DOST study links milkfish gut bacteria to fry survival

By Rizza B. Ramoran, DOST-PCAARRD S&T Media Services

ILOILO CITY — A project funded by DOST-PCAARRD and implemented by the University of the Philippines Visayas found that wild and hatchery-produced milkfish fry carry different gut bacteria, which researchers said may help explain why wild fry generally survive better.

The researchers also identified a probiotic candidate from wild fry that increased survival among infected fish in controlled laboratory tests.

The Department of Science and Technology’s Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development funded the UPV project, titled “Improvement of Milkfish Larval Rearing and Nursery Culture through Gut Metagenome, Transcriptome Analysis, and Gut Microbial Community Manipulations.”

PCAARRD said in 2023 that the country needed nearly 4 billion milkfish fry a year, domestic hatcheries could produce no more than 800 million, and about 75% of national requirements were imported from Indonesia.

Wild and hatchery fry differ

Researchers used metagenomic and transcriptomic analyses to examine microbial communities in the guts of milkfish larvae and juveniles and their possible roles in nutrition, metabolism, and immune defense.

Wild-sourced fry are generally considered hardier than hatchery-produced fry, but their limited supply cannot meet national production needs.

The project said hatchery fry often show deformities and low survival, which may be linked to broodstock, environmental conditions, and other pressures during early development.

Researchers found that wild fry had different and more diverse gut bacteria than hatchery-raised fry, with the source of the fish associated with the composition of the gut microorganisms.

The wild-fry microbiome contained more genes with predicted functions related to carbohydrate and energy use, vitamins, other nutrients, and immune defense, while hatchery fry showed more microbial functions involving fat metabolism and other metabolic processes.

Researchers said these microbiome differences may partly account for the generally better performance of wild fry.

Probiotic candidate raises survival

The team isolated Pediococcus pentosaceus, a species in the Lactobacillaceae family, from wild-caught milkfish fry.

In controlled laboratory tests, the isolate inhibited Vibrio parahaemolyticus and Vibrio harveyi, two bacteria that can cause disease in commercially important fish.

The team then added the probiotic isolate to commercial feed and compared it with regular feed without the supplement.

Fry that received the supplemented feed had higher survival and fewer signs of disease after exposure to a known milkfish pathogen, according to the project.

The project reported a statistically significant increase in survival among infected fish given feed containing the P. pentosaceus strain.

The release did not provide the sample size, survival percentages, pathogen used in the feed challenge, or statistical test behind the finding.

Researchers also isolated three additional probiotic candidates from the guts of wild-caught fry.

The team is screening the three isolates and testing the dosage and application frequency needed for milkfish fry.

Researchers said the data may help identify microorganisms found in wild fry but absent from hatchery-raised fry and support practices that promote gut microbial diversity and improve fry health and quality.