al., 2015)
Conclusions
Currently, usage of organic acids in the field of aquaculture and in the aquafeed industry has picked up and the farmers are greatly interested to increase the yield through the culture-based system and by commercializing the use of organic acids in aquaculture resulting in enhancement of both growth performance and control diseases. As evident from the research reviewed, many studies have reported that organic acids, their salts or blends in feed improved growth, feed utilization, gut health and disease resistance in aquatic animals. Generally, reports on organic acids have noted improving growth performance and nutrient utilization in aquaculture. The growth factor appears to be dependent on the type of organic acids used and host specific (Ng and Koh, 2011).
It is anticipated that the use of dietary organic acids as functional feed additives in aqua feeds to be used in aquaculture will increase markedly in the near future. Available information on the beneficial effects of dietary inclusion of organic acids and their salts on fish and shell fish are less experimented. Their performance is inconsistent and appear to vary among fish species, fish size or age, and the types and levels of organic acids and salts or their combinations. Additional factors include the compositions of experimental diets, buffering capacities of dietary ingredients, culture and feed management, and water quality. Despite the discrepancies among the published data, it appears that organic acids and/or their salts have good potential as dietary supplements to improve growth performance, feed utilization efficiency and nutrient digestibility. They alter gut microflora populations and increase the disease resistance of aquaculture species. However, more research is needed to better understand the mechanisms of the potential beneficial effects of these compounds and their mixture. The issue now is one of education and enforcement, especially in many developing nations where a significant amount of aquaculture production still comes from small-scale farmers who may be unaware of the dangers posed by the excessive use and misuse of antibiotics. The economic window for the incorporation of organic acids in aqua feeds is therefore narrow. Convincing farmers and aquafeed manufacturers regarding the actual benefits of organic acid supplementation is crucial and this can only be done through scientifically proven, laboratory- and field-based research data.
