Large Mola Mola Sunfish swimming in the ocean — Large Mola Mola Sunfish | Photo by Martin Voeller

The ocean is one of those places that has captured our imagination since the dawn of humanity. Anyone who’s ever stood by the shore or sailed across open water has probably looked out—or down—and wondered what incredible creatures might be lurking below. Among the most fascinating are the ocean’s true giants: the whale shark, the largest fish in the world, and the giant oceanic manta ray, one of the biggest rays to ever glide through the seas.

These massive fish aren’t just impressive because of their size; they play a huge role in keeping marine ecosystems balanced. Whale sharks help manage plankton blooms, while manta rays support reef health by feeding on tiny organisms. But despite their size and ancient presence in our oceans, these species are facing growing threats. Overfishing, habitat loss, and bycatch have put tremendous pressure on their populations. Even the mighty whale shark isn’t immune, and the giant manta ray often finds itself at risk from human activity. However, many of these species are threatened by human activities such as overfishing and habitat destruction. The whale shark, despite its size, is still vulnerable to fishing and habitat loss, while the giant oceanic manta ray often falls victim to bycatch and illegal hunting.

Let’s dive deeper into the world of these ocean giants, discover what makes them so important, and look at what’s being done to help protect them for future generations.

1. Whale Shark – Up to 18 Meters (21.5 tons)

The whale shark, easily the largest fish species in the world, inhabits warm tropical waters around Australia, the Philippines, and Mexico, while the beluga sturgeon holds the title of the world's largest freshwater fish. Despite their imposing size, these gentle plankton feeders pose no threat to humans. Whale sharks are essential in controlling plankton populations, keeping ocean ecosystems balanced. Interestingly, their spot patterns are unique, much like human fingerprints, and are used by scientists to identify individuals and track migration routes.

Whale Shark in the clearest water imaginable over coral reef at Ningaloo — Whale Shark, Ningaloo Reef

2. Basking Shark – Up to 12 Meters (5.2 tons)

Another colossal but harmless creature, and one of many shark species out there , the basking shark is recognized by its massive mouth, often wide open as it filters plankton from ocean waters, making it a filter feeder. Basking sharks face heavy commercial exploitation due to extensive fishing practices for food and other products. Preferring temperate seas, this slow-moving shark is frequently spotted near the surface, appearing to “bask” in the sunlight.

Basking Shark Underwater with the mouth open — Basking Shark | Via Getty Images

3. Beluga Sturgeon – Up to 7.2 Meters (1.5 tons)

The beluga sturgeon, one of the biggest species found primarily in the Caspian and Black Seas, holds the title of the world’s largest freshwater fish. Famous for producing highly prized beluga caviar, overfishing and habitat loss have critically endangered this ancient fish. Their prehistoric appearance is fitting since sturgeon ancestors date back over 200 million years, emphasizing their importance to biodiversity and conservation.

a Beluga Sturgeon fish swimming underwater in a lake during migration season — Beluga Sturgeon | Zocha

4. Giant Oceanic Manta Ray – Up to 7 Meters (3 tons)

Giant oceanic mantas are graceful, intelligent, and among the most beloved majestic creatures by divers. Found gliding through tropical and subtropical oceans, these filter feeders feed mainly on plankton. Unfortunately, these gentle giants often suffer from ocean debris entanglement, a subject highlighted in my article about How Ocean Pollution Affects Sea Animals.

two Giant Oceanic Manta Ray swimming underwater near the bottom of the ocean — Giant Oceanic Manta Ray | Via Getty Images

5. Greenland Shark – Up to 7 Meters (1 ton)

A slow-moving, deep-sea dweller, the Greenland shark is one of the ocean’s oldest vertebrates, with some individuals living over 400 years. These ancient sharks inhabit the deep blue, cold Arctic waters, as well as temperate oceans, adding to the ocean’s mystery. Found in cold Arctic waters, their longevity and adaptability continue to intrigue marine biologists. Greenland sharks demonstrate how size and age do not always correlate with danger, a concept also explored in Bull Shark Species: 12 Interesting Facts to Know Them Better.

