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The How and Why of Arctic Char in Ireland.

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The How and Why of Arctic Char in Ireland.
Myles Kelly

The distribution of arctic char. Populations outside the circle are landlocked, relict populations.

The arctic char, Salvelinus alpinus, is an anadromous fish. In this respect it is rather like the salmon and sea trout, i.e. it has two modes of life. The adult fish and parr migrate to the sea when certain environmental and/or endocrinal triggers are set. They feed until they get the urge to spawn and again seek out freshwater, perhaps the stream where it first hatched from the egg. This is the scenario for arctic char in their "normal" home range, above 60° Latitude. Here they can attain great weights and form a major part of commercial and subsistence fisheries for the indigenous peoples of Alaska, Canada, Greenland etc. (See Char Biology).

The arctic char in Ireland and most of Europe are regarded as relict species, left overs from a different era. In Ireland most people are familiar with the Burren, Co. Clare. This is a vast karst landscape which is host to a unique assemblage of plants. Plants which today are only found in the Mediterranean coast, the Alps and even further afield. These rare species of plant are also relict species, left over from the last Ice Age. As was the char.

About 13,000 years ago the climate in Ireland began to improve. For the last number of thousand of years most of the nortern hemisphere had been beset by enormous glaciers making the life nearly impossible. But it would be wrong to consider Ireland at this time to be a polar desert. During the very coldest phases there may have been little other than small patches of arctic-alpine plants and perhaps beetles in ponds or amongst the vegetable debris. Most of the time conditions would have been relatively genial with ample fodder for reindeer, giant elk and wooly mammoths.

Arctic char would have run the rivers to spawn. But as conditions improved they would have slowly retreated with the colder conditions northwards. As they went, they would have continued to feed in the sea and return to rivers and streams to spawn. So their retreat from Ireland would been gradual and slow, like the melting of the glaciers.

As the Ice melted and shrank back the landscape began to dramatically alter. Fifteen thousand years ago, as Ireland experienced the end of the advancing ice sheets, sea level was about 130m below its present level. As the ice melted sea level rose until it was about 4m more than today (eustatic sea level change). But, the enormous weight of the ice was becoming less of an influence on Ireland. The land relieved of its burden began to rise (isostatic lift). Initially the rate of sea level rise would have been greater than the eustatic lift. The changing currents and tides would have had a dramatic effect on the coastline. Then the rate of isostatic lift increased and features such as beaches were lifted high above sea level, up to 20m in Northern Ireland.

Some of the changes would have been more dramatic than others taking perhaps only a couple of years to affect the landscape. These are changes that would have led to arctic char becoming trapped in Ireland. Rivers may have stopped running to the sea and instead filled large lakes, or the the streams running from them may have become impassable. Spawning and mending adults may have become cut off from the sea along with fry and parr. These fish would have adapted to their new conditions or died. Initially the trapped char probably lived like Irelands present brown trout, living in rich (though colder) lakes and spawning in feeder streams. But the gradually rising temperatures would have caused many of the populations problems, heat stress would have weeded out those which could not adapt. Today char in Ireland are only found in cold, deep lakes, such as the corrie lakes in the more mountainous areas. It is not unlikely that these cold lakes provided a refuge for this arctic species. Unfortunately these lakes are characterised by being rather poor in nutrients and the char developed into a much smaller fish than that which exists further north today. Even in the richer lakes where the char hung on, L. Mask, L. Conn, L. Neagh and L. Melvin for example, the fish became a smaller race, perhaps due to the relative warmth the experienced.

A typical char from Lough Talt. This is an example of a char which feeds primarily on plankton including invertabrate fauna found in the mid waters such as chironomid pupae etc. Their mode of feeding also changed. Almost all trapped populations which are known today are strict planktivores. There are some races which are benthic feeders and some which do both, but in general the char changed from a large pisciverous predator to a diminutive invertebrate feeder with a correspondingly small mouth. These changes have been
noted in Scotland, England, Wales, Mainland Europe and Iceland .
It is believed that one of the reasons the char became a smaller fish is that when conditions and the landscape allowed, brown trout colonised many of the same waters as the char. Perhaps the char was confined to the cool waters by rising temperatures, could not run to the sea due to a lost behaviour and the more aggressive trout competitivley excluded it from the prime feeding zones, forcing it to adapt to life as a midwater fish.

Char were not the only fish trapped in this manner. It is believed that the Sonaghen, Gillaroo and Ferox trout of L. Melvin became cut off from the sea for similar reasons. These trout would have been derived from ancient sea trout stocks (migratory trout). The Sonaghen also became a midwater fish, perhaps evolving tihs behaviour due to competitive exclusion by the other forms of trout, and eventually becoming a seperate species. Modern brown trout (plain old Salmo trutta) are also in the lake, these fish may have arrived when conditions became approproate in more recent millenia.