Extreme weather clouds the water for our river fish

ANOTHER LIFE:AS THE GREAT Atlantic swells approached Ireland last Sunday, rising under low pressure and driven by the wind, their rolling turbulence sent vibrations – microseisms – humming ahead through the coastal lakes and rivers.

A tingling intimation of floods and swollen estuaries touched thousands of adult eels already primed for migration, their bellies silvered, their systems saltproofed. As our hillside spinney bowed to the gale in a midnight blizzard of leaves I pictured the long procession flashing out from the river below, setting course south for Sargasso.

On the same night, up at Burrishoole, in the hills above Clew Bay, the eels crowding out of Lough Feeagh were being counted out and weighed through the mill-race traps of the salmon research station. Odd things have been happening to them. Although numbers have decreased, there are more and bigger female eels in the flow, so the total weight of those trapped remains more or less the same – this from an ill-nourished watershed in which eels have waited up to 57 years to be big and mature enough for the long ocean swim. Migrating numbers have been falling as ocean changes have drastically reduced the baby eels returning to western Europe, but the lakes and rivers of Ireland are warming and may now actually be good for Anguilla anguilla.

The signs are far less propitious for the salmon and sea trout whose comings and goings and welfare have been Burrishoole Fishery’s main business since 1955. Now part of the Marine Institute, it monitors the whole aquatic environment of land, water and air. As Atlantic weather systems sweep in to affect its lakes and streams, Burrishoole offers pivotal research on what a warming world will do to freshwater life.

Salmon, trout and eels were among the small range of cold-water fish that swam into Ireland’s rivers at the end of the Ice Age. But while the eels are at the northern edge of their range, where extra warmth will help them grow to their optimum, salmon and trout have a different thermal tolerance. This is more marked in salmon, but temperature is still the key to the timing and duration of most stages of their lives, from the emergence of fry from the spawning gravels, to the size of parr in freshwater, to their emigration, as smolt, to the sea.

Water temperatures in the Burrishoole catchment, rarely exceeding 23 degrees in summer, reached almost 35 degrees in the Black river in June 2007. One Scottish upland river has already warmed by up to 2 degrees in winter, and higher winter temperatures at Burrishoole, when salmon eggs were incubating in gravel beds or hatched into parr, have been linked to lower survival. Higher temperatures change water chemistry and the content of dissolved oxygen.

These are some of the findings in a new report* on the prospects for Burrishoole’s salmon, trout and eels, the work of a 16-strong team of researchers led by climate scientists at NUI Maynooth and drawn from Trinity and Burrishoole itself. Along with its importance in European research, it will be closely read by fishery managers right down the west coast, already troubled by the threat of bigger downpours, flash floods and summer droughts in the short spate rivers between mountains and sea.

A big part of its job was to translate projections of global warming this century (between 2 and 6.3 degrees for Europe, according to the European Environment Agency) to regional and local probabilities. Assuming that a mixed-up world makes a slow and fragmented response to curbing CO2, it bravely postpones the worst outcomes to the 2080s or after. By then, in its model, Burrishoole’s maritime summers may be some 2 degrees warmer and its autumns and winters warming by rather more. At the same time winter rainfall goes up by a fifth and summer rain comes down by much the same or even more.

This sharp seasonality of drought and deluge, so different from Ireland’s usual moist regime, must affect salmon and sea trout adapted to more accommodating patterns of river flow. Drought can strand them in the estuaries; floods can wash their precious spawning gravels away and send pulses of acidic water rushing down from conifer forestry.

During the team’s two years of research the Burrishoole valleys had their own “extreme rainfall event”. On July 2nd last year, after six weeks of dry settled weather, about 50mm – two inches – of rain descended on Buckagh mountain in two evening hours, while rain gauges only a few kilometres away recorded as little as 16mm. Pouring off dry ground into the river valleys, the deluge drowned water-level recorders in a once-in-250-years flood and brought sand, gravel, boulders and trees to block culverts, bridges and roads. “Initial surveying of the Srahrevagh river,” says the report, “indicated massive mortality among juvenile salmonids and involuntary downstream migration of adult trout.” Research will continue on “resilience and recovery”.

*You can download the Rescale report at marine.ie

Eye on nature

In Deerpark Wood in Virginia, in Co Cavan, I came across a tree with most of its bark missing, and with a lot of dead bark around it. It looked like some animal had been either chewing through it or sharpening its claws on it.

Anne Comney, Ratoath, Co Meath

I was most likely a badger rooting out and feeding on the insects behind the dead bark, and sharpening its claws.

Here in the Glens of Antrim buzzards are a common sight; we often see them in groups of up to four. But they are quite noisy: the cry is unmistakable. As they attract so much attention to themselves, how do they manage to catch their prey? Why do rooks gather in particularly large flocks when the weather gets colder?

Brian Scott, Cushendall, Co Antrim

Buzzards often eat carrion where sound doesn’t matter. They hunt alone and do not call. Rooks roost together in rookeries because of the advantages of numbers in detecting danger. They forage in flocks and spread out over a field to get the optimum benefit from a source of food.

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