Greenland shark swimming, Somniosus microcephalus swims into the artificial light — Greenland Shark | Via Getty Images

6. Ocean Sunfish (Mola mola) – Up to 3.3 Meters (2.3 tons)

The bizarre-looking ocean sunfish holds the title for the largest bony fish in the sea, making it one of the ocean giants. Often found basking on the ocean surface (thus its name), sunfish consume jellyfish, helping regulate their populations. Their unusual shape and behavior highlight nature’s incredible evolutionary diversity.

a mola mola fish swimming underwater — Ocean Sunfish | Photo by Jfibu

7. Giant Freshwater Stingray – Up to 5 Meters with tail (600 kg)

Living primarily in Southeast Asia’s murky rivers, giant stingrays are challenging to study due to their elusive nature. Another notable species in Southeast Asia is the giant barb, the largest species of carp, which can grow up to 10 feet and weigh up to 550 pounds. It spends most of its time buried in sandy riverbeds. This species underscores the importance of conserving freshwater habitats, ecosystems crucial for countless species.

a Giant Freshwater Stingray on a big tank and a human trying to touch it — Giant Freshwater Stingray | Barry Rogge, CC BY 2.0, via Wikimedia Commons

8. Mekong Giant Catfish – Up to 3 Meters (350 kg)

Native to Southeast Asia’s Mekong River Basin, the Mekong giant catfish is among the world’s largest freshwater fish. Threatened by dam construction and overfishing, significant conservation efforts aim to preserve this gentle herbivore. Its vulnerability highlights broader ecological issues surrounding river management and human impacts, emphasizing the need to maintain ecological balance.

a close up of an endangered Mekong Giant Catfish — Mekong Giant Catfish | Photo by Bill Roque

9. Arapaima – Up to 3 Meters (220 kg)

Arapaimas, or “pirarucu,” can reach a maximum length of up to 3 meters and reside in oxygen-poor waters of the Amazon basin. These giants navigate the winding rivers of the Amazon basin, showcasing their adaptability. They’ve evolved a fascinating ability to breathe air directly from the surface, showcasing adaptation to challenging environments. However, their size and popularity in sport fishing have made them susceptible to overfishing, emphasizing sustainable fishing practices’ importance.

close up of an Arapaima swimming — Arapaima | Via Getty Images

10. Alligator Gar – Up to 3 Meters (137 kg)

Native to waterways in the southern United States, the prehistoric-looking alligator gar is one of the largest species of gar and a powerful apex predator. The alligator gar’s appearance is reminiscent of its ancient ancestors, as evidenced by the fossil record. With an appearance reminiscent of dinosaurs, they’re important for controlling fish populations and maintaining balanced freshwater ecosystems. Despite their intimidating looks, they pose no danger to humans.

an Alligator gar swimming underwater — Alligator Gar | Vua Wirestock

11. Paddlefish – Up to 2.2 Meters (90 kg)

The paddlefish’s unique paddle-shaped snout helps it detect plankton in murky waters of the Mississippi River Basin. While primarily found in the Mississippi River Basin, paddlefish have also been spotted in rivers along the west coast of North America. As a member of the sturgeon family, it represents a significant evolutionary success story. As one of North America’s oldest fish lineages, it represents a significant evolutionary success story. Today, habitat loss and pollution threaten its survival, underscoring the necessity for freshwater conservation efforts.

a Paddlefish swimming surrounded by other fish — Paddlefish | Photo by W Rangel

12. Sharptail Mola (Masturus lanceolatus) – Up to 3 Meters (2 tons)

This lesser-known relative of the ocean sunfish, the southern sunfish, inhabits deep, warm temperate seas. Rarely observed, much about the sharptail mola remains a mystery. They, too, consume jellyfish, helping maintain healthy marine ecosystems and demonstrating the ocean’s still-unraveling mysteries.

a sharptail mola swimming in the water — Sharptail Mola | Hectonichus, CC BY-SA 3.0, via Wikimedia Commons

Freshwater Giants: The Unsung Heroes of Rivers and Lakes

When most people think about massive fish, ocean giants usually come to mind. But freshwater ecosystems are also home to some of the most impressive—and often overlooked—species on the planet. Take the Mekong giant catfish, for example. This gentle giant inhabits Southeast Asia's Mekong River, reaching lengths of up to 10 feet and weighing as much as 660 pounds. Equally awe-inspiring is the beluga sturgeon, native primarily to the Caspian Sea. Some beluga sturgeons have grown up to 24 feet long and weighed over 3,500 pounds, making them truly colossal freshwater residents.

Unfortunately, many freshwater giants are critically endangered, largely due to habitat loss, pollution, and unsustainable fishing practices. Beluga sturgeons have been driven close to extinction because of overfishing for their valuable caviar. Similarly, Mekong giant catfish populations have plummeted due to dam construction and excessive fishing pressure. Conservationists emphasize the urgent need for action, such as protecting river habitats and establishing sustainable fisheries. By addressing these threats proactively, we can ensure these remarkable fish continue to thrive and keep our freshwater ecosystems healthy.

