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The purity of the breed

By Peter Lapsley

Peter Lapsley investigates the genes of wild brown trout and how it affects the characteristics of diploid and triploid trout, ferox, gillaroo, dollaghan, sonaghan and marbled trout.

Strong stomach: Ireland's gillaroo trout.
Strong stomach: Ireland's gillaroo trout.
Even small wild fish - far more 'stream-wise' than farmed fish - have no difficulty in holding their own against stocked trout.
Even small wild fish - far more 'stream-wise' than farmed fish - have no difficulty in holding their own against stocked trout.
Sonaghan: Lough Melvin's plankton feeders.
Sonaghan: Lough Melvin's plankton feeders.

It seems reasonable to suppose that farmed brown trout stocked into rivers will necessarily discomfort those rivers’ wild trout – that they will harass the wild fish, dislodging them from their lies; that they may prey on small wild fish, particularly if they themselves are large; that they may disturb wild trouts’ spawning redds; or worse, that they may mate with wild fish, diluting the genetic integrity of wild trout populations.

It was these concerns, and especially the last one, which led the Environment Agency’s (EA) National Trout & Grayling Fisheries Strategy, published in 2002, to propose severe restrictions on the use of (fertile) diploid trout for stocking rivers. It envisaged the banning of all stocking in ‘wild fish’ waters, in which trout stocks were reliant on natural production. And it said that river fisheries that were reliant on stocking but also had significant, self-sustaining wild trout populations should be stocked only with (sterile) all female triploid trout.

Unsurprisingly, this provoked a furious response from riparian owners and river keepers who demanded to see the evidence for the introduction of such a strategy. In truth, there was none. The EA was simply applying the ‘precautionary principle’ – ‘if you don’t know the answers, err on the side of caution’

To its credit, the EA, proved to be in listening mode and took up the challenge. It placed the Strategy on hold and commissioned the Game Conservancy Trust (GCT, now the Game & Wildlife Conservation Trust ) to conduct four studies into the consequences of stocking rivers with trout, and to compare the performances of diploid and triploid trout and their effects on wild brown trout. The research, which was conducted in phases between 2002 and 2006, was jointly funded by the EA, the GCT and a number of individuals. It was conducted on the Rivers Arrow and Monnow, both upland, spate rivers, and on the River Allen in Dorset and the Wiltshire Avon, both lowland chalkstreams.

Intriguingly, the outcomes show the suppositions set out at the beginning of this article to be completely wrong on both upland and lowland rivers, chiefly because wild trout – far more stream-wise than farmed fish – have no difficulty in holding their own.

There was no statistically significant drop in abundance or growth of wild fish when stocking took place. Stocking did not cause the displacement of wild fish. Fish formed a very small part of the diets of both stocked and wild fish, and bullheads, stone loach and minnows were the predominant species found in the stomachs of the few trout that did occasionally take fish. The growth of stocked fish was negligible.

The study of stocked trout interference with wild trout spawning on the upland rivers was plagued by otters. In the lowland study however, although a few all-female diploids did move into the spawning areas, they went less far into them than did wild trout and, importantly, made those incursions about a month earlier. There was no evident interference with wild fish spawning by stocked fish.

Wild genes
Interestingly, there is strong evidence for long-term maintenance of the genetic integrity of wild fish in research done by Professor Andy Ferguson of the School of Biology and Biochemistry at Queen’s University in Belfast. It is to be found in his paper, Reproductive isolation and genetic differentiation of ferox trout from sympatric brown trout in Loch Awe and Loch Laggan, Scotland, which he co-wrote with Dr Paulo Prodohl and published in the Journal of Fish Biology in 2006.

The paper argues for the formal classification of ferox trout as ‘Salmo ferox Jardine, 1835’, following the established taxonomic practice of crediting a species to the first person to have identified it. As it shows, true ferox trout have a number of defining characteristics. They grow comparatively slowly early in their lives, are late-maturing, long-lived and pisciverous, and are reproductively isolated and genetically distinct from other brown trout living alongside them. They feed on invertebrates until they are about 30cm (12in) long and then switch to a fish diet, usually – perhaps exclusively – of charr. It is presumably no coincidence that they are found only in lakes in which charr are present.

