Why so many Irish trout species? It's the lough of the draw

ANOTHER LIFE:IT'S CLOSE ON 60 YEARS since Ethna's father, Seamus MacManus of Westport, won the beautiful bronze shield now hanging on our wall – the first An Tostal trophy for trout angling on Lough Mask in Co Mayo in 1953.

The exact weight of his fish seems forgotten, but it is fair to assume it came nowhere near the amazing 11kg of the monster ferox trout coaxed to the net last weekend from the depths of neighbouring Lough Corrib.

The moths have long since shredded Seamus’s prized wallet of flies (in that neglect, I fear, I was a disappointing son-in-law), but the bait for last Saturday’s great armful of trout was a blast-frozen roach, again marking out the ferox as the one species of trout that eats fish. Indeed, as the prize specimen goes off to the taxidermist to be stuffed for an eager Clonbur pub, a little chunk of its DNA seems sure to be booked for the attention of science at Queen’s University Belfast.

There, led by Dr Andrew Ferguson, a young team of geneticists have been investigating not only the several species of fish we call trout but also how, and from where, they first came into Ireland’s rivers. It has been a long and complex detective story, for, as 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.”

The classic demonstration of the diversity of Ireland’s trout is to be found in Lough Melvin, at the border of Leitrim and Fermanagh. Here, in something like a valley of lost tribes, the three best-known kinds of “brown” Irish trout live virtually separate lives, their names coined by gentleman anglers in Victorian times.

“The gillaroo,” wrote Justice Kingsmill Moore in A Man May Fish, “is the panther of the water, the loveliest of our fish.” It was, he thought, what Gerard Manley Hopkins had in mind for his image of “Rose-moles all in stipple upon trout that swim”. In Melvin, the gillaroo, richly spotted and blotched with red, orange and sienna, noses hungrily after caddis larvae and water snails on the bed of the lake.

Above the gillaroo, in midwater, the more sombre dark-finned sonaghen dash after water fleas and midge pupae. And in distant shadows the ferocious-looking ferox, with black spots and lots of large teeth, stalk the lake’s Arctic charr and small brown trout.

With their different feeding habits, and separate spawning grounds in the lake and its feeder streams, they preserve genetic distinctions with origins in distant waters before the last Ice Age.

As the glaciers thawed, and Ireland’s rivers were open again to fish, all the colonising species were anadromous, conditioned to feed in the sea but spawn in rivers. As an Ireland free of ice lifted up again, most brown trout lost the seagoing habit or found themselves cut off in lakes or above waterfalls. They took on different colourings, as camouflage in different waters, but largely kept the genetic distinctions of their origins.

Where had they come from? The latest genetic study by the Queen’s team, led by Ferguson’s student Niall McKeown, analysed the DNA of 3,636 trout from 83 lakes and rivers in Ireland and Britain. It suggested at least five “potential glacial refuges” from which they had travelled to colonise the islands, including one west of Ireland and another in the Celtic Sea.

Many of today’s trout populations show mixtures of very different biological origins – a pattern uncommon in nature. The gillaroo and sonaghen of Melvin had probably diverged before they reached the lake. The ferox are found only in a few of our lakes (Corrib, Erne, Leane, Mask and Melvin) and show a strong association with a particular “clade”, or genetic branch on the evolutionary tree.

Their fish-chasing habits, late maturity and long lives (many live to more than 10 years, and one from Lough Mask was 15) all indicate a common ancestor.

The “mosaic-like distribution” of lineages, as the Queen’s team puts it, creates special problems for conserving the various species of trout. Taking good care of Melvin, as a treasure house of biodiversity, is an obvious priority and has been a personal mission of Ferguson’s. But the new genetic evidence, with its story of randomly mixed populations, seems to offer almost too much help in defining further trout species.

“How do we know,” Ferguson asked years ago, “whether an organism is common or rare if its species or other unit cannot be defined or recognised?” His point was not to waste conservation resources by focusing on supposedly rare fish that are simply variations on common species.

Priority, says the latest study, “should be based on the biological characteristics of local populations rather than solely on evolutionary lineages.”

Bird-savvy readers will have spotted that the stonechat in last week’s drawing was wrongly captioned as the wheatear. Sorry for the confusion

Eye on nature

I saw short, red growths like small horns on some lime leaves. Is it a disease?

Katherine Power, Lucan, Co Dublin

From the photograph you sent, it appears they are lime-leaf galls caused by a 0.2mm mite, Eriophyes tiliae. It feeds in spring on the lower side of the leaf, excreting a chemical that causes the galls to grow on the upper surface, then lays an egg in each gall.

I noticed a lot of larvae of some type in Lough Key. What is the spiky case that covers them?

Rebecca Langrish, Dromahair, Co Leitrim

They are the larvae of caddis flies, which spin a protective case around them of plants, rocks and leaves, open at both ends so they can draw in oxygenated water.

I heard the most amazing birdsong outside my window and looked out to see a lovely, slender linnet with a pink speckled breast.

Justin Doyle, Virginia, Co Cavan

Our resident mute swans, on Lough Lene, appeared on May 20th with nine cygnets. More usually they have four or five.

Liam O’Flanagan, Castlepollard, Co Westmeath

Older, more experienced pairs have been known to produce up to 11 cygnets, but that is very unusual.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo, or email viney@anu.ie. Please include a postal address