Ferox Trout - what does the future hold?

A few years ago, when fishing the Bladnoch in SW Scotland, some locals told me that during a population survey a "brown" trout had been caught and released that weighed 8 lbs. Bearing in mind that on the Bladnoch, brown trout do not often exceed 1 lb, this account caused me to speculate on whether ferox trout were more widespread than is commonly supposed. For a wild trout to achieve this weight in insect-poor waters would have meant that it was largely piscivorous. The question is, do such trout reach such sizes because of a change in feeding behaviour or are such trout a different species?

Ferox trout can reach very large sizes (20 lbs plus is not uncommon), attain a great age (23 years has been recorded), are often morphologically different to brown trout and are piscivorous, specialised in feeding on arctic charr, small trout and salmon parr. Ferox trout have been found in several Scottish, Irish and Cumbrian lakes as well as Scandinavia and Switzerland, and the general consensus seems to be that the the landlocking of lakes caused by glaciation created physical barriers to migration for both prey, such as charr, and trout, and hence set up the conditions for speciation.  Interestingly I am not aware that ferox trout have been found in the deep Welsh lakes such as Vyrnwy, although mention is sometimes made of Welsh ferox.

So perhaps ferox trout inhabit some UK rivers but, given certain behavioural characteristics, are seldom seen? The existence of "freak" trout in certain UK rivers is well catalogued. "Kings of the weir, the great Thames Trout" were by all accounts very large.  However, the more you look into this possibility, the more complicated the biology becomes and the more uncertain the position.

Firstly, what do we mean by a different species of trout? As Ferguson shows, the classification of the salmonids, brown trout in particular, has been chaotic and it is probably more helpful to think in terms of a "species complex". Kottelat (Biologia 1997, 52) has proposed 25 species of trout in Europe. Since the advancements of DNA sequencing techniques, it is now possible to measure the relatedness of different types of trout. As Ferguson says trout are  very varied. There are 5 times more genetic diversity in brown trout in Ireland than in the human population in the world. However, despite this huge genetic diversity, it is possible to detect whether a trout type, a genotype, is reproductively separate from another. Ferguson's lab has shown that the ferox trout in Lough Melvin in Ireland, and now Lochs Awe and Laggan in Scotland, are reproductively separate and he has suggested a species name, Salmo Ferox.

Secondly, not all trout displaying ferox characteristics appear to be reproductively separated. For example the ferox trout in Corrib and Mask (Ferguson personal communication) appear to have "ferox" genes but can reproduce with non-ferox trout. Ferguson says  that this interbreeding may have been caused by human interference.

Thirdly, given the genetic diversity even within a small locality, the odd "monster" trout is perhaps to be expected and we do not need to ascribe a theory of speciation to account for them.

This complicated picture presents problems for conservation policy. On the one hand where ferox trout are reproductively distinct, then the ferox is very vulnerable to small environmental changes particularly in relation to its feeding specialisation and spawning behaviour. On the other hand where ferox trout interbreed with normal brown trout, then perhaps these trout are less vulnerable. However, the difficulty is that very little is known about the extent to which "ferox genes" contribute to the behaviour of these trout. Until we know more, conservation policy should err on the side of caution.

So what of our Bladnoch trout and other reports of super large trout in rivers?  It is unlikely that a separate species of trout would go undetected on UK rivers, given the fishing pressure. The existence of a separate species implies the existence of sufficient numbers of trout to make the population biologically viable. The techniques employed by pike fishermen would inevitably catch such trout if they existed in any numbers. It could be argued that the reason we don't see such trout is that their numbers have been decimated. However, fishing in UK rivers has been intensively practised for hundreds of years, and there is little evidence from the records that would suggest a separate species.

What is probably happening is that from time to time trout will change its feeding behaviour in response to something and become more piscivorous, and thereby grow rapidly. However such trout are unlikely to survive for longer than the normal 4 or 5 years and what we are seeing is the outer limits of a normal distribution of traits in a fish that is itself very genetically diverse.

As for the ferox of the Corrib, the message is that we need to adopt approaches that do not potentially endanger these top predators, and since little is yet known about the biology of these fabled trout, caution should be used. Catch and release should be practised but even this is not without its dangers as safely releasing a ferox that has been caught trolling is not that easy.