How deep does a seal dive?

Reading time 4 minutes · Part of: Knowledge base

The short answer

A Dutch common or grey seal dives on average 25 metres deep and stays underwater 5 to 10 minutes. In the relatively shallow North Sea they simply cannot go much deeper. Where the water is deeper they do: a harbour seal off Greenland was recorded at 631 metres, and grey seals at up to 477 metres. Dives of fifteen to twenty minutes occur.

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), animals simply cannot dive deep.

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. High CO₂ tolerance

Where we feel an irresistible urge to breathe with rising CO₂, seals respond far less strongly to CO₂. That lets them stay underwater for long periods without the urge to breathe forcing them up — 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 record holders: elephant seals and the Weddell seal

The true deep divers among seals are the elephant seals. A southern elephant seal was recorded diving to 2,388 metres, a northern elephant seal to 1,735 metres; half-hour dives are routine for them. The Antarctic Weddell seal (Leptonychotes weddellii) dives to about 600 metres under the pack ice and can stay under for more than an hour, hunting fish and squid.

These deep divers use the same adaptations as our seals, only more extreme: a very large blood volume, huge amounts of myoglobin in the muscles and a strong drop in heart rate during the dive.

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