The key to the history of our freshwater fishes lies in the series of Ice Ages which covered Britain in varying degrees through the Pleistocene period (1.8 million to 10,000 years ago). During the times of ice, the vast majority of what is now Britain was frozen and uninhabitable, but during interglacial periods – when the ice retreated northwards and the weather warmed up a little – many fish species that we would be familiar with colonized the rivers and lakes left by the melting ice.
The last major ice age occurred about 50,000 years ago, when ice from the Arctic advanced to smother much of northern Europe, including Britain. The mountainous regions of southern Europe, such as the Alps and the Pyrenees, were also surrounded by ice, so that the climate of the whole of Europe turned much colder. During this period few — if any — freshwater fish existed in Britain and those that had swum in before were wiped out by the cold or migrated south through the sea to warmer areas.
Ice out, fish in
About thirteen to fifteen thousand years ago the ice cap retreated and freshwater fish once more found parts of what was to become Britain habitable. The countryside left clear of ice was still cold and barren, like much of the Arctic tundra today. To us it would have seemed wild and inhospitable -but fish soon moved in.
The first type of fish to appear in this wasteland would have been from the Euryhaline group. These are fish that flourish in cold water, whether running or still. They include char, eels, houting, lampreys, mullet, powan, salmon, shad, trout, ven-dace, smelt, sticklebacks, sturgeon and even flounders.
Most found their way from the sea into the newly created rivers running from the retreating ice cap to the sea. They are all from families that can tolerate a certain amount of salt and they were the first fish our earliest ancestors caught and ate.
As the ice retreated it gouged out great hollows in what is now Scotland, Wales, the Lake District and parts of Ireland. The glaciers melted, filling these hollows with water and creating huge lakes. Many of the fish that entered the rivers swam into these lakes and began to live there, some of them even being cut off from the sea for ever as the land rose (relieved from the crushing weight of the ice) or trapped in waters enclosed by great deposits of soil and rock scraped up by the glaciers and then dumped as they melted. In this way we now have our Arctic char which are completely landlocked, unlike their cousins in North America and Greenland that migrate from fresh water to the sea. The same is also true for the whitefish species.
Warmer water, more fish
As the ice moved away north, the weather warmed and the lakes and rivers in what is now southern England became more attractive for other types of fish. This second group, referred to as the Stenohaline type, numbers such species as barbel, bleak, bream, chub, dace, grayling, gudgeon, minnows, perch, pike, roach, ruffe, tench, and the now extinct burbot. These fish did not enter our waters by sea for they can’t tolerate salt. Rather, they came by a ‘land’ route.
At that time, around thirteen thousand years ago, the southern counties were connected to northern Europe by a land bridge that ran across what is now the English Channel. Indeed, it’s possible that the Thames and even the Trent were tributaries of the Rhine and that fish from that colossal river system simply moved gradually over the land bridge into our own waters. Even if the Thames had not been connected to the Rhine, fish would have spilt on to the floodplains of northern France and then spread easily to southern Britain by flood, birds or man himself.
This process took thousands of years, and the Rhine itself was populated with fish from the Danube Basin, an area that had escaped the worst of the glacial freezing.
Reaching the limits
Both types of fish, the Euryhaline and the Stenohaline, were limited in their spread in
Biitain. The first group found the south too warm for their liking, so they tended to keep to the north and west where the weather was cooler and the water was still cold close to the ice cap.
In the same way, the fish that had come over from Europe found the north too cold for them and they tended to remain in the more mellow south. Hence, Ireland, Scotland and large areas of the south-west did not receive any of these species.
In fact, Britain as a whole did badly. Today we have just over 50 freshwater species compared with around 200 in Europe as a whole. A major reason for this is again the impact of the ice cap. It continued to melt as it retreated north, and as it melted the level of the seas rose. Gradually that vital land bridge between England and Europe was submerged and further entry of more fish species became impossible.
Making the most of it
What nature does, and does very well, is to spread the species already in existence. For example, in times of flood a river bursts its banks and takes the fish stocks to adjacent low-lying meadows. Ditches and ponds therefore become stocked with bream, perch, roach or whatever the river holds. This is not a rare occurrence – it happens even today in most winters. When a mighty river like the Wye or Trent rises in a serious flood, fish can be carried for dozens of miles. Sometimes we treat with scepticism the tale of fish being spread by herons and other wading birds, but this really does happen. Herons have a real feast during the spring when fish are spawning in the shallows. It is then that they do most of their wading, and the slimy, sticky eggs of many fish species adhere to their legs. Herons have quite a wide range and when they fly off to the next lake or river, these eggs wash off and, if conditions are suitable, hatch out and a new population of that species is established. Fish stocking in this way does not happen overnight, but if a species advances just ten or so miles every couple of spawning years, it can make very wide advances over the centuries.
This, then, would have been the situation in the British Isles when the Romans landed (Julius Caesar invaded in 55BC). Roman anglers (the Romans were very keen fishermen) would have found bream, chub, dace, roach and tench and so on scattered around the southern counties as far north as Northumberland and as far west as Shropshire and Devon. And intrepid Roman anglers moving into Wales, Scotland, Ireland or Cornwall would probably have found salmon in plenty, many brown trout, char if they could catch them and eels virtually everywhere.
Today anglers obviously have a far greater variety of fish to pursue. Many species have appeared in these islands during the last 2000 years as nature was given a helping hand by man. Some were brought in for food — others for sport.