Ferox Trout

Where are ferox found?   Ferox biology & habits  Genetics  Conservation Further Information Academic references


Ferox trout are a fantastic example of the genetic diversity that exists within wild brown trout.  Since the term ferox was first coined in 1835 by the renowned angler scientist Sir William Jardine, various authors have attempted to define what exactly a ferox is, for example:

Campbell (1979) offered the following definition:

‘Ferox are long-lived, late-maturing, piscivorous brown trout which in Britain and Ireland, are often present in large, deep glacier-formed lakes containing Arctic charr or whitefish species’

The status of ferox within the brown trout (Salmo trutta) species complex epitomises the debate about what is and what isn’t a species (see Ferguson, 2004). There can be no doubt however that ferox can be spectacularly different from other brown trout; in life history, genetics and habits.

Catch & release of ferox trout: A video produced by Ferox 85; a dedicated group of ferox biologists:

Where are ferox found?

Ferox trout are found throughout the brown trout’s native range, wherever there are large glacial lakes.  In Britain they are found mainly in Scotland and Ireland but there are also thought to be populations in some Welsh lakes and in the English Lake District. In Campbell’s (1979) study, ferox were mainly found in lochs of over 100 ha with Arctic charr present, however ferox do occasionally occur in smaller lochs such as the 20ha Loch Bhrotain, in Inverness.

                                Argyll Fisheries Trust biologist, Alan Kettle-White with a 30lb Ferox

Ferox biology & habits

All young trout feed chiefly on invertebrates when young. After ferox reach a length of 30 – 35 cm, a change occurs, with a switch to an almost entirely piscivorous (fish-based) diet. This change is size-related rather than age-related and has a dramatic effect on ferox growth (Mangel & Abrahams, 2001); tagging studies have recorded a 10lb (4.5 kg) increase in weight in 4 years! Ferox, particularly in the cold, acidic lochs of the Scottish highlands, are long lived (a 23-year-old fish from Loch Killin, Inverness being the oldest on record). This fact, combined with their protein-rich diet, translates into much larger sizes than the average brown trout angler will ever encounter. The current British rod-caught record is a 14.4 kg (31.7lb) fish from Loch Awe, Scotland, a renowned ferox loch.

Tagging work on ferox in Perthshire has found that ferox will make wide ranging movements, probably searching out shoals of baitfish such as Arctic charr. Ferox have been found to congregate around Loch outlets during the salmon smolt run. Ferox are active during the day, and have been recorded making dives down to 30 metres, possibly in pursuit of charr. The presumably means that ferox are active pursuit predators as opposed to ambush predators using cover to attack prey.

Arctic charr (see picture on right) are of great importance to ferox diet, particularly in Scottish lochs. Where present,  whitefish species (coregonids) are commonly taken, for example in Loch Lomond.  Other brown trout will also occasionally be taken (this why ferox have long been labelled as cannibalistic browns), as will coarse fish such as perch. In the Irish limestone loughs such as Corrib, ferox have switched from a charr-dominated diet to a roach-dominated diet following the extinction of charr after the introduction of the latter.                                                                                                                                                

Ferox with arctic charr prey

However, it is not entirely unknown for ‘normal’ brown trout to shift to a fish diet, or grow to large sizes for that matter.  So how are ferox different?


Currently, scientific opinion is divided over the status of ferox as a separate species from brown trout.  Modern genetic techniques have allowed some fascinating insights and raised fundamental questions about whether our conventional description of a species is adequate to describe the diversity that exists.  For example, studies on Lough Melvin (north west Ireland) have identified three distinct forms of brown trout: sonaghen, gillaroo and ferox; these forms are genetically distinct from each other and are likely to have remained so since colonising the lough from separate refuge areas following the last ice age 10 – 15,000 years ago.  They have remained distinct by spawning in different locations and adopting different habits.  The ferox spawn in larger in-flowing rivers (in similar locations to salmon), the sonaghen spawn in smaller inflowing streams and the gillaroo spawn in the river flowing out of the lough.  In fact, naturally occurring ferox-salmon hybrids are more common that ferox-sonaghen hybrids, indicating the degree of separation between the different forms of trout.

These genetic differences between trout in the same lake have also been found in Lochs Awe and Laggan in Scotland. Indeed, ferox in these different lakes (Melvin, Awe and Laggan) are more genetically similar to each other than to other trout in the same lakes! Initial studies have suggested that the ferox of Loughs Mask, Corrib and Erne also have the same genetic characteristics as Melvin ferox.  This suggests a common ferox lineage colonised these waters after the last glacial retreat.

Some fisheries biologists, however, consider ferox to be brown trout that have simply adopted a different life history to others in the same water. It is the case that ferox in other lochs may not follow the Melvin/Awe/Laggan lineage; for example there is no evidence at present to suggest that the ferox of Loch Rannoch are genetically distinct. This may be due to ‘ordinary’ brown trout becoming piscivorous, colonisation by other post-glacial lineage(s) of ferox different to those of Melvin/Awe/Laggan, or simply the lack of research.  What is not in doubt is that further study is needed to throw light onto the origins of the ferox in other lakes.


