In response to the country’s insufficient supply of milkfish fry, a project is exploring milkfish gut microbiota that can help improve the survival, growth, and health of hatchery-produced fry.
The project, “Improvement of Milkfish Larval Rearing and Nursery Culture through Gut Metagenome, Transcriptome Analysis, and Gut Microbial Community Manipulations” was funded by Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development of the Department of Science and Technology (DOST-PCAARRD). Implemented by the University of the Philippines Visayas (UPV), the project aims to expound, using metagenomic and transcriptomic techniques, the microbial factors and their roles in the physiology of milkfish larvae and juveniles. The results will be used to design appropriate rearing protocols to enhance the hatchery productivity of milkfish.
Wild-sourced milkfish fry are generally considered more robust than hatchery-produced fry, although their limited supply cannot meet national production requirements. Hatchery-produced fry often experience deformities and low survival rates, which the project noted may be linked to broodstock-related factors, environmental conditions, and other challenges during early development.
The project studied the microorganism present in the guts of milkfish larvae. The researchers used advanced laboratory techniques, including metagenomic and transcriptomic analysis, to understand how these microorganisms affect milkfish nutrition, metabolism, and ability to fight disease. Wild and hatchery-produced fry have different gut bacteria. It is hypothesized that the poor performance of the hatchery-reared milkfish fry as compared to the wild caught fry could be attributed to the gut associated microflora that helps in nutrient digestion, utilization, and supply of microbial metabolites that enhance immune responses. The project has shown that the gut bacteria of wild milkfish larvae are different and more diverse than those present in hatchery-raised larvae. Also, the project team found that the source of the fish, whether from the wild or from a hatchery, is associated with the composition and types of microorganisms living in their gut.
The gut bacteria of wild milkfish fry were found to be richer in genes with predicted functions related to carbohydrate and energy use, vitamins, and other nutrients. They also showed greater representation of functions associated with the immune system and disease defense. Meanwhile, hatchery-produced fry showed greater representation of microbial functions involved in fat metabolism and other metabolic processes. These findings suggest that differences in the gut microbiome may partly contribute to the generally better performance of wild fry.
As part of the objectives, the project also isolated bacteria from wild caught milkfish fry that could potentially be used as probiotics. Among the bacteria isolated, a candidate probiotic, Pediococcus pentosaceus belonging to the Lactobacillus bacterial family group, was shown to exhibit inhibitory activities against milkfish bacterial pathogens. Results demonstrated that, in controlled laboratory tests, this probiotic suppressed Vibrio parahaemolyticus and Vibrio harveyi, two bacteria that can cause disease in economically important fish.
The researchers then added the selected probiotic bacterial isolate to commercial feed and tested the supplemented feed on milkfish fry. Based on the results, the fry with the supplemented feed exhibited higher survival rates and fewer signs of disease after being exposed to a known milkfish pathogen compared with fish that received regular feed without the supplemented probiotics.
In particular, the tested P. pentosaceus strain showed strong potential for improving the survival of milkfish fry and was further tested against a known bacterial pathogen. The experimental results showed a statistically significant increase in the survival of infected fish. Additionally, the project has successfully isolated three more promising bacterial probiotics candidates from the gut of wild caught milkfish fry. These probiotics isolates are being screened and optimized for dosage and application frequency to maximize their efficacy as probiotics for milkfish fry.
The metagenomic and transcriptomic data gathered from wild and hatchery-produced fry could also eventually help researchers to identify microorganisms that are present in the wild fry but absent in the hatchery-reared fry. This information generated by the project could be the basis of hatchery culture innovations and practices that can promote and maintain the gut microbial diversity of hatchery-reared milkfish similar to that of the wild fry.
The generated information is also important in developing hatchery technologies and protocols that allows the use of specific probiotics isolated from the wild milkfish gut with gut health promoting functionalities that can improve the health and quality of hatchery-produced milkfish fry.