What makes an apex predator: the ferox trout

I have to admit, the topic of this research really floats my boat (as you may have noticed from the latest Salmo Science Spot)…. I spent several years trying to convince people that perched at the top of the Loch Ness food web was not an elusive plesiosaur but something much, much more sexy. Ferox! So I’m really pleased that the newly doctored Martin Hughes has taken time out to précis his completed PhD, but it does mean I’ll have to review the WTT ferox pages with his new findings.

The brown trout, Salmo trutta, is an incredibly diverse species. Individuals from the same population can adopt completely different life history strategies, which are often given vernacular names. For example, some S. trutta remain in small freshwater streams their entire lives; these are referred to as resident trout. Others migrate into large rivers or lakes to complete their life cycle and retain the name brown trout, but some that migrate into estuarine waters are referred to as slob trout and others that fully migrate into marine waters before returning to natal streams to spawn are called sea trout. One relatively understudied life history is that of ferox trout. Ferox’ which is Latin for Fierce’ is aptly used for these large piscivorous trout which can grow to large size (14kg UK record) and are exceptionally long lived (23 years oldest UK record- reports of 39 years old in Norway). Their impressive growth potential and life span combined with their rarity and near mythical status only adds to the allure to both anglers and scientists alike.

Astonishingly, even in the 21st century we know relatively little about these incredible fish. Genetic studies throughout the 1980s in Ireland only added to the mystery surrounding ferox trout. Pioneering research by Andy Ferguson and his colleagues at Queens University Belfast found some populations of ferox trout were actually part of an ancient lineage of trout, believed to be one of the original lineages that invaded the British Isles after the glaciers retreated approximately 12,000 years ago. This raised interesting questions surrounding the evolutionary history of ferox trout and their taxonomic classification. In 2008, ferox trout were reclassified as Salmo ferox which was recognised by the IUCN. The IUCN however classified Salmo ferox on the Red List of Threatened Species under the category of Data Deficient’ meaning the risk of extinction of ferox trout could not be assessed due to a lack of scientific data. In other words, these potentially vulnerable and genetically distinct fish are not afforded any sort of protection!

I was first introduced to ferox trout during my undergraduate degree at the University of Glasgow and after a meeting with the enthusiastic Alistair Thorne at the Marine Scotland Faskally Laboratory, I was hooked. I based my undergraduate dissertation on ferox trout and working alongside another ferox trout legend, Alan Kettle-White from the Argyll Fisheries Trust (and WTT Trustee), I collected data on ferox trout and brown trout from Loch Awe.

Hughes ferox wild trout trust blog awe

A mirror still Loch Awe in the summer of 2011 (just think of the midges…)

My undergraduate research led directly into a postgraduate degree which was funded by a large EU project called Integrated Aquatic Resource Management Between Ireland, Northern Ireland and Scotland’ or IBIS (http://​www​.loughs​-agency​.org/​i​b​i​s​-​p​r​o​j​ects/). As an IBIS PhD student, I was tasked with conducting investigatory research into ferox trout which would inform the scientific community, fishery managers and policy makers. I designed my research to help inform stakeholders on the geographic distribution of ferox trout, gain a greater understanding of their basic underlying biology (including physiology and behaviour), and also investigate differences between brown trout and ferox trout co-habiting the same lake.

Hughes ferox wild trout trust blog

The IBIS project partners the Loughs Agency, Queens University Belfast and Glasgow Univeristy, funded by the European Union.

My first research project on ferox trout distribution was based on an extensive literature review and analysis of angling books going back hundreds of years. After almost 2 years of research, I found 192 lakes in Scotland have evidence of supporting a ferox trout population. I also investigated the environmental characteristics associated with these ferox trout populations and found a strong correlation between lake size, lake depth, and the presence of Arctic charr Salvelinus alpinus. The strong relationship between ferox trout, habitat size (lake size and depth) and prey species (Arctic charr) are indicative of most apex predator populations. This research was published in the Journal of Fish biology in 2015 and tells us two main things:

  1. Ferox trout populations are relatively rare given the amount of lakes in Scotland they could be found in, and
  2. Ferox trout, where they do exist are most likely fulfilling the role of an apex predator. Given the disproportionate importance that apex predators play in ecosystems globally, ferox trout are likely to be providing an essential ecological service in those lakes.

The main focus of my research was to investigate underlying biological differences between ferox trout and brown trout. Traditionally this is done using an experimental design called a common garden experiment’. Common garden experiments, as the name suggests, are often used with plants, where plants are raised in a common environment (same temperature, same light exposure, same water availability etc.) so any observed differences between plants are inherited differences and not differences caused by the environment. Essentially you are controlling the external environment at all times. The same can be done for animals raised from birth. However, the logistics and animal husbandry involved makes such an experiment much more complex, and combined with the rarity of ferox trout populations, this was a monumental task. In fact, there were no published records of anyone having collected eggs from spawning ferox trout.

