Submitted 11 May 2016
Accepted 3 October 2016
Published 1 November 2016
Joseph L. Thorley,
Additional Information and
Declarations can be found on
2016 Thorne et al.
Creative Commons CC-BY 4.0
The abundance of large, piscivorous
Ferox Trout (Salmo trutta) in Loch
Alastair Thorne1, Alisdair I. MacDonald1and Joseph L. Thorley2
1Freshwater Laboratory, Marine Scotland, Pitlochry, Scotland
2Poisson Consulting, Nelson, British Columbia, Canada
Background. Ferox Trout are large, long-lived piscivorous Brown Trout (Salmo trutta).
Due to their exceptionally large size, Ferox Trout are highly sought after by anglers while
their life-history strategy, which includes delayed maturation, multiphasic growth and
extended longevity, is of interest to ecological and evolutionary modelers. However,
despite their recreational and theoretical importance, little is known about the typical
abundance of Ferox Trout.
Methods. To rectify this situation a 16 year angling-based mark-recapture study was
conducted on Loch Rannoch, which at 19 km2is one of the largest lakes in the
Results. A hierarchical Bayesian Jolly-Seber analysis of the data suggest that if individual
differences in catchability are negligible the population of Ferox Trout in Loch Rannoch
in 2009 was approximately 71 fish. The results also suggest that a single, often
unaccompanied, highly-experienced angler was able to catch roughly 8% of the available
fish on an annual basis.
Discussion. It is recommended that anglers adopt a precautionary approach and release
all trout with a fork length ≥400 mm caught by trolling in Loch Rannoch. There is an
urgent need to assess the status of Ferox Trout in other lakes.
Subjects Aquaculture, Fisheries and Fish Science, Conservation Biology, Ecology, Statistics
Keywords Survival, Hierarchical, Bayesian, Exploitation, Jolly-Seber, Abundance, Ferox Trout,
Brown Trout, Piscivorous
Due to its large size and distinctive appearance, the Ferox Trout was originally considered
its own species, Salmo ferox (Jardine, 1834); an appellation that was lost when all the
forms of Brown Trout were lumped into Salmo trutta. More recently, Duguid, Ferguson &
Prodohl (2006) have demonstrated that Ferox Trout in Lochs Melvin (Ireland), Awe and
Laggan (Scotland) are reproductively isolated and genetically distinct from their sympatric
conspecifics and together form a monophyletic grouping. Based on this evidence, Duguid,
Ferguson & Prodohl (2006) argue that the scientific name S. ferox should be resurrected.
Ferox Trout are characterized by their large size and extended longevity. The British
rod caught record is 14.4 kg (31 lb 12 oz) and the oldest recorded individual was estimated
to be 23 ±1 years of age based on scale annuli (Campbell, 1979). The consensus view is
that Ferox Trout achieve their large size by forgoing spawning until they are big enough
How to cite this article Thorne et al. (2016), The abundance of large, piscivorous Ferox Trout (Salmo trutta) in Loch Rannoch, Scotland.
PeerJ 4:e2646; DOI 10.7717/peerj.2646
to switch to a primarily piscivorous diet at which point they experience an increased
growth rate (Campbell, 1971;Campbell, 1979;Went, 1979). The resultant higher survival
and fecundity is assumed to compensate for the lost spawning opportunities (Mangel,
1996;Mangel & Abrahams, 2001).
Comparative lake studies (Campbell, 1971;Campbell, 1979) and ecological models
(Mangel, 1996;Mangel & Abrahams, 2001) indicate that Ferox Trout require a large
(>1 km2) oligotrophic lake and an abundant population of Arctic Charr (Salvelinus
alpinus). The ecological models also suggest that under such conditions, Ferox Trout should
constitute approximately 5% of the total Brown Trout population (Mangel & Abrahams,
2001). However, there is a lack of robust estimates of the abundance of Ferox Trout or
assessments of the potential for angling to impact individual populations. The reasons
for this knowledge gap were clearly stated by Duguid, Ferguson & Prodohl (2006, p. 90).
One of the main difficulties in attempting a detailed ferox study is obtaining sufficient
specimens. Ferox densities are believed to be low, and their large size and usual
distribution deep in the water column makes angling the only practical way to obtain
fish. Only a small number of ferox, however, are caught from any lake in a single year
even by anglers specializing in ferox capture.
