Imagine sitting on the cliffs of Dover contemplating the sea on a crisp spring day. Today your eye would be drawn by the crawling shapes of cargo vessels, ferries and fishing boats.

Wind back the clock to the seventh century, however, and the scene would be very different. Instead of shipping, you would watch the passage of great whales on their northward migration from African wintering grounds to Arctic feeding areas. At the season's peak, over a thousand whales might pass in a day.

Today few whales are sighted in the English Channel, because we have decimated their numbers by hunting.

The slaughter began in the Bay of Biscay and English Channel around the ninth century and, by the early Middle Ages these abundant animals sustained a vigorous whale fishery that was conducted from coastal bays and inlets along their migration routes. Records suggest that numbers were declining as long ago as the 12th and 14th centuries.

The depletion of those stocks offers a good explanation for why Basques whalers were so quick to exploit newly-discovered Arctic and Canadian whale populations in the late 16th and early 17th centuries.

Over the following centuries – in Scotland right up until after the second world war – whales were pursued relentlessly. Those left are a small fraction of former numbers.

By the 18th century, for example, the Atlantic grey whale had been driven to extinction. Nowadays, despite being protected, the northern right whale is down to the last few hundred animals and faces the same fate.

How do we know how big whale populations once were? Whaling records, historians and others all describe the abundance of these beautiful creatures. One 16th century writer reported how whales were “ever present, familiar guests” around the coasts of Scandinavia.

Nowadays we also have DNA studies, showing a level of genetic diversity that could only have been achieved by huge numbers of animals.

How different the seas must have been then, in both spectacle and ecology, but it is not just whales that have dwindled over the centuries.

Our propensity to pursue marine wildlife extended beyond whales to porpoises, dolphins, basking sharks, angel sharks, tunny, skate and halibut and a host of other ocean megafauna.

Bone remains from medieval times tell of a Humber Estuary population of bottlenose dolphins that disappeared for good over a hundred years ago.

In the 18th century, porpoises were described as so common they sometimes darkened the sea as they rose to draw breath. Large predators were sustained by populations of prey fish, pilchards, herring, sprat and others, far greater in abundance than those present today.

In the United States, an unexpected consequence of the depletion of large sharks, like tigers and hammerheads, has recently been uncovered. When the big sharks disappeared one of their former prey items, cownose rays, flourished, in turn munching their way through any bay scallops they could find. Few would have predicted that shark fishing could cause the collapse of a lucrative scallop fishery.

Grey whales are submarine bulldozers, feeding on clams and other animals buried in the seabed. In the Pacific, historic populations of grey whales numbering near 100,000 animals once raised as much sediment in the Arctic as is dumped today by the equivalent of 12 Yukon Rivers.

Steve Palumbi of Stanford University estimates that nutrients in this sediment would have fuelled plankton blooms that would feed a million seabirds. There are no Grey Whales left in the Atlantic, but their role as ecological engineers has been replaced by prawn trawls that raise millions of tons of sediment as they sweep back and forth in chilly northern seas.

It is difficult to know in how many other ways the ecology of our seas has been restructured as a result of hunting and fishing. Historical ecologists will argue over this subject for years to come. For the rest of us, the loss of the seas’ spectacular megafauna is a matter for sadness and regret.

Callum Roberts, Professor of Marine Conservation, Environment Department, University of York, York, YO10 5DD. Email: cr10@york.ac.uk

More information on historical losses of marine megafauna

http://www.york.ac.uk/res/unnatural-history-of-the-sea

The Wildlife Trusts marine megafauna campaign

http://www.wildlifetrusts.org/?section=marinebill:seasofplenty

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