I began my quest for ferox trout eggs in October 2013, in the Highlands around Loch Maree, with the help of Peter Cunningham of the Wester Ross Fisheries Trust. During the following weeks of sampling numerous rivers in the Loch Maree catchment, we caught lots of brown trout but not a single ferox. It turned out to be one of the mildest Octobers on record and the rivers were extremely low. It looked like we might miss our opportunity to catch any spawning ferox trout for another year which would change the planned research considerably.

We identified the river most likely to support spawning ferox trout and waited for the rains to arrive. In early November, the first substantial overnight rainfall was forecast and we assembled a team to be on the river first thing in the morning. Within 2 hours we had captured 3 female ferox trout and a number of males, including a huge male fish which looked like he’d been through the wars, and won! We quickly transported the fish to nearby holding tanks before spawning them to create full sibling groups and hybrid crosses between ferox trout and brown trout.

There is a critical water hardening’ period after fertilisation in which eggs cannot be moved after a 48-hour period or they perish as they become sensitive to movement and light. The water temperature of the eggs also had to be kept cooler than 8 degrees centigrade at all times or I’d risk losing the clutches. After successful fertilisation I stored the precious cargo in flasks surrounded by ice and water in a small blue ford van. With the windows down and AC on I left Wester Ross at 9pm and drove down to the Scottish Centre for Ecology and the Natural Environment (SCENE) on Loch Lomond, wearing two scarfs, a thermal hat and a pair of gloves, wincing at every bump in the road. I arrived at the field station about 3am and began to carefully transport the eggs into specialised temperature-controlled fridges. After an hour of careful negotiation, the final clutch was placed into the temperature control unit.

hughes ferox wild trout trust blog

Ferox trout eggs at a crucial stage of development, the eyed’ stage.

For three months I visited the lab to check the eggs (including Christmas day!) and was ecstatic (relieved) to find they had survived the journey and the embryos were developing well. On the 5th of January 2014, most of the eggs began to hatch and for first time we had juvenile ferox trout in a common garden experiment. Now the real work could begin.

Over the following year all fish were held at the same water temperature and fed the same amount of food allowing me to measure differences in yolk sac size, head shape morphology, metabolism, lipid deposition and mortality rate.

hughes ferox alevins wild trout trust blog

3 day old hatchlings known as alevins. The ferox trout had much larger yolks sacs which is unsurprising as larger females produce larger eggs with larger yolk sacs. What is important, however, is the larger the yolk sac the higher the rate of survival.

Perhaps one of the most interesting experiments we conducted was investigating the dominance behaviour between juvenile brown trout and ferox trout. As juvenile trout form dominance hierarchies in natal streams, with the most dominant individuals enjoying better feeding opportunities, we postulated ferox trout would be more dominant than brown trout. Our reasoning behind this hypothesis was in order to feed on other fish, ferox trout must reach a large size quickly. It is therefore more important for a juvenile ferox trout to reach a large size by acquiring more food than it is for a juvenile brown trout. Our results were staggering! Ferox trout were more dominant than brown trout in 90% of the trials.

hughes ferox

Photograph of fish behaviour footage. On the left is a submissive brown trout juvenile and on the right a light-coloured, dominant ferox juvenile.

Given we had controlled environmental effects and controlled the size of each individual (each fish was matched to within 2mm length of one another), this behaviour is likely inherited from ferox parents. If this behaviour is replicated in the wild, it suggests that juvenile ferox trout may be predetermined to become large piscivorous trout, which supports the evidence provided by Ferguson that ferox trout may be distinct from other brown trout.

The results from the other physiological experiments will be published later this year and therefore cannot be discussed ahead of time. However, more interesting distinctions have been discovered. Like all good research, I have more questions than I started off with and I would like to thank all who have been involved in the research and continue to support the project. If you would like to find out more information, please like the The Ferox Project’ FaceBook page where I will continue to update the progress of the research, follow me on Twitter @DrMartinHughes and watch the full behaviour video behaviour video here (https://​www​.youtube​.com/​w​a​t​c​h​?​v​=​h​H​B​I​q​H​y​I​e​0​s​&​f​e​a​t​u​r​e​=​y​o​u​tu.be)

Martin Hughes (m.​hughes.​4@​research.​gla.​ac.​uk)

Related publications

Hughes, M.R., Dodd, J.A., Maitland, P.S. and Adams, C.E., 2016. Lake bathymetry and species occurrence predict the distribution of a lacustrine apex predator. Journal of fish biology.

Hughes, M.R., Van Leeuwen, T.E., Cunningham, P.D. and Adams, C.E., 2016. Parentally acquired differences in resource acquisition ability between brown trout from alternative life history parentage. Ecology of Freshwater Fish.