Nearly half of Iloilo River fish catch is gloria tilapia

By Rjay Zuriaga Castor

ILOILO CITY — A baseline fisheries inventory along the 12.8-kilometer Iloilo River estuary found that nonnative cichlids have established a heavy presence across the waterway, accounting for more than half of all fish caught and posing a growing ecological threat to native aquatic life.

The study, slated for the July–December 2026 issue of The Philippine Journal of Fisheries, presents the first baseline assessment of fish diversity and abundance in the urbanizing estuarine system.

It was written by Precious Nikola B. Gaurana, Harold M. Monteclaro, Sheila Mae S. Santander-de Leon, and Liberty Napilan-Espectato of the University of the Philippines Visayas (UPV) College of Fisheries and Ocean Sciences.

The Department of Science and Technology’s Accelerated Science and Technology Human Resource Development Program funded the research.

Researchers inventoried catches from small-scale fishers using hook-and-line gear, filter nets, and gill nets at four sites stretching from Drilon Bridge near the river mouth to Old Sooc Bridge upstream.

Field inventories conducted between March and November 2022 recorded 514 fish individuals representing 24 species and 19 families, with a cumulative sample biomass of 25,544.5 grams.

Although native fish made up 83% of the species recorded, or 20 of 24, introduced cichlids dominated both numerical abundance and total sample weight.

Nonnative cichlids accounted for 60.32% of all captured specimens and 56.31% of total biomass.

“Catch composition in the Iloilo River estuary is influenced by environmental factors alongside impacts from infrastructure development and the presence of invasive cichlids,” the study said.

“The increasing prevalence of nonnative fish species is attributed to direct introductions through stock enhancement activities involving cichlid species and fisheries management measures, including fishing gear regulation,” it added.

The invasion is led by the blackchin tilapia (Sarotherodon melanotheron), locally known as “gloria tilapia,” which made up 47.28% of the total catch and 47.01% of total biomass.

The study said its morphological identification of the species marks the first documented report of S. melanotheron in Western Visayas.

A second introduced cichlid, the Nile tilapia (Oreochromis niloticus), locally called “Drilon tilapia,” ranked as the third most abundant species overall at 12.26% of the total catch.

S. melanotheron and O. niloticus were the only two species found at all four sampling sites along the river.

The researchers believe the cichlids entered the river through two routes: overflow from surrounding fishponds during harvest, and intentional introduction through stock-enhancement activities led by local fisheries managers.

The study noted that hybrids of O. niloticus were commonly cultured in ponds surrounding the Iloilo River estuary.

O. niloticus and the Mozambique tilapia (Oreochromis mossambicus), a third cichlid recorded in the river, were introduced to the Philippines in the 1950s to improve food stability and nutrition, according to the study.

“As a result, cichlids continue to threaten the sustainability of the Iloilo River estuary as a fishing and breeding ground for commercially important fish species,” the study emphasized.

Local fishers told researchers that the spread of blackchin tilapia has hurt wild populations of the native Asian seabass (Lates calcarifer) and the native Sulu shrimp (Metapenaeus suluensis), locally known as balaskugay or pasayan.

The Sulu shrimp was a target of now-prohibited traditional fishing gear and remains popular as bait among recreational anglers on the river, the study said.

Despite the pressure from invasive species, the estuary continues to support key native populations, led by the Pacific tarpon (Megalops cyprinoides), or bulan-bulan, which ranked second in overall abundance with 152 individuals, or 29.57% of the total catch and 30.3% of sample biomass.

Nearly all tarpon were caught at the upstream site between Carpenter’s Bridge and Old Sooc Bridge, where the species slightly outweighed blackchin tilapia in biomass.

Overall, the researchers found moderate diversity, low dominance, and an unstable community structure, which they said may indicate the early prevalence of the three introduced cichlid species.

Policy recommendations

To prevent further ecological degradation and protect local livelihoods, the researchers called for policy updates and stronger community engagement.

Local fisheries management in the Iloilo River estuary is primarily governed by Iloilo City Regulation Ordinance No. 2003-233, or the Basic Fishery Ordinance, as amended by Regulation Ordinance No. 2005-228 and Regulation Ordinance No. 2009-247.

The study noted that motorized push nets, or trol, and filter nets, or tangab, are now banned in the estuary, and that the ban has reduced fishing pressure.

Rather than a total prohibition on these gears, the researchers suggested a closed fishing season during peak spawning periods as a potentially more effective management strategy.

They also cited a 2019 UPV graduate thesis by A.H. Bantiquete that identified increasing the mesh size of motorized push nets as a viable alternative to an outright ban.

Such modifications could allow regulated fishing to continue while reducing the capture of juvenile fish, balancing conservation goals with the livelihoods of local fishers, the researchers said.

They also emphasized that effective conservation requires incorporating the perceptions of local resource users, particularly traditional fishing communities facing livelihood displacement.

“The results of this study can serve as a basis for continued research aimed at enhancing the effectiveness of the Iloilo River estuary as a fishing ground as well as a breeding ground for commercially important fish species and as a sustainable source of livelihood for traditional fishers,” the researchers said.

“Effective policies, monitoring activities, proper enforcement, and stronger fisherfolk participation are essential to preserve the estuary’s biodiversity and support sustainable fisheries,” they added.