How deep does a seal dive?

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The short answer

A Dutch common or grey seal dives on average 25 metres deep and stays underwater 5 to 10 minutes. The deepest recorded dive for our species is around 100 metres, with duration records up to 20 minutes. Most foraging dives are far less spectacular though: 10 to 50 metres, a few minutes long, in the relatively shallow North Sea.

How deep, how long?

Since the 1980s researchers have fitted seals with satellite and depth-logger tags that store depth, duration and heart rate per dive. The result: seals are extraordinarily flexible divers. On a single day the same seal can make thousands of shallow "shuttle dives" in a fishing ground, plus a handful of deep exploratory dives.

The grey seal generally dives deeper and longer than the common — fitting its preference for larger prey like cod, which live in deeper water. In the relatively shallow Waddenzee (averaging 2 to 20 metres deep), most animals simply have no need for 100 metres.

The physiology: four adaptations

A human can, at best, hold their breath for 2 to 3 minutes. A well-trained freediver reaches 6 to 8 minutes and depths to 100 metres — with significant blackout risks. A seal does this daily, while hunting along the way. Four adaptations make it possible:

1. High myoglobin content

A seal's muscles are loaded with myoglobin, the protein that stores oxygen in muscle cells. Their muscles are so dark red they appear almost black. Per kilo of muscle tissue they store up to ten times more oxygen than human muscle. On top of that, their blood volume is larger (1.5× per kilo) and the haemoglobin content is higher. A seal is, physiologically, a walking oxygen tank.

2. Bradycardia (heart rate reduction)

At the start of a dive, a seal's heart rate drops from 100 to sometimes only 10 beats per minute. This "diving reflex" (similar to what occurs in humans in cold water, but much stronger) saves oxygen because the heart does less work. Only when the animal surfaces does the heart rate jump back up.

3. Blood redistribution

During the dive, blood vessels in the skin, gut and extremities constrict. Blood is redistributed to the core areas that need oxygen most: brain and heart. Muscles work in that period anaerobically on the stored myoglobin oxygen. On surfacing, circulation is restored and the built-up lactic acid is cleared.

4. Weak breathing reflex

Where we feel an irresistible urge to breathe with rising CO₂, that reflex is largely suppressed in seals. They breathe consciously, not reflexively. That lets them stay underwater for long periods without panic or automatic inhaling — a prerequisite for both deep diving and sleeping underwater.

And the lungs?

A common question: don't seals get decompression sickness when they surface quickly? The answer is no. Their lungs are built differently: under increasing pressure the air sacs deliberately collapse (alveolar collapse) and the air stays only in the airways, where no gas exchange takes place. No nitrogen in the blood under pressure means no bubbles on relaxation. Human divers with compressed-air tanks therefore have problems that seals simply do not.

The world record: the Weddell seal

Our North Atlantic species are world-class divers, but not world record holders. That title belongs to the Weddell seal (Leptonychotes weddellii) of Antarctica: measured dives to 600 metres deep and up to 80 minutes long. Under the pack ice, in total darkness and 1.8°C water, this species hunts polar cod and squid. It is the deepest measured record among seals, putting them in the same league as ambitious whale species.

Only recently has it become clear how they do it: in addition to all the adaptations above, Weddell seals have twice the blood volume per kilo and a specially adapted myoglobin that doesn't clump at high concentrations — an evolutionary chemical trick that scientists are now trying to mimic for human medicine.

More about the Antarctic relatives at species worldwide; on anatomy itself at seal anatomy.