Invasive ornamental fish threatens Laguna lake's ecosystem — study

MANILA, Philippines — A new study warned that Lake Sampaloc, the largest of the seven crater lakes in San Pablo, Laguna, is facing a significant ecological threat from the proliferation of Flowerhorn cichlids, ornamental fish popularly known as “Flowerhorn fish” (scientific name: Amphilophus spp.).

In a research published on March 20 by biologists Janice Ragaza and Hannah Gasmen of Ateneo de Manila University, the researchers detailed how these ornamental fish have transitioned from aquarium pets to well-established and ecologically concerning invasive/non-native cichlids.

According to Ragaza and Gasmen, Flowerhorns are fish bred for ornamental purposes and prized in the aquarium trade for their vivid gold coloration and distinct nuchal humps.

Citing local accounts, the researchers said the fish first entered Lake Sampaloc’s ecosystem after escaping from breeding facilities during a major typhoon.

The study highlights that Flowerhorns possess traits that promote ecological dominance, including extreme territorial aggression and high reproductive potential.

The researchers said the fish outcompete native species by monopolizing feeding sites and breeding grounds.

“Their aggressive behavior and high reproductive potential threatens native ichthyofauna and destabilizes existing food web dynamics (Nasution et al., 2022),” the study read.

“Collectively, these factors highlight the growing ecological stress on Lake Sampaloc’s native fish populations and the urgent need for integrated management strategies that address both unintentional introductions and deliberate aquaculture of invasive ornamental species,” it added.

The impact on local fauna has been alarming, according to the researchers.

The fish “Ayungin,” a native species once common in the lake and a staple of local diets, is now rarely encountered and has been classified as almost endangered.

Other native species, such as the “Bakuli” and “Biya,” are also being increasingly outnumbered by these invasive cichlids.

“Biologically, Lake Sampaloc supports a moderately diverse fish assemblage, with twelve recorded species representing three orders and eight families (Briones et al., 2016). This community comprises Fenced and Feral: Farming and Spread of Flowerhorn (Amphilophus spp.) in Lake Sampaloc, Native species such as Leiopotherapon plumbeus (Ayungin), Giuris margaritacea (Bakuli), and Glossogobius spp. (Biya) persist but are increasingly outnumbered by non-native fishes like the most commercially farmed Oreochromis spp. (Tilapia)”

According to Ragaza and Gasmen, the Flowerhorn’s phenotypic plasticity has allowed it to thrive in degraded environments.

Citing different studies, they pointed out that Lake Sampaloc currently faces a “mounting pollution burden,” including trace organic pollutants such as pharmaceuticals and industrial compounds, as well as high concentrations of microplastics.

While these conditions stress sensitive native species, Flowerhorns remain well-established, potentially benefiting from the nutrient-rich runoff produced by intensive tilapia farming.

“These dynamic conditions not only destabilize the native biota but also strengthen the competitive advantage of Flowerhorn cichlids, whose phenotypic plasticity and physiological tolerance enable survival and dominance under anthropogenically stressed environments,” the researchers said.

In an interview with Philstar.com, Gasmen showed the difference in the catch between Ayungin and Flowerhorn fish in the lake.

She showed that Flowerhorns significantly outnumbered Ayungin in the catch.

When asked why authorities should help resolve the threat, she said the fish compete for essential resources with native fish in the lake.

“Lake Sampaloc does support native fish species and one of the most important is the ayungin, which is currently classified as vulnerable,” Gasmen said.

“Over time, this competition can place additional pressure on native fish populations, potentially reducing their abundance and contributing to the decline of the lake's overall biodiversity,” she added.

Despite the threat to the lake’s ecology, fisherfolk in Lake Sampaloc have turned the threat into an economic opportunity.

In an interview with Philstar.com, Gasmen said wild Flowerhorn entering tilapia pens are retained and sold; some pens now intentionally stock them

“While residents of Barangay IV-A do not consume the fish, visiting locals from Laguna Province and nearby areas were observed catching Flowerhorn cichlids during recreational fishing and incorporating them into home-cooked meals,” she said.

“These individuals depicted the taste and texture of the fish as fattier and oilier than tilapia, giving it a distinct culinary profile,” she added.

However, Gasmen warned the public not to consume the fish without “risk assessment.”

“We do not recommend consumption pending risk assessment for parasites and contaminants; if consumed, fish must be thoroughly cooked,” Gasmen said.

When asked what should be done to mitigate the fish’s proliferation, Gasmen explained that the fish gather around fish cages and shallow, vegetated areas, allowing for their straightforward removal through community efforts.

“It can be processed into a potential fishmeal ingredient so it can potentially be integrated into feed formulations for cultured species," Gasmen said.

Gasmen also pointed out that methods are available to evaluate the threat posed by non-native freshwater fish invasions. One example is the Aquatic Species Invasiveness Screening Kit (AS-ISK), which she said was successfully used in a 2024 assessment of Lake Naujan in Oriental Mindoro.

“Standardized invasion risk assessment tools, such as the Aquatic Species Invasiveness Screening Kit (AS-ISK) applied by Gilles et al. (2024), could be incorporated into national freshwater management programs to identify high-risk non-native fishes before they become established,” Gasmen said.

“Combined with continuous ecological monitoring, habitat restoration, public education, and collaboration among agencies such as the DENR, BFAR, LLDA, local government units, and academic institutions, these measures would strengthen the conservation of Philippine freshwater ecosystems while balancing biodiversity protection with sustainable livelihoods,” she added.

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