Imagine a sea filled with huge fish. One in which cod were twenty times more abundant than they are today, their lithe green bodies pressed close amid seaweed thickets; where predatory packs of dogfish twenty miles long crowded haddock shoals into bays and coasts.

Further out to sea schools of giant bluefin tuna would have preyed on plentiful herring, pilchard and sprats while skates the size of dining tables glided like the shadows of passing clouds across the seabed and leopard spotted wolf-fish with bulbous brows and toothy grins, guarded almost every cave and ledge of coastal reefs.

These are not the waters of some far-off place, but the seas of the United Kingdom in the early 19th century, before the onset of industrial fishing.

Eye-witness accounts give us a window to the past that can open our eyes to the extraordinary vitality and fecundity of our seas.

Oliver Goldsmith reported in 1776 that the porpoise is found “in such vast numbers, in all parts of the sea that surrounds this kingdom, that they are sometimes noxious to seamen, when they sail in small vessels. In some places they almost darken the water as they rise to take breath.”

Goldsmith was also struck by the arrival of the herring shoals around UK coasts in spring and early summer, a spectacle that must have ranked as one of the world’s greatest wildlife phenomena.

He said, “The whole water seems alive; and is seen so black with them to a great distance, that the number seems inexhaustible…Millions of enemies appear to thin their squadrons. The fin-fish [fin whales] and the cachalot [sperm whale] swallow barrels at a yawn; the porpus [porpoise], the grampus [pilot whale], the shark, and the whole numerous tribe of dogfish, find them an easy prey, and desist from making war upon each other…and the birds devour what quantities they please.

“By these enemies the herrings are cooped up into so close a body, that a shovel or any hollow vessel put into the water, takes them up without farther trouble.”

Those days are now gone. Our seas have been stripped of most of their wildlife by over a century of barely controlled industrial exploitation.

Giant skates, angel sharks and sturgeon have almost disappeared. Bottlenose dolphins are down to the last few hundred individuals. Whales were a common sight in the past but today are rarely seen following centuries of depredation by whalers.

Only a few species do better today than in the early 19th century. Seals and seabirds like gannets and guillemots, for example, have prospered following protection from hunting and egg collecting. Prawns and scallops have grown in numbers as the ranks of their predators thinned. But overall, the picture is grim. Taken together, the weight of large bodied fish present in our seas has fallen by around 95% since the 1830s. The scale of these losses is hard for us to grasp today because much of it happened before our lifetimes.

All is not lost, however, because most of these species are still present somewhere in our seas. We have it within our power to restore the abundance and productivity UK waters once supported.

Many scientists now recommend, myself among them, that we should set aside 30% of UK seas in a network of marine reserves that are off limits to exploitation.

These reserves would provide havens in which fish and their habitats could recover. And because big fish produce many times more offspring than small ones, reserves could help breathe life back into surrounding fishing grounds, helping to recover the fortunes of fishers too. All it would take is the right political leadership.

Professor Roberts' award-winning book, The Unnatural History of the Sea, documents the effects of 1,000 years of fishing and hunting on the world’s marine life. www.york.ac.uk/res/unnatural-history-of-the-sea

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