Fish in the Ocean Ecosystem: Vital for Balance and Biodiversity

Fish aren't just fascinating, they're essential. They form a fundamental part of marine ecosystems, supporting countless other marine creatures. The whale shark, for instance, consumes vast amounts of plankton and small fish, playing a key role in controlling these populations and maintaining ecological balance. Similarly, the giant oceanic manta ray helps sustain coral reef ecosystems by filtering plankton from surrounding waters.

Losing these marine giants would disrupt entire food chains, impacting seabirds, marine mammals, and even smaller fish species. This interconnectedness underscores the importance of conservation efforts, including protecting marine habitats and advocating for sustainable fishing practices. Preserving these species isn't just beneficial—it's essential for the ocean’s health, biodiversity, and resilience, allowing future generations to continue enjoying a vibrant, balanced marine world.

Are Bigger Fish More Dangerous?

There's a common misconception linking size with danger in the ocean, but surprisingly, this isn't always true. The ocean’s biggest fish, such as the peaceful whale shark and the gentle basking shark, pose absolutely no threat to humans—they survive primarily on plankton. In contrast, some smaller marine predators, like bull sharks, are notably more aggressive, often hunting larger prey and posing a higher risk to humans. My detailed article "Bull Shark Species: 12 Interesting Facts to Know Them Better" further explores how aggression and danger aren’t always related to a creature's size but instead its species-specific behavior.

Recognizing this difference helps us appreciate marine biodiversity more accurately, breaking down stereotypes and fostering respect for all ocean life.

Threats Facing the World's Largest Fish Species

Despite their essential ecological roles, many large fish species are critically threatened by human activities. Freshwater giants like the Mekong giant catfish face severe threats from dam constructions and overfishing, resulting in dramatically declining populations. Once abundant, these magnificent creatures are now at risk of extinction, underscoring how human impact on freshwater ecosystems demands immediate attention and careful management.

In marine environments, the situation is equally concerning. Even the gentle whale shark, the largest fish on Earth, struggles with declining populations due to habitat disruption, pollution, and accidental capture in fishing nets. Many other large marine species, such as the basking shark and top predators like the tiger shark, also face pressures from overfishing, bycatch, and habitat loss. By understanding the diets and habitats of these giants, such as exploring what whale sharks eat—we can better formulate conservation strategies to protect them.

One highly effective approach is the establishment of Marine Protected Areas (MPAs). MPAs create safe havens where fishing and other disruptive activities are restricted, allowing large fish populations to recover and ecosystems to thrive. Studies have shown that well-managed MPAs can significantly boost fish biomass, enhance biodiversity, and protect critical habitats vital for the survival of species like whale sharks and tiger sharks. Expanding and properly enforcing these protected areas is key to long-term ocean health and species conservation.

Supporting marine conservation initiatives, advocating for stricter fishing regulations, and reducing ocean pollution are vital steps toward preserving these incredible species. Protecting large fish isn't just about saving individual species, it’s about safeguarding the entire ocean ecosystem and ensuring biodiversity for future generations.

Frequently Asked Questions about the Biggest Fish:

Who is the largest fish in the world?

The whale shark is the world’s largest fish, growing up to 18 meters and weighing over 20 tons.

The largest known specimen of a whale shark measured 18.8 meters in length.

What is the largest fish that is not a shark?

The giant oceanic manta ray, reaching up to 7 meters in width, is the largest non-shark fish.

What is the largest fish ever to exist?

The extinct Leedsichthys problematicus, a giant bony fish from the Jurassic period, estimated up to 16 meters long.

What is the largest fish in Australia?

Whale sharks, commonly spotted at Ningaloo Reef in Western Australia, are Australia’s largest fish.

What is the largest known specimen of a fish?

The largest known specimen of a beluga sturgeon measures 23.6 ft long and weighs 1,571 kg.

Conservation Matters: Protecting Marine Giants

The world’s largest fish aren’t merely impressive—they play essential roles in their ecosystems. Whether filtering plankton, maintaining balanced predator-prey dynamics, or supporting biodiversity, their conservation matters greatly, including for species like tiger sharks. Challenges such as ocean pollution, climate change, and overfishing continue threatening these threatened species, making informed conservation actions crucial.

Top 12 Biggest Fish in the World: Giants Beneath the Waves