At 19 km2, Loch Rannoch, which is situated in central Scotland, is one of the largest
lakes in the United Kingdom. It was chosen for the current study due to its long history
of producing Ferox Trout (Campbell, 1971;Campbell, 1979). Whether the Ferox Trout
in Loch Rannoch are sufficiently isolated and genetically distinct to be considered a
separate species (Duguid, Ferguson & Prodohl, 2006) is unknown. Consequently, Ferox
Trout were identified based on their large size and capture method—trolled dead baits and
lures (Campbell, 1971;Campbell, 1979;Went, 1979;Grey et al., 2002). As well as Brown and
Ferox Trout, Loch Rannoch also contains three ecologically and morphologically distinct
forms of Arctic charr (Verspoor et al., 2010).
In 1994, the first author (AT)—a highly-experienced ferox angler—began tagging and
releasing all Ferox Trout captured by himself or his boat companion on Loch Rannoch.
He continued this practise for 16 years. The paper uses the resultant dataset to estimate
the abundance of Ferox Trout in Loch Rannoch. Although the current dataset represents a
unique opportunity to better understand the life history of this top-level piscivore, the data
are nonetheless sparse. Consequently, they are analyzed using Bayesian methods which
provide statistically unbiased estimates irrespective of sample size (Kéry & Schaub, 2011;
Royle & Dorazio, 2008).
Loch Rannoch, which is located in Highland Perthshire (Latitude: 56.685 Longitude:
−4.321), has a length of 15.1 km, width of 1.8 km and a maximum depth of 134 m. It
is oligotrophic with a stony shoreline and lies in a catchment dominated by mixed relict
deciduous and coniferous woodlands with areas of rough grazing and marginal cultivation.
Murray & Pullar (1904) provide a more complete description of its physical characteristics.
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 2/12
Figure 1 Map of Ferox Trout Caught by Angling on Loch Rannoch between 1994 and 2009. The 69 ini-
tial captures are indicated by black circles and the 11 inter-annual recaptures by red triangles. Consecutive
recaptures of the same individual are linked by black lines. The coordinates are for UTM Zone 30N (EPSG
32630). Map data from Land Cover of Scotland data, MLURI 1993.
Loch Rannoch is part of the Tummel Valley Hydro Electric generation complex and has
been a hydroelectric reservoir since 1928, when Rannoch Power Station began to receive
water from Loch Ericht. A low barrage at the eastern end of the loch limits the change in
water level to a maximum of 2.74 m.
As well as Brown and Ferox Trout, the loch contains at least seven other species of fish:
Arctic Charr, Atlantic Salmon (Salmo salar), Pike (Esox lucius), Perch (Perca fluviatilis),
Eels (Anguilla anguilla), Three-Spined Sticklebacks (Gasterosteus aculeatus) and Minnows
Fish capture and tagging
Between 1994 and 2009, AT tagged and released all Ferox Trout captured by himself or
his boat companion while angling on Loch Rannoch (Fig. 1). In the absence of any genetic
data, a Ferox Trout was deemed to be any member of the Brown trout species complex
that was caught by trolling with a fork length ≥400 mm. A fork length of 400 mm was
chosen as this is considered to be the upper length threshold for the inferred switch to
piscivory (Campbell, 1971;Campbell, 1979). All the fish were caught during the angling
season (March 15 to October 6) by licensed anglers using permitted angling methods.
The research was conducted within the framework of the UK 1986 Animals Scientific
The Ferox Trout were angled by trolling mounted dead baits and lures behind a boat at
differing depths and speeds (Greer, 1995). The dead baits (usually Brown Trout or Arctic
Charr) were mounted to impart fish-like movement. An echo sounder was used to search
the contours of the loch bottom for drop-offs and likely fish holding areas and to ascertain
fishing depth. Typically, one entire circuit of the loch’s shoreline excluding the shallow
west end, which has an area of 3 km2, was undertaken on each visit.
Hooked fish were played with care and netted directly into a large tank of water before
being carefully unhooked. The fish was then transferred into a large fine-mesh keep net
(net pen), on the shore closest to the point of capture, where it was allowed to recover
before processing. After recovering, the fish was removed from the keep net and placed
in a tank containing water and anesthetic (0.05% aqueous solution of 2-pheoxyethanol).
