Whale sharks are literally scarred by human activity

The sight of a whale shark, the largest fish alive today, leaves most people humbled. Its slow, graceful movements and gentle nature often make them symbols of the ocean’s mystery. Yet behind this serene image lies a struggle for survival.

Populations have fallen sharply, and their biology makes recovery slow. The findings of a new study from Indonesian waters not only reveal the scale of human impacts but also highlight why protecting whale sharks is vital for ocean health.

Whale shark decline accelerates

Over the last 75 years, global whale shark numbers have dropped by more than half. In the Indo-Pacific, declines are even steeper.

Hunting, habitat disruption, and entanglements push whale sharks closer to extinction. Because they take up to 30 years to mature, populations rebound very slowly.

Large-bodied species like whale sharks face unique risks in ecosystems since they grow slowly, reproduce late, and invest heavily in fewer offspring.

The researchers noted that the disappearance of such giants can ripple through marine systems.

Injuries trace back to people

In the Bird’s Head Seascape of West Papua, the research team found 62 percent of whale sharks bore scars or wounds, mostly from human actions.

“We found that scars and injuries were mainly from anthropogenic causes, such as collisions with ‘bagans’ – traditional fishing platforms with lift nets – and whale shark-watching tour boats,” said Dr. Edy Setyawan of the Elasmobranch Institute Indonesia.

“Relatively harmless minor abrasions were the most common. Serious injuries from natural causes such as predator attacks, or from boat propellers were much less common.”

Patterns in ecology show that when human activities concentrate near feeding or breeding grounds, injury rates increase significantly.

According to the study, direct human pressures such as vessel strikes and fishing gear interactions remain among the greatest global threats to megafauna. Whale sharks are no exception.

Following the gentle giants

Between 2010 and 2023, the scientists tracked whale sharks across Cenderawasih Bay, Kaimana, Raja Ampat, and Fakfak. This biodiversity hotspot hosts 26 marine protected areas. Using the distinctive spot patterns of each shark, the researchers identified 268 individuals.

Nearly all sightings occurred near bagans, where whale sharks fed on baitfish like anchovies and sprats. Most were juvenile males, four to five meters long. More than half were resighted, including one young male recorded 34 times over three years.

From an ecological lens, these observations support a classic theory: young individuals often use coastal waters rich in food, while adults migrate offshore into deeper, less accessible habitats.

Such ontogenetic shifts are common across marine megafauna and are essential for population survival.

Injuries reshape populations

Of 206 sharks with scars, over 80 percent carried human-caused injuries. Serious wounds, including lacerations and blunt trauma, were rare but concerning. More often, sharks bore superficial abrasions from rubbing against fishing gear and boats.

Studies across marine ecology emphasize that while minor wounds may not kill, cumulative injuries reduce overall fitness.

Injured animals often change behavior, avoid key feeding zones, or expend more energy healing. Over time, this shapes population dynamics.

Female whale sharks live in deep seas

A lingering question is the absence of females and older individuals, since previous studies show that adult whale sharks – especially females – prefer the deep ocean where they feed on prey like krill and schooling fish.

By contrast, younger males remain near shore in shallow, plankton-rich waters that help them grow quickly.

“Our own satellite tracking data also show that females and adults frequently use deep sea features such as canyons and seamounts,” said study co-author Mochamad Iqbal Herwata Putra.

Ecological theory explains this pattern: sexual segregation often arises in large marine vertebrates, with males and females exploiting different ecological niches to reduce competition and improve survival chances. This strategy is a driver of population resilience across species.

Tourism brings dangers to whale sharks

Staying near coasts, whale sharks offer strong potential for ecotourism.

“Whale sharks in Cenderawasih Bay and Triton Bay (Kaimana) had high rates of residency and resighting, indicating that they should be viewed as valuable tourism assets for local communities and governments,” said Dr. Mark Erdmann of Rewild.

However, unmanaged wildlife tourism can create a risky cycle: attraction leads to proximity, which heightens disturbance and injury risk.

Small changes, big impact

The researchers recommend small but effective changes, such as modifying fishing platforms to remove sharp edges.

“We aim to work with the management authorities of the marine protected areas to develop regulations to require slight modifications to the bagans, including the removal of any sharp edges from boat outriggers and net frames,” said Erdmann. “We believe those changes will greatly reduce scarring of whale sharks in the region.”

Ecological models highlight that even minor adjustments in human practices can dramatically reduce stress on threatened populations.

Protecting whale sharks is more than conserving one species. It safeguards a keystone component of marine food webs, maintains ecosystem balance, and secures benefits for communities who depend on healthy seas.

The study is published in the journal Frontiers in Marine Science.

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