Pike may not be invader species

 The pike, with its torpedo-shaped enamelled green body, strong tail, huge eyes and a large mouth armed with a fearsome set of teeth make the freshwater fish one of Ireland's most efficient hunters.

SCIENTISTS from Inland Fisheries Ireland, led by Debbi Pedreschi of UCD, have studied the history of pike in Ireland.

Their findings put the cat, or at any rate the fish, among the proverbial pigeons; a new theory on the origins of our ‘freshwater tiger’ has emerged.

The pike, up to now, was regarded as an introduced alien species. The Irish name ‘gailliasc’, the ‘foreign fish’, seemed to attest to that. No illustrations of this large lake-dweller appear in medieval manuscripts, although the salmon and the eel feature prominently. The first written references to pike appeared in the 16th century, suggesting that this sport fish arrived here around then.

That pike were introduced seemed a plausible theory. So-called ‘diadromous’ species, such as eels salmonids and lampreys, can move between fresh and salt water. When the ice covering this part of the world melted, 8,000 years ago, they were able to cross the sea and enter Irish rivers. Pike, and the other ‘coarse’ fish, would die if they ventured into the ocean. They couldn’t reach Ireland, or so it was thought.

Pedreschi and her team took tissue samples from 752 pike, at 15 locations, and compared their DNA with that of fish from Britain and mainland Europe. A curious genetic picture emerged. It shows that pike reached Ireland and Britain soon after the end of the last ice age, about 8,000 years ago. Nor did all of our pike necessarily originate in Britain: their genetics suggest some may have come directly from mainland Europe.

The first human colonisers were arriving in Ireland at around that time. It seems inconceivable, however, that semi-nomadic hunter-gatherers would have imported live fish. Pike, therefore, must have got here under their own steam. Although they are a freshwater species, they have some tolerance of salt water. In the Baltic, a semi-freshwater sea, they hunt herring. Perhaps what we now call the Irish Sea was a large brackish lagoon back then, similar to today’s Baltic. Rapidly melting ice could have created such a water body. If so, pike might have gained access to Irish rivers by passing through it.

This theory has its critics. Brown trout colonised our rivers as soon as the melting of the ice allowed them to do so. In due course trout communities, isolated from each other over the millennia in our rivers and lakes, evolved their own unique features. The Irish brown trout population became a patchwork of genetically distinct groups. If pike arrived in Ireland 8,000 years ago, a similar genetic divergence between its communities should have developed but there is only limited evidence of this. However, pike show remarkably little genetic variation between populations throughout their range; the species seems to have gone through a bottleneck, surviving the last ice age in only a few locations and leaving few survivors. As a predator at the top of the food chain, its numbers would have been much lower than those of the species on which it feeds.

The next significant change in the pike’s fortunes occurred 3,500 to 4,000 years ago; the Irish and British populations went their separate ways. The Irish Sea may have remained fairly brackish until around then. Once it became truly marine it presented, in Pedreschi’s phrase, ‘an impassable barrier to a freshwater fish’. This might account for the divergence between the Irish and British populations.

Then, during the Middle Ages, more pike seem to have been introduced. This may have occurred in early Norman times. The barons brought rabbits, hedgehogs, fallow deer and pheasants to Ireland. It seems that they also imported pike.

As an invasive species, which eats trout, it is often persecuted. If, however, the pike arrived here without human assistance, it has to be reclassified as a native species and protected accordingly.

* Genetic Structure of Pike and their History in Ireland. Debbi Pedreschi et al. Inland Fisheries Ireland, 2013.


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