8.84 kg (19.5 lb) ferox caught by David Greenwood, Loch Awe



The International Union for the Conservation of Nature (IUCN) and the Freshwater Fish Specialist Group regard ferox as a separate species. Ferox is listed in the IUCN Red List of Threatened Species with its status being given as “Data Deficient” (www.iucnredlist.org/details/summary/135577/0 ). Whether or not ferox are considered a distinct species, their low numbers (at least in Scottish lochs) and distinct genetic characteristics (as in Loch Melvin) warrant enhanced conservation status. Large, predatory fish like this are prized by anglers yet only a tiny proportion of the trout in a loch are ferox; the ferox trait is therefore valuable and rare. There is some justification in the call for ferox to be accorded species status rather than the less valued (in an EU context) sub-species status. This echoes calls by researchers for effective conservation of brown trout to be based on genetic differences between populations. Threats to ferox trout include:

- Overfishing: tag-and-release studies in Loch Rannoch suggest that ferox densities in these nutrient-poor lochs are likely to be low. Even low intensity ferox angling therefore has the capacity to impact on ferox numbers. Catch-and-release is therefore of utmost importance and the same studies suggested that angling mortality is acceptably low. Because of their longevity, the impact of exploitation is likely to be greater than it would be for a shorter loved species. 

- Spawning tributaries: Spawning areas for ferox may be fairly specific and limited. In Lough Melvin, ferox achieve reproductive isolation by spawning in a single, deep river.  Obstructions can block access to spawning tributaries and outflow dams on Lochs Rannoch and Laggan may have negatively impacted on ferox recruitment. In Scandinavia, a number of studies before and after hydropower development have been carried out and are fairly applicable to the British Isles. In upland areas, intensive commercial forestry can also severely limit primary productivity in streams and increase acidity to dangerous levels during peak flood events.

- Introduced species: In relatively pristine post-glacial fish communities like Lough Melvin in Ireland, ferox hold the top predator position. Introduction of pike in these lakes could lead to the replacement of ferox as the top predator. Pike and ferox coexist in Loch Awe and more study is needed into the relationship between these top predators. Potentially more serious is the introduction of the filter feeding zebra mussels into Lough Melvin. These are highly efficient filter feeders and their introduction could have disastrous effects on the lakes native planktivores (e.g. sonaghen trout and charr) that ferox rely on. Ferox are adapted open water pelagic predators. The replacement of charr for example by littoral or benthic feeding fish could therefore have unforeseen consequences for ferox - again, a precautionary approach and more research is needed.

Stocking: The dangers of stocking fertile, hatchery-bred fish are well documented elsewhere on this website. Stocking could threaten the genetic integrity of the ferox stock and reduce or alter ferox characteristics. Supportive breeding schemes (using broodstock from the wild) also pose a threat through inadvertent crossing of genetically distinct forms of trout.

Further information

Click the following links for other useful information on Ferox biology & conservation:

(This page has been compiled with the kind assistance of Dr. Ron Greer, Dr Alan Kettle - White and professor Andy Ferguson. Photographs courtesy of Alan Kettle - White, Aya Thorne and David Greenwood.)


Academic references

Campbell, R. N. Ferox trout, Salmo trutta L., and charr, Salvelinus alpinus (L.), in Scottish lochs. Journal of Fish Biology 14, 1–29 (1979).

Cawdery, S. A. . & Ferguson, A. Origins and differentiation of three sympatric species of trout (Salmo trutta) in Lough Melvin. Polskiie Archiwum Hyrdobiologii 35, 267 – 277 (1988).

Duguid, R.A., Ferguson, A. & Prodohl, P. Reproductive isolation and genetic differentiation of ferox trout from sympatric brown trout in Loch Awe and Loch Laggan, Scotland. Journal of Fish Biology 69, 89–114 (2006).

Ferguson, A. The Importance of Identifying Conservation Units: Brown Trout and Pollan Biodiversity in Ireland. Biology & Environment: Proceedings of the Royal Irish Academy 104, 33–41 (2004).

Ferguson, A. & Taggart, J. B. Genetic differentiation among the sympatric brown trout (Salmo trutta) populations of Lough Melvin, Ireland. Biological Journal of the Linnean Society 43, 221–237 (1991).

Greer, R. B., Thorne, A. & Macdonald, A. Ferox trout : A Predator worthy of Pursuit and Protection. Proc. 2002 ISACF meeting (2002).

Greer, R.B. (1995). Ferox Trout & Arctic Charr; A predator, its pursuit & prey. Swanhill, Shrewsbury.

Grey, J., Thackeray, S., Jones, R. & Shine, A. Ferox Trout (Salmo trutta) as Russian dolls: complementary gut content and stable isotope analyses of the Loch Ness foodweb. Freshwater Biology 47, 1235–1243 (2002). To view the paper, click here. (with kind permission of Dr. Jonathan Grey).

Grey, J. Ontogeny and dietary specialization in brown trout (Salmo trutta L.) from Loch Ness, Scotland, examined using stable isotopes of carbon and nitrogen. Ecology of Freshwater Fish 2001: 10: 168–176. To view the paper click here (with kind permission of Dr. Jonathan Grey).

Hardie, R.P. Ferox and Char Parts I and II, Coch-y-Bonddu Books, Machynlleth (re-print of Part I first published 1940 and recently discovered manuscript of Part II)

Mangel, M. & Abrahams, M.V. Age and longevity in fish, with consideration of the ferox trout. Experimental Gerontology 36, 765-790 (2001).