Milkfish scales glint silver in the sunlight, a natural feature that helps them blend into the water. But aside from camouflage, milkfish growth depends on light. Under natural conditions, fish are exposed to an equal cycle of 12 hours of light and 12 hours of darkness, reflecting conditions in the wild.

Locally known as bangus, milkfish is deeply woven into the daily life and food culture of Filipinos. It is a vital source of income for many coastal and inland fishing communities. A study in 1999 cited that the Philippines has been one of the pioneers of milkfish aquaculture since the early 1970s, and up to today, thousands of fisherfolk depend on a steady supply of healthy milkfish fingerlings to sustain their livelihoods.

Recognizing the continuing challenges in fingerling production, and to help fisherfolk produce stronger, healthier fingerlings, DA-NFRDI, through the Brackishwater Fisheries Research and Development Center (BFRDC) in Lala, Lanao del Norte, conducted a study that explored a simple but innovative way to help farmers produce more fingerlings in a shorter time—by using light.

Crucial nursery phase

Before milkfish reach market size, they must first pass through the fingerling stage, one of the most critical phases in farming. Slow growth or high mortality during this period can mean lost income and longer waiting times for fisherfolk. Even modest improvements in fingerling survival and growth can bring significant benefits to communities that depend on aquaculture for their livelihoods.

Understanding photoperiod manipulation

Photoperiod manipulation may sound technical, but the idea is simple. It refers to the length of time fish are exposed to light and darkness in a day. In natural conditions, fish experience around 12 hours of daylight and 12 hours of night. Photoperiod manipulation changes this by extending the hours of light using artificial illumination. By extending the lighting period in nursery ponds, phototactic plankton are drawn, providing additional natural food for the fish and allowing them to feed for longer periods.

Milkfish are daytime feeders. Meaning, they depend on light to see and capture food. When it gets dark, feeding activity slows down. By providing additional light at night, they are given more time to eat, promoting growth. In this study, LED lights were used to extend photoperiod exposure without changing or compromising the water quality.

Shedding light on milkfish growth

Milkfish fry, measuring 1.5-1.7 centimeters at the start, were raised in net cages inside a pond with uniform stocking rates. One group followed the normal day-night cycle (12L:12D), while two other groups received extended lighting, either 18 hours or continuous 24-hour light.

LED lights provided the extra illumination, and simple barriers, like black polypropylene sacks, prevented light from affecting other groups. Over the 30-day period, the fry were closely monitored for growth, survival, feeding, and water quality, ensuring accurate tracking of the effects of extended light exposure.

Faster growth, higher survival

Milkfish raised under longer light exposure grew faster and reached fingerling size within a month, making them ready for sale or transfer to grow-out ponds. Among the groups, those given extended hours of light showed better growth compared to fish raised under normal day-and-night conditions. Overall, increased lighting helped the fish grow bigger in a shorter period of time.

Survival rates were also significantly higher in lighted treatments, compared to those under natural light conditions. From a technical standpoint, survival rates above 80 percent are considered very good for milkfish nursery rearing as recommended by experts, indicating a stable and favorable environment. For farmers, this means more fingerlings harvested and fewer losses during the nursery phase.

Plankton abundance and feeding activity

One of the important technical observations of the study was the increase in natural food organisms. Artificial light attracted plankton, particularly zooplankton, which serve as an important food source for milkfish fry. With extended lighting, plankton remained available even during nighttime hours, allowing fish to continue feeding beyond daylight.

Analysis of gut contents indicated a broader range of food items in fish reared under longer light exposure, suggesting more consistent feeding and better utilization of natural food in the system.

Light, temperature, and water quality

One concern with using artificial lighting is whether it might raise water temperature and cause stress to the fish. To address this, water conditions were regularly checked throughout the rearing period. The results showed that the added lighting did not cause harmful increases in water temperature, and the fish remained within a safe and comfortable range for growth.

Other important water conditions were also observed and stayed generally stable during the study. Changes in temperature and salinity were mainly linked to seasonal weather patterns rather than the use of artificial light, with warmer conditions during the dry season and lower salinity during the rainy months.

Higher income potential for fisherfolk

A cost-and-return analysis showed that despite the added cost of LED lights and electricity, extended photoperiod treatments resulted in lower overall production costs and higher net income.

Faster growth shortened the nursery period, while higher survival increased the number of saleable fingerlings. For fisherfolk, this means earlier harvests, reduced losses, and better profit margins.

Lighting the path forward

While the results are promising, further testing in larger pond systems is still needed before the technology can be widely cascaded and adopted. Nevertheless, this study on photoperiod manipulation is a practical, low-cost innovation with strong potential to improve milkfish fingerling production.

As the research continues, NFRDI aims to verify this technology in larger production areas and further optimize lighting strategies, ensuring that the silver glint of milkfish continues to shine in Filipino ponds and on Filipino tables, for years to come. ### (Ann Dominique del Valle)