The question that must niggle away in the back of one’s mind, of course, is how so distinct a sub-species could have come to be present in waters as far apart as Lough Melvin in Ireland and Lochs Awe and Laggan in Scotland. The answer may lie in the fact the as recently as 15,000 years ago there were no fish at all in British or Irish lakes or rivers, because those waters were buried beneath 13,000ft of ice. All our freshwater fish came in from the sea after the ice cap had receded.

Several distinct salmonid species arrived during that period, including the Atlantic salmon, the brown trout, the Arctic charr and the grayling. It may not be unreasonable to suppose that brown trout did not arrive either in a single wave or as a single species, but at various times and as a number of already distinct sub-species, some finding homes in Ireland and some in Scotland. If that is so, then the purity of the several breeds would appear to have been maintained for many millennia simply because, although they are capable of inter-breeding, each of the sub-species favours a different type of spawning ground and has continued to breed separately from the others.

Salmo ferox is by no means the only sub-species of brown trout to be found in Britain and Ireland. Gillaroo and sonaghan are similarly distinct, as presumably are dollaghan.

Gillaroo (Salmo trutta Linnaeus, 1758) are found chiefly, perhaps exclusively, in the Shannon system in Ireland, the Galway lakes, and Loughs Mask, Corrib, Neagh and Melvin. A striking fish, with large crimson and vermillion spots ringed with white on a golden background, it is almost exclusively a bottom feeder, living chiefly on snails, freshwater shrimps and caddis larvae. Its digestive system has developed thick, muscular walls to cope with so crunchy a diet.

Sonaghan (Salmo trutta nigripinnis), which share Lough Melvin with ferox trout, gillaroo and ‘ordinary’ trout and sea trout, are distinctive both in terms of appearance and behaviour. Small fish, averaging less than a pound in weight, they have dark, blue-grey backs, very distinctive pitch black fins (including an unusually large caudal or tail fin), a few black spots and, generally, even fewer dull red ones.

Sonaghan are unlike any other brown trout sub-species in that they spend the late spring, summer and early autumn in very deep water, feeding on clouds of zooplankton, wandering along with their mouths open, filtering the tiny crustacean from the water. As the zooplankton die back in the autumn, sonaghan may move up in the water to feed on aquatic and terrestrial invertebrates. But, apart from a brief spawning period early in the winter, they rarely if ever leave deep water.

Probably no less distinct a species are dollaghan, the ‘freshwater sea trout’ of Lough Neagh in Northern Ireland, about which John Todd wrote an excellent article, Searching for the big, fat fellow in the October issue of FF&FT.

Spending much of their adult lives growing fit and fat on the rich feeding to be had in the lough, and running up the half dozen or so rivers that feed it come spawning time, dollaghan demonstrate very clearly the way in which a trout’s view of the world differs from ours. We define ‘sea trout’ as brown trout that migrate to the sea to feed. Trout don’t see it like that. Those that migrate simply head downstream in search of better feeding, caring not one jot whether the water in which they find it is salt or fresh. When they run back upstream to spawn, dollaghan are as fit, fat and bright as any ‘sea trout’ and even share the sea trout’s shyness in daylight.

There is little doubt that ferox trout, gillaroo, sonaghan, dollaghan and, indeed, ‘normal’ brown trout can interbreed. Brown trout can even mate successfully with salmon, albeit on very rare occasions. (Genetic analysis of a 10lb ‘sea trout’ caught on the River Doon in July 2007 showed it to be a trout-salmon hybrid – a cross between female trout and a male salmon. The fish, which looked like a sea trout, had originally been tagged in the Tyne, its river of origin.)

But such crosses are extremely rare and are really no more than a distraction. The fact of the matter is that ferox trout, gillaroo, sonaghan and dollaghan have maintained their genetic integrity simply by spawning in established and distinctly separate areas.

All of which raises a number of interesting questions. As far as I am aware, those four salmonid sub-species are the only ones in Britain to have been subjected to detailed genetic research. Could it be, for example, that Loch Leven trout, Kennet greenbacks or the black-finned trout of Wales are similarly distinct sub-species? And, stretching the envelope even further, could it be that salmonids – in this case brown trout – have far more accurate ‘SatNav’ systems than we realise. It is well known that salmon and sea trout return unerringly to their natal rivers to spawn, but I wonder whether they, and non-migratory brown trout, may not make more precise choices than that, heading specifically for the parts of those rivers – or even for the particular redds – in which they were hatched. But that is pure speculation.

