Aquatic organisms have an intimate relationship with their farming environment. Therefore, the administration of probiotics must be carefully considered in this sector.
Probiotics are live microorganisms that, when ingested, promote the health of aquatic species. They also strengthen the ecosystem by reducing ammonia nitrogen and improving feed conversion ratios. Traditionally, most probiotic studies on fish have involved dietary supplementation.
1. Effect of Probiotics on Immune System
Various studies have revealed that probiotics can stimulate the immune system in aquatic animals, thus increasing resistance to infections. For example, a study showed that feeding young tilapia with a mono-and multiprobiotic containing Bacillus strains significantly improved their intestinal and serum immunological parameters. Another study found that dietary Pediococcus acidilactici boosted the immune system of rainbow trout (Oncorhynchus mykiss) and increased their survival rate.
Moreover, probiotics can also act as an alternative to antibiotics and chemicals in aquaculture. They can help to improve water quality, increase the stress tolerance of fishes, and generate high-quality livestock. Therefore, it is important to investigate the effects of probiotics on the immune system of aquatic organisms in order to use them as a means of increasing their production and productivity.
The most common method of probiotic administration in aquaculture is through the supplementation of rearing water. This method has been proven to be effective at all stages of the life cycle of fish, and it can be used from the first day of hatching. However, this method has limitations during the early larval stage due to the immature digestive tract of these small animals. In addition, it is difficult to administer probiotics through feed, as the digestive tracts of fishes are usually closed at this time.
Recently, researchers have started to explore the possibility of probiotics being administered directly through the water column in aquaculture systems. This method has been shown to be very effective and efficient, but there are still some issues that need to be addressed. For example, there is no current information on the transcriptomic changes induced by probiotics administered through water, and this is something that needs to be investigated in the future.
2. Effect of Probiotics on Growth
In aquaculture, probiotics have been used to increase growth and immunity of aquatic organisms. This has been achieved by administering the microorganisms as dietary supplements, or directly added into the water column of the system. The effectiveness of these probiotics is dependent on the administration route, the type of microorganism, and the conditions under which it is administered.
Studies have shown that the use of probiotics improves the digestibility of feed and the survival of aquatic organisms. It also improves resistance to stress and promotes reproduction. Furthermore, the microorganisms can synthesize complex B vitamins which help to improve the nutritional status of the fish. This can be beneficial for aquaculture as it reduces the need for expensive supplements and additives to the feed.
Recently, the effect of probiotics on the immune and metabolic systems of teleost fishes have been explored. For example, a single dose of the bacterium Bacillus subtilis at concentrations of 106-108 cells per gram of feed resulted in a significant increase in gonadosomatic index, fecundity, and viability of female fish. The bacterial strain also caused an increase in the expression of antioxidative enzymes, such as catalase and superoxide dismutase in the fish.
However, the effects of probiotics on teleost fishes have been highly variable. This could be due to a number of factors, including the water quality, temperature, and pH of the fish tank, the binding strength between the probiotic and the gut mucosal surface, and the duration of the probiotic supplement diet.
3. Effect of Probiotics on Survival
In aquaculture, the use of balanced probiotic dietary supplements is very common in order to increase production rates and flesh quality of edible fishes. However, introducing probiotics into feed is not an easy task due to their low stability in water and their requirement for a specific type of feed. In this context, direct addition of probiotics into rearing water has been shown to have positive effects on a variety of parameters such as immune function, growth performance, and survival (see Figure 1).
Recently, the application of a mixed Bacillus species probiotic in larviculture of the common snook (Centropomus undecimalis) has shown to increase larval survival up to 2.5 times when compared with control systems without the probiotic. These results were attributed to the ability of the probiotic strains to degrade pristane, a terpenoid alkane, produced naturally by some zooplankton52. It has also been demonstrated that some Bacillus strains produce biosurfactants, which interfere with quorum sensing and reduce the virulence of pathogenic bacteria53.
In addition, the addition of probiotics in the rearing water has been shown to decrease abiotic stress in larvae by decreasing nitric oxide and ammonia production and increasing oxygen levels. Moreover, probiotics have been shown to be capable of increasing the mRNA expression of antioxidative enzymes in response to abiotic stressors in aquatic organisms54.
4. Effect of Probiotics on Reproduction
As aquaculture contributes half of the global animal protein production, any disturbances in reproduction can lead to significant economic losses. The traditional use of antibiotics for fish diseases has tended to aggravate the problem by increasing the resistance of microorganisms. Therefore, scientists have been searching for eco-friendly methods to inhibit disease and enhance the health of animals.
One way to do this is to administer probiotics to the environment in the form of feed or water. Studies show that the addition of probiotics to feed significantly increases fish weight and survival. The use of probiotics also enhances flesh quality, the production rate of fishes, and the immunity of fishes. In addition, the addition of probiotics to the rearing water can purify waste water in the pond and reduce ammonia levels.
However, the administration of probiotics through feeding has some limitations in the early larval stages because of the immature digestive tracts of young fish. The addition of probiotics directly to the rearing water is therefore a better method in this stage.
Recently, the effect of probiotics on reproductive dysfunction has received attention in aquaculture research. In this regard, it has been found that the administration of certain probiotics can improve the physiology and immunology of broodstock animals by increasing the activity of reproductive enzymes and transcription of reproductive genes. This leads to improved breeding performance in a species of edible fish such as Nile tilapia (Oreochromis niloticus).