Deep-Sea Fish and Fish Oil: Biology, Sources, and What the Evidence Says

Deep-sea fish inhabit the aphotic layers of the ocean and display adaptations such as large mouths and bioluminescence. Commercial fish oils are typically derived from small oily pelagic species, though shark liver has been used historically. EPA and DHA in marine oils have potential benefits for heart health, brain development, and inflammation, but clinical trial results vary by dose and population. Choose tested products and consult a clinician for therapeutic use.

What counts as "deep sea"?

The ocean layers are often described by how much light reaches them. The sunlit (photic or euphotic) zone extends roughly to about 200 meters. Below that is the disphotic zone (twilight) and then the aphotic or "deep-sea" zones, where sunlight no longer supports photosynthesis. Many deep-sea fishes live well below ~200 m; some species occur at depths of several thousand meters.

How deep-sea fish are adapted

Life in permanent darkness has driven clear adaptations. Many deep-sea species have large mouths, extendable stomachs, reduced or highly sensitive eyes, and bioluminescent organs used for hunting and communication. Typical deep-sea examples include anglerfishes, viperfishes, lanternfishes and certain deep-water sharks. Food is sparse: animals rely on falling organic matter from surface waters, predation, or chemosynthetic ecosystems around vents and seeps.

Where fish oil comes from

"Fish oil" broadly refers to oils rich in long-chain omega-3 fatty acids (EPA and DHA). Commercial fish oil is most often extracted from small, oily, pelagic species such as anchovies, sardines, and mackerel because they are abundant and tend to bioaccumulate fewer contaminants. Historically, shark liver oil (rich in squalene and other lipids) has also been used, but many modern supplements use marine finfish or are purified concentrates.

Extraction typically involves cooking and pressing whole fish or rendering livers, followed by refining steps (molecular distillation, purification) to concentrate EPA/DHA and reduce contaminants.

Health evidence and safety

EPA and DHA have been linked to benefits for cardiovascular health, brain development, and inflammation. Clinical trial results vary by dose, formulation, and patient population. For example, a high-dose, prescription EPA preparation showed a reduction in major cardiovascular events in a trial of statin-treated patients with elevated triglycerides. Other studies and meta-analyses report mixed results, so benefits depend on how much and what form of omega-3 is taken.

Safety considerations matter: large predatory species can concentrate mercury and other pollutants, while small oily fish tend to have lower levels. Shark liver products may contain unique compounds and different contaminant profiles. Look for third-party testing (USP, NSF, IFOS) and follow medical advice for therapeutic dosing.

Practical guidance

Eating fatty fish (for example, small oily species) twice weekly remains a common dietary recommendation for general health. If you consider supplements, check the EPA/DHA content, verify purity testing, and consult a clinician for high-dose or prescription options - especially if you have existing heart disease or high triglycerides.