Michael Viney: how trout disappeared from my favourite river

New research shows sea trout have more lice the closer they swim to salmon farms

My favourite stretch of Mayo river flows between ivied oaks in a hollow below the bog and onwards under Louisburgh’s bridge to the sea. The Bunowen is perfect for sea trout – quite a short river, a bit acid, low in nutrients, running up to stony shallows in the moor. In the hollow, it glides over eight mossy boulder weirs, built more than a century ago.

Until the late 1980s, it was possible to count, in early autumn, a dozen or more small white trout in each of the eight weir pools. Last autumn there was none to be seen. The otter that used to sleep downstairs in a badger’s sett on the bank has long gone off to fish in the sea.

The Bunowen’s sad story is a coda to the sea trout crash in 1989-1990 in a number of western rivers with salmon farms offshore. This was also when, just over the mountain, anglers fishing the Bundorragha found pools full of returning sea trout. They were thin and listless, weakened by skin-grazing copepods, the “sea lice”, picked up round the cages of swirling farmed salmon.

Long-term research

Such suggestions were then resisted as unproven and malign. Last month, Inland Fisheries Ireland welcomed new, long-term research in Ireland, Scotland and Norway. It finds that sea trout carry a lot more lice the closer they swim to marine salmon farms. This reduces their weight or even kills them at sea. (A pdf can be downloaded from fisheriesireland.ie.)


The intervening years have brought better inspection, management and lice control. At least some rivers have seen a rise in sea trout numbers, if far from their previous abundance. Delphi Lodge fishery, for example, which includes the Bundorragha, reported anglers landing 190 sea trout in August – an improvement coinciding “with very low lice levels in Killary this year”.

But bad examples persist. They have helped to halt further inshore salmon farm development, as in the large assembly of cages promoted by An Bord Iascaigh Mhara (BIM) for Galway Bay.

Young sea-going trout among brown trout populations set off to feed in coastal waters in early spring, their scales turning silvery and bodies set to cope with salt water. Most feed in coastal waters, but some may range up to 300km before returning in late summer (as “finnock”) or staying away a whole year or more to grow even bigger (as “maidens”).

Such migration is called anadromy, shaped at least some 14,000 years ago. As ice melted from land and rivers, both salmon and trout, the salmonids, swam from the ocean into fresh water. Rivers promised greater safety for breeding, with migration to feed and grow in the sea.

Time and geography created distinct brown trout races in separate rivers and lakes. But the genes for anadromy persisted, switched on variably by local environments, population sizes and freshwater food supply.

Scientific neglect

There was long scientific neglect of the sea trout, despite its value to angling and a flavour often superior to salmon. The sea-louse crisis in Connacht showed its vulnerability and its potential as a signal of environmental change.

Efforts to close the big gaps in knowledge of sea trout genetics and ecology have included the Celtic Sea Trout Project, its report just published online by Inland Fisheries Ireland. This was the first multidisciplinary study of the sea trout in the Irish Sea and of their many home rivers on the mountainous coast of Wales and the lowlands of eastern Ireland.

The “identity” of the different river stocks fall into nine big groups within the Irish Sea. Each group is genetically distinct, with a DNA history arising from the nature of their rivers.

Sea trout abundance varies over time between rivers and regions, but short rivers, with low alkalinity and easy access to good spawning and nursery areas were seen as the more productive. The trout’s growth at sea depends a lot on temperature, as well as supply of sand eels and sprats, so that those from more northerly rivers come back smaller than fish in the warmer south.

There were no salmon farms in the research project’s area, so that lice on the sea trout were found at natural levels. But the severe damage to the stocks of some Connacht rivers, even to local stock extinction, was what sparked the current research into one of the world’s most valuable and biologically interesting fish.

More than 25 years ago, Dr Mark Costello, then a young Irish marine biologist, now globally eminent, sent me his paper on the potential of native wrasse as "cleaner" fish. They feed on the lice of caged salmon, an alternative to having them washed off the fish with insecticide. Marine Harvest's Irish salmon farms have begun to try it, both with wrasse and with lumpfish bred by NUI Galway at its Carna research station. The eggs of the latter, conceivably, could end up as supermarket "caviar".

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks