A study of brown trout shows there is more genetic variation among the fish than in human populations
Martin O’Grady is probably the country’s foremost expert on brown trout. As senior research officer with Inland Fisheries Ireland, he has spent most of his working life travelling the country, monitoring fish stocks, assessing water quality and lecturing on conservation.
To describe him as passionate about the subject misses the point. He once chased several tagged fish down the river Cong in Mayo in a helicopter just to pinpoint their spawning ground.
Yet nothing in all this prepared him for the welter of new data which has landed on his desk in recent times.
A series of genetic studies on Irish trout populations has revealed a heretofore unknown complexity to the fish and its spawning rituals, which is set to transform conservation for decades to come.
The research, conducted by Inland Fisheries Ireland in combination with academics from University College Dublin and more recently Queen’s University Belfast, reveals there is more genetic variation in Ireland’s brown trout population than there is between populations in the human race.
O’Grady and his team have already identified 13 different varieties of brown trout in Lough Corrib, each with its own discrete genetic make-up and spawning ritual.
“Because trout return home to spawn they are distinct genetically and recognisable, which makes them ideal candidates for DNA analysis,” he says.
Thin and streamlined like a herring, croneen trout are a pelagic fish, designed to swim long distances to spawn. The croneen population in the Lough Derg catchment of the Shannon’s main stem retreat to the small Camcor river in Co Offaly, 50 kilometres away, to spawn.
Another trout variety in the river Suir, which hasn’t yet been named, was found to travel 78km from the river Nier in Co Waterford, where it is born, down to the main stem of the Suir.
“We had no idea this was going on. We used to think trout came down from nearby streams and tributaries and stayed relatively local, near the mouth of the inlets in which they were born, but that’s not the case at all,” he says.
Missing link
O’Grady admits to being startled by some of the findings. “It’s like the missing link. We’ve always had a huge amount of data in terms of morphology, stream-water chemistry, general ecology and fish numbers, but this has opened our eyes to how they move.”
Perhaps the most significant discovery for conservationists relates to the curious population dynamics of Ireland’s most common freshwater fish.
In the western fishery of Lough Mask, for instance, the team’s study found spawning trout in the small eight-mile Owen Brin river, one of the smallest Mask sub-catchments, supplied more than 40 per cent of the adult population of trout in the lake.
In contrast, the trout born in the biggest sub-catchment, the 60-mile long Robe river, contributed just 4.5 per cent of the lake’s adult population.
Damage to the Owen Brin sub-catchment, as a result of pollution, for example, would demolish fish stocks in the lake as it is the “backbone of the Mask fishery”, O’Grady says. “If you do a sum plotting the wetted area of stream against its contribution to the adult stock of trout, it’s crazy. The pattern is all over the place.”
The studies show this topsy-turvy pattern is repeated in waterways across the country. So what is going on?
“What we’ve found is the fish spawning in these small gravely streams are forced to migrate because there’s not enough food to sustain them while the trout in the larger rivers and sub-catchments are staying put because food is plentiful.”
In the past, conservationists might have uniformly enhanced trout habitats in these sub-catchments in the hope of bolstering the adult stock in the main fishery.
“What these findings allow us to do is to target our conservation programmes more carefully,” he says.
In the course of these studies, geneticists have also discovered peculiar “private alleles” – bits of genetic material that do not appear to be in other sub species of the population – associated with trout varieties.
There is a particular type of trout in Lough Corrib’s Grange river that has a large amount of genetic material which geneticists can’t seem to find in other brown trout. “This material may or may not be important. We just don’t know yet.” The more genetic variety a species has the better equipped it is to respond to ecological changes such as global warming, he says.
Ireland - a world fish superpower
If the world was mapped according to premier brown trout fisheries, Ireland would rank as the pre- eminent superpower.
Of the 13 lakes in western Europe classified by the EU as ecologically significant brown trout fisheries, 12 are in Ireland. To put this in context, there is only one trout fishery of consequence in the whole of Britain - Loch Leven in Scotland.
According to Martin O'Grady of Inland Fisheries Ireland, our waterways are blessed with a unique morphology that makes the island a veritable Shangri-La for trout.
The shallow limestone waters across Ireland support a rich plant life, which provides a productive base for the insects, snails and shrimp that trout feed on. In mainland Europe, many of the big lakes are too deep to sustain a comparable level of life.
The climate here also ensures the lakes remain cold, rarely exceeding 20 degrees even in the summer months, which is crucial for a cold-blooded animal such as the brown trout.
O'Grady says the biggest threat to Ireland's brown trout is global warming. "One of the reasons why big continental lakes don't have the same trout stock is because they get too hot in the summer. A lethal water temperature for a trout is about 23 degrees."
There is also another vital element . Since the Ice Age only came to an end in Ireland about 12,000 years ago, the only fish able to colonise the inland waterways were those that came in from the sea or the ones left by retreating waters. "As a result . . . we don't have very many species of fish, but the indigenous species we do have do very well because they suffer less from competition and predation."
O'Grady rates the river Suir as the best brown trout fishery in the world.
