'The gillaroo," wrote Justice Kingsmill Moore, in his classic of fly-angling, A Man May Fish, "is the panther of the water, the loveliest of our fish. On a background of satin wood are scattered spots and blotches of red, orange, umber and burnt sienna, so thickly as to touch and interfuse.
Manley Hopkins must, I think, have had in mind the gillaroo when he wrote: "Rose-moles all in stipple upon trout that swim." Half a century on, the gillaroo of Lough Melvin, at the border of Co Leitrim with Co Fermanagh, still nose hungrily after caddis larvae and water-snails on the bed of the lake. Above them, in mid-water, dark-finned sonaghen dash after water-fleas and midge-pupae. In distant shadows, lean and ferocious-looking ferox, with black spots and lots of large teeth, stalk the lake's population of Arctic charr.
Gillaroo, sonaghen and ferox are the lake's unique trio of trout, living beside ordinary brown trout, sea trout, salmon and charr in one of the few pristine salmonid lakes left in north-west Europe. Long distinguished by their common names, and more recently by genetic fingerprinting, they are now identified, by DNA examination, as three distinct species - not just "forms" or "types" of the brown trout, Salmo trutta.
The genetic research has been led by Dr Andrew Ferguson, of Queen's University, Belfast, who has spent some 25 years studying Melvin and its unique array of fish. He has found the gillaroo, sonaghen and ferox not only looking different, eating different things and following different lifestyles, but homing to widely-separated spawning areas in Melvin's tributaries, so that there is no interbreeding.
His confirmation of their DNA diversity has, however, arrived at a time of unprecedented threat to the lake, after thousands of years of a stable ecosystem. April is the high point of Melvin's fishing season, and Dr Ferguson fears that this spring's anglers could bring with them the alien plague of the zebra mussel, already at high density in Lower Lough Erne, a mere 10 kilometres distant.
The mussel, Dreissena polymorpha, native to south-eastern Europe, arrived in Ireland in the mid-1990s on the hulls of old canal boats brought from England and has now spread through much of the Shannon/Erne system and beyond. It reproduces prodigiously, spreading like a living lava to smother native species and filtering the water of the plankton that feeds fish, such as sonaghen and charr. As water clears and more light reaches lake or river-bed, entire ecosystems begin to change.
The angling club at Garrison has expanded the supply of Melvin's own boats and there is a scheme for washing boats and engines brought from outside. But precautions against importing the zebra mussel need clinical care: not just hosing and scrubbing, but draining the bilges, emptying the bait cans and mopping out any other water that could carry the mussel's free-swimming larvae.
So far, Lough Melvin has survived like a valley of lost tribes. More than 20 kilometres square and glacially deepened in places to some 50 metres, it was populated with fish that came in from the sea as the ice thawed away from Ireland about 13,000 years ago. It is miraculously unspoiled by fertiliser run-off, sewage or silage pollution, over-fishing, or any temptation to "improve" the lake by re-stocking with hatchery trout.
Its designation as a cross-border "Special Area of Conservation" goes far to guard its water quality. But it now also needs urgent protection from the zebra mussel and nothing less than a total ban on outside water craft seems likely to provide that. Any angler heading for Melvin with a boat on a trailer should look hard at his ecological conscience.
Meanwhile, Andrew Ferguson continues to confront what he and other ecologists call the "chaotic state" of systematising and naming Europe's freshwater fishes: notably the salmonids and, in particular, the brown trout.
He has faced a long-standing scientific reluctance to split the brown trout into further species or sub-species. Homing to natal rivers and often physically isolated from each other, they can form distinct populations, often with highly variable body colours, and Victorian gentleman anglers, not to mention the early fish biologists, proposed dozens of different species in Europe.
Lumping them all together as brown trout, Dr Ferguson argues, has served to distort the EU's species-based conservation policies. The widespread "brown trout" is not a threatened species calling for conservation, but a lake with species such as Melvin's becomes a treasure of biodiversity.
Recognition of Melvin's "remarkable fish community" comes high, as it happens, in the new Dúchas booklet, Living with Nature, which gives an overview of how and why conservation sites are selected - but still no trout species is listed there under the EU Habitats Directive.
Even away from Melvin, there are fish unique to Ireland. Most remarkable is the pollan, the Irish descendant of an Arctic species of whitefish, Coregonus autumnalis, which got left behind post-glacially. Its three populations, in Lough Neagh, Lough Erne and the Shannon, have been isolated from each other for several thousand generations, and the pollan, in Dr Ferguson's view, is now "one of the most unique components of the Irish fauna overall."
Melvin shares the big, long-lived and "ancestral" ferox trout with Corrib, Erne and Mask. Lough Neagh has brown trout, known locally as dollaghan, and salmon-trout. But even beyond the differences to be expected in isolated lake populations, genetics also distinguish the sea trout from adjacent rivers, so precise is their natal homing.
Indeed, as Andrew Ferguson likes to point out, "there is some five times more genetic diversity among brown trout populations in Ireland than among human populations throughout the world".
But, for the moment, as anglers launch their boats on Melvin, could we at least spare a thought for the gillaroo, the sonaghen and the ferox?
Living with Nature, which also lists the names and telephone numbers of more than 70 local conservation rangers, is available from Dúchas, 7 Ely Place, Dublin 2.
I live in a house right beside the river Blackwater, south of Cappoquin. The river is tidal outside my house to a depth of about 12 feet at spring tide, but there is only a very faint trace of salt in the water. Frequently, in the early morning, I see a heron sitting at the end of the quay, head sunk, like a little bent, crabby old man, waiting for the tide to go down so that he can get down to some fishing. Recently, I saw him plunge off the quay into six or seven feet of water after a fish. He disappeared with a mighty splash and surfaced, floundered up to where he could take wing again, flew back to the quay, where he ate his catch, and regained his "little-old-man" pose. I have never seen nor heard of a heron taking a high dive after a fish into deep water. Has he been taking a course in fishing from the gannets?
Desmond Hall, Villierstown, Co Waterford
Herons will dive, plunge and also swim after fish.
I heard a chiffchaff in my garden in Co Limerick on St Patrick's day. Is this the first of this year's visitors? I also saw a martin on March 20th near where I live.
Elaine O'Malley, Castleconnell, Co Limerick
Your chiffchaff was an early visitor. They start arriving in early March, but most of them come in early May.
Eye on Nature is edited by Michael Viney, who welcomes observations sent to him at Thallabawn, Carrowniskey PO, Westport, Co Mayo. E-mail: viney@anu.ie. Observations sent by e-mail should be accompanied by postal address.