TAWILIS IN TROUBLE: Thomasian study reveals feeding shifts in endangered Sardinella tawilis of Lake Taal

Sardinella tawilis, the world’s only freshwater sardine, is found exclusively in Lake Taal in Batangas—a lake formed by a rare geological event: the collapse of a volcanic caldera. 

Once a marine inlet, the lake was sealed off and gradually turned fresh, forcing marine species to adapt or disappear. Tawilis did the latter. 

Today, however, this species is endangered, and its feeding ecology tells a story of a lake under pressure.

Led by researchers from the University of Santo Tomas, a new study set out to answer one question: What happened to the lake’s most iconic fish?

According to the study, between 1999 and 2011, tawilis harvests declined by nearly 90%. By 2018, the species had lost over half its population and earned a spot on the International Union for Conservation of Nature (IUCN) Red List as endangered. 

The reasons were many: overfishing, pollution, aquaculture, habitat loss, and increasing competition from invasive species. And then in 2020, Lake Taal erupted.

The lake’s chemistry changed overnight. Ashfall clouded the water, oxygen levels dropped, and visibility worsened. For a species already in crisis, this disruption posed another serious blow.

So, they looked not at the surface—but inside the fish itself.

To understand what was happening beneath the surface, researchers turned to two investigative tools in aquatic ecology: Gut Content Analysis (GCA) and Stable Isotope Analysis (SIA).

These methods offer complementary views; one short-term, one long-term, into the feeding ecology of a species.

GCA is like taking a peek at what the fish just ate. Scientists examine the actual contents of the fish’s stomach to identify its most recent meals. It gives an immediate snapshot of diet but only reflects the last few hours or days before capture.

SIA, on the other hand, goes deeper. By analyzing the chemical composition of tissues, specifically the ratios of stable carbon and nitrogen isotopes, scientists can reconstruct what a fish has been eating over weeks or even months. Different types of prey leave distinct isotopic “fingerprints” in the body. SIA shows not just what a fish ate, but how energy flows through the food web over time.

By combining both techniques, the researchers can uncover both short-term feeding habits and long-term ecological changes.

According to their findings, before the eruption, GCA results showed tawilis fed mainly on calanoid copepods—large, energy-rich zooplankton that lived in the lake’s deeper, clearer layers.

But after the eruption, those prey nearly disappeared. In their place came smaller, less nutritious organisms like cyclopoid copepods and tiny cladocerans such as Moina micrura, Bosmina fatalis, and Ceriodaphnia cornuta, all different species of water fleas. These now make up over 90% of the sardine’s diet.

SIA confirmed the pattern. It showed that the fish had adapted over time to rely increasingly on low-energy prey that dominate murky surface waters.

Why the shift?

It’s not about taste—it’s about survival in clouded waters.

According to the study, tawilis is a visual feeder, relying on clear water to hunt. But after the eruption, ash and nutrients turned the lake turbid. Light no longer reached the depths. The prey that once thrived in deep, clear waters became harder to spot. So tawilis turned upward, shallower, and toward what it could still see: slower, less mobile zooplankton that had adapted better to murky conditions.

But these new prey are less nutritious, smaller in size, and could lead to declining reproductive success and slower growth if sustained long-term.

Still, in terms of size and spawning, tawilis has held its ground. 

According to post-eruption measurements, on average, the fish still measured 11.08 cm long and weighed about 12.73 grams. The timing of its peak spawning season also stayed the same, with the gonadosomatic index (GSI)—a measure of reproductive readiness—peaking in February for both males (GSI: 4) and females (GSI: 5). The fish remains outwardly the same, but its ecosystem tells another story. 

Tawilis sits in the middle of Lake Taal’s food web. It’s both predator and prey—controlling zooplankton populations and feeding larger fish and birds. A change in its diet reflects a wider ecological imbalance.

According to the research, the lake has been trending toward eutrophication, or nutrient overload, for years. The study attributes this to human-caused pressures, especially the excessive feed and waste from fish cages, runoff from nearby farms, and untreated domestic sewage from communities around the lake. 

These inputs have led to high turbidity, more microbial activity, and lower oxygen levels— signs of a nutrient-overloaded lake.

As early as before 2020, Lake Taal was already showing these symptoms. But when Taal Volcano erupted, the ash and chemicals it released into the lake intensified the damage, accelerating a decline that was already in motion

But it wasn’t just the prey  that changed. The study found that plankton—the tiny creatures at the bottom of the food web—varied between the north and south basins of the lake. 

In the north basin, where most fish cages are located, the water was dirtier and more polluted, making it harder for healthy plankton communities to thrive. 

Meanwhile, the south basin was cleaner and deeper, but it didn’t have enough food or safe spaces for plankton and fish to recover. 

This shows how different parts of Lake Taal are being affected in different ways, and how both human activity and natural features shape what lives there

The Thomasian study offered not just knowledge, it offered direction.

It urged to protect spawning grounds and enforce seasonal fishing bans, limit aquaculture expansion and regulate waste entering the lake, promote ecosystem-based management — protecting not just the fish, but the water, plankton, and balance it depends on—  and raise awareness among communities, schools, and visitors about the uniqueness, and urgency, of tawilis conservation.

Because once it’s gone, it’s gone for good.

Tawilis continues to swim—through warmer water, cloudier currents, and a diet not of its choosing. Its eyes see less. Its mouth finds less. But it swims still.

In it lives a message for all: ecosystems change slowly, then suddenly. And the ones that feel it first are the smallest—the quiet swimmers, the invisible grazers, the overlooked links in the chain.