After the meltdown

Taking a short-cut through the dunes last week, I found myself crunching over hundreds of tiny white snails, scattered like hail-stones across the turf. I pocketed some for a later session with the snail book, but without much hope of excitement: the really rare snails in places like this are ones you need to get down on your hands and knees to find, often in the marshy bits between the dunes and the first fields.

And sometimes, to be certain of a species, a snail-hunter has even to start dissecting. Catinella arenaria, for example, a tiny, amber-coloured snail found at a few places on European coasts, the machair-land of Mayo included, can be told for sure only by a little arrow-shaped mark on the end of its penis (no, honestly).

Vertigo geyeri is another very rare snail - barrel-shaped and less than two millimetres across - that survives in a few limey marshes and fens in the west and the midlands. A new site for it was discovered only last month, on Killaun Bog, near Birr in Co Offaly, a fascinating piece of wet and regenerating cutaway bog, very lush and fen-like.

Vertigo's classic habitat is the "lagg" of a raised bog - the belt of fen that originally encircled the growing dome of mosses. There's very little of this to be seen now, so much drainage and turf-cutting having gone on, and what is really important about V. geyeri is its presence in Offaly for probably some 10,000 years since the last ice age. It's to watery places one must go, indeed, to connect with survivors of the raw rock-and-tundra landscape of post-glacial Ireland, pocked with lakes and flushed by silty, gravel-laden rivers. This is one of the insights brought home by a trio of important new books on Ireland's rivers, lakes and wetlands. They were produced by Irish scientists for last summer's big international freshwater conference in Dublin and are now available from the Marine Institute.

Among the "glacial relicts" still found in Co Kerry, for example, is a gleaming green hawker dragonfly, the northern emerald, and a community of stoneflies at a high mountain lake which have specially short wings to prevent them being blown away. The west of Ireland has glacial water beetles and caddis flies, and the old fens and lakeshores in the midlands have many interesting water beetles, sometimes flightless, which are rare or extinct in Britain.

The more glamorous part of our freshwater post-glacial heritage is, of course, the suite of fish that swam in from a chilly ocean to become the first native species of our rivers and lakes. All are under threat from various human activities, and one or two may even face extinction in this island.

This is specially explored in a review of research on the ecology and genetics of salmon and trout, Arctic char and pollan co-authored by no fewer than six of our top fishery scientists and included in Studies in Irish Limnology.

For the Atlantic salmon, which recolonised our rivers at the first thaw some 13,000 years ago, the latest hazard is the risk of interbreeding with escaped farmed fish, to produce offspring with lower chances of survival. "Interactions" of this sort are already happening in rivers in Donegal and Antrim, and farmed escapees of distant and unknown origin have been sneaking into Mayo's Erriff River.

To find out what happens when wild and farmed salmon interbreed, an "escape" situation was simulated with native and Norwegian fish in a closed stretch of river in the Burrishoole system, run by the Salmon Research Agency. The hybrid progeny showed a good return rate as two-sea-winter salmon, but their ultimate performance and genetic impact will need long-term tracking of the families.

If Irish salmon are powerfully faithful to their birth-rivers, the populations of brown trout show an extraordinary genetic diversity. For a start, they range between an "ancient" race and a "modern" race that may have been split by glaciation as many as 80,000 years ago. But there are also scores of populations keeping their particular gene-mix to themselves, even within the same network of rivers and lakes.

Indeed, it seems that there is five times more genetic variation among Irish brown trout populations than among human populations from all over the world. This makes problems in deciding which populations need conserving. A key target must be Lough Melvin, Co Leitrim, shared by three types of trout - gillaroo, sonaghen and ferox - that keep their genes and lifestyles quite distinct. The ferox, a large, long-living, fish-eating trout, also swims in many of the large lakes of the west as a genetically precious remnant of the first post-glacial colonisation.

Char and pollan are both Arctic fish that once were widespread in our waterways. The introduction of pike may have helped extinguish the big char from lakes like Ennel and Lowel, but modern eutrophication is what finished them in Lough Conn, and they continue to decline in other western lakes.

The pollan, a herring-like whitefish unique in western Europe, has survived the worst period of pollution in Lough Neagh, and this remains its stronghold. Elsewhere it has all but disappeared, assailed by pike, competition from roach and insidious eutrophication (even the invading zebra mussel threatens to smother its last spawning-beds in the Shannon lakes).

This glacial inheritance offers just one angle on the rich mass of information in the three new books. They were written by scientists for scientists, but very accessibly, and the Marine Institute is right to seek a wider audience for them.

Ireland's Freshwaters, by Dr Julian Reynolds of Trinity is a clear and wide-ranging introduction to the subject, with colour photo- graphs (£7.50). Studies in Irish Limnology, edited by Prof Paul Giller of NUI Cork, has chapters by specialists on everything from parasites to peatlands (£15). And Studies of Irish Rivers and Lakes, edited by Christopher Moriarty, is a very thorough account of our waterways and wetlands by the scientists who have been working on them (£15)