The point that needs making, and which both the GCT research and Professor Ferguson’s have made very clearly, is that the risk that stocked, fertile, diploid fish will undermine the genetic integrity of wild trout populations is actually very low. (Research done in continental Europe shows the extent of genetic changes in wild brown trout populations to be much less than might be anticipated given the scale of stocking and the fact that it has been carried out for over a hundred years in some cases.)

Which brings us to two different but closely related questions. The first is what, in Britain, is a ‘wild brown trout’? There can scarcely be a river in the British Isles that has not been stocked with fertile diploid fish at some time in the past hundred years or so. If wild and stocked fish ever interbreed, as they probably do, if only occasionally, it would seem not unreasonable to argue that the ‘damage’ must already have been done – although the outcomes from the European studies mentioned above suggest that that damage may well be negligible.

The second is whether (sterile) triploid fish are fit for purpose from the angler’s perspective. As part of their research, the GCT asked anglers at a fishery in the Salisbury area to fill in return forms. The fishery was stocked with equal numbers of separately marked diploid and triploid fish. The return forms invited the anglers to use a 1-5 scale to comment on the visual condition and fighting ability of each fish caught. The fishery rules were ‘dry fly only’ until July 1, which also enabled the GCT team to assess the diploids’ and triploids’ respective willingness to take dry flies.

Loss of appetite?
The outcomes showed no significant difference between the two groups of fish in any of these matters. They were judged by the anglers to be similar in visual condition, to have similar fighting qualities and to be equally willing to take dry flies.

But those outcomes differ somewhat from some of the anecdotal evidence.

At the Test & Itchen Association’s EGM last October, Mick Lunn, perhaps the country’s most experienced and respected river keeper, gave a very objective and coherent account of his experience of triploids. They rose freely during the early part of the season, he said, but tended to sit on the bottom once the Mayfly hatch had finished, only coming to the surface for large artificials. He wondered whether this might be due, in part at least, to shortage of natural flies on the chalk streams, and whether the fact that triploids do not need to produce ova may lead to a lack of appetite.

Wild Trout Trust co-founder, Richard Slocock, another very experienced fishery manager has, for the past two years, been managing a stretch of the Frome well downstream of Dorchester. It is not naturally a wild trout fishery, so he has stocked it with diploids and triploids in parallel. He says it has become increasingly evident to him that triploid retention is markedly poorer than diploid retention, and that the triploids (which have come from two different fish farms) are markedly more ‘bottom-dwelling’ and less free rising than their diploid cousins.

There are, of course, other factors to be taken into account, not least those identified by Professor Norman Maclean, Emeritus Professor of Genetics at the University of Southampton, in his response to the EA’s most recent consultation on the subject. Having discussed the issues with a number of experts, he has concluded that many fish farms currently producing diploid brown trout for stocking would have difficulty in finding the space and mastering the technology required to produce triploids. Triploids have to be hatched and reared separately from diploids, and they are normally produced by pressure-shocking the ova which, in inexpert hands, can lead to high egg mortality or to a high incidence of diploids amongst fish supposed to be triploids.

Professor Maclean notes also that, because of the production costs involved, which include their greater susceptibility to malformation and disease, triploid trout will necessarily cost up to 25% more than comparable diploids. He suggests, too, that because triploids are less tolerant of high water temperatures than diploids, climate change gives cause for concern for future triploid production.

The EA has a difficult tightrope to walk in all this. Their role is to protect and improve the environment. This being so, we must recognise that, where trout are concerned, they are really interested only in wild fish. On the other hand, they support angling as a sport and must therefore take anglers’ interests into account. While most fly fishers would probably prefer to catch wild brown trout, we are a pragmatic lot. We recognise that, if the supply of fishing is to meet the demand, there is – and will continue to be - a need for rivers to be stocked, especially in the most populous parts of the country. Were the EA to be seen by anglers and fishery managers to be enforcing change too quickly, basing their decisions on flawed or flimsy evidence, that would be unhelpful to everyone.

It seems to me, therefore, that the EA would do better to observe the “if it ain’t broke, don’t fix it” principle, rather than an over-cautious precautionary one. Which is to say that they should maintain the status quo, allowing the stocking of both diploids and triploids provided they are of a size appropriate to the water and introduced in only reasonable numbers, encourage more research into trout behaviour and the genetic integrity of wild brown trout stocks, and insist on change only if and when there is real evidence of harm, actual or impending.

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