Ocean Sunfish (Mola mola): Welfare Challenges at Sea and in Captivity

The ocean sunfish is the world's heaviest bony fish and a frequent bycatch victim. Its welfare in aquariums and fisheries raises important questions about large pelagic fish.

Key Facts

Ocean sunfish can weigh up to 2,300 kg — the heaviest bony fish on record
They feed primarily on jellyfish and other gelatinous zooplankton
Sunfish are frequently entangled in drift nets and caught as bycatch in tuna fisheries
They are kept in a small number of major public aquariums with specialised facilities
Sunfish undergo enormous growth — from 2.5 mm at hatching to over 2 metres
They spend much of their time basking at the surface, likely for thermoregulation

Welfare Considerations

Ocean sunfish are among the most common megafish bycatch in Mediterranean and North Atlantic fisheries. Entanglement in drift nets can result in drowning or severe injury. When caught in purse seine nets, their large size means they can be crushed by other fish. In aquariums, sunfish require massive tanks — the Monterey Bay Aquarium pioneered their captive care — and early captive mortality was very high. Current best-practice aquariums still report significant challenges with feeding, stress from tankmates, and glass collision injuries.

What You Can Do

Support campaigns to reduce drift net and purse seine bycatch of large pelagic fish
Advocate for improved sunfish bycatch reporting requirements in tuna fisheries
Visit public aquariums that maintain sunfish only when they demonstrate high welfare standards
Donate to marine conservation organisations working on large pelagic fish bycatch reduction