When the fish was sufficiently sedated its fork length and wet mass were obtained. The
adipose fin was then clipped to aid in the identification of recaptures. In addition, all but
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 3/12
Table 1 Initial captures and subsequent recaptures of Angled Loch Rannoch Ferox Trout by year.
Captures Year 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09
7 1994 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0
6 1995 0 0 0 0 0 0 0 0 0 0 0 0 0 0
5 1996 0 0 0 0 0 0 0 0 0 0 0 0 0
2 1997 0 0 0 0 0 0 0 0 0 0 0 0
5 1998 0 0 0 1 0 0 0 1 0 0 0
2 1999 0 0 0 0 0 0 0 0 0 0
12 2000 0 1 1 0 0 0 0 0 0
6 2001 1 0 1 1 0 0 0 0
3 2002 0 0 0 0 0 0 0
4 2003 0 0 0 0 0 0
2 2004 0 0 0 0 0
2 2005 0 1 0 1
1 2006 0 0 0
1 2007 0 0
2 2008 0
one fish (F63) was externally tagged using a Carlin, dart or anchor tag. The tags included
the text ‘‘REWARD’’ and a telephone number for reporting. The reward value which was
not printed on the tag was five British pounds. The type of tag used depended on which
type was available at the time. After tagging, the fish was returned to the keep net to recover
and then released from the shore. The entire procedure typically took less than 30 min.
The capture location was estimated using a 1:5,000 map.
Five anglers, including AIM, accompanied AT on one or more occasions. On average
AT spent 10 days boat angling per year for approximately 10 h per day while fishing three
rods although detailed logs of angling effort were not kept. The boat, outboard, rods, reels,
line type and dead bait set-up remained constant throughout the study.
Two fish (F53 and F58), which were both recaught once, were excluded from the study
because they had a deformed spine and jaw, respectively. After the further exclusion of four
intra-annual recaptures, the data set contained information on 80 encounters involving 69
different Ferox Trout (Table 1); seven of which were recaught in at least one subsequent
Hierarchical Bayesian model
The abundance, annual survival and probability of (re)capture were estimated from
the mark-recapture data using a hierarchical Bayesian Jolly-Seber (JS) model (Kéry &
Schaub, 2011). The model was the superpopulation implementation of Schwarz & Arnason
(1996) in the form of a state-space model with data augmentation (Kéry & Schaub, 2011).
Based on preliminary analyses the augmented data set was fixed at 1,000 (genuine and
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 4/12
pseudo-) individuals. The zero-inflation of the augmented data set was modeled as an
inclusion probability (ψ). Due to the sparsity of data, the annual survival (S) and the
probability of (re)capture (p) were assumed to be constant. The only remaining primary
parameter was the probability of an individual recruiting to the population at the start of
the first year (ρ1). The prior probability distributions for ψ,S,pand ρ1were all uniform
distributions between zero and one. The hierarchical Bayesian JS state-space model made
the following assumptions:
1. Every individual in the population had the same constant probability of
2. Every individual in the population had the same constant probability of surviving (S).
3. Previously captured individuals were correctly identified.
4. The number of individuals recruiting to the population at the start of each year (B)
5. Sampling is instantaneous.
The posterior distributions of the parameters were estimated using a Monte Carlo Markov
Chain (MCMC) algorithm. To guard against non-convergence of the MCMC process, five
chains were run, starting at randomly selected initial values. Each chain was run for at least
105iterations with the first half of the chains discarded for burn-in followed by further
thinning to leave at least 10,000 samples. Convergence was confirmed by ensuring that the
Brooks-Gelman–Rubin convergence diagnostic was ˆ
R≤1.05 for each of the parameters
in the model (Brooks & Gelman, 1998;Kéry & Schaub, 2011). The reported point estimates
are the mean and the 95% credible intervals (CRIs) are the 2.5 and 97.5% quantiles
The analyses were performed using R version 3.3.1 (R Core Team, 2015), JAGS 4.2.0
(Plummer, 2003) and the ranmrdata and ranmr R packages, which were developed
specifically for this paper. Article S1 provides instructions on how to download the
packages and replicate the analysis.
The (re)captured fish varied from 400 to 825 mm in length and from 0.62 to 7.41 kg
in mass (Fig. 2). Although two large recaptures appeared to senesce (as evidenced by a
decline in mass with increasing length), there was no obvious effect of previous capture
on body condition (Fig. 2). Tag loss was only recorded for one of the individuals: F21
on its second recapture eight years after it was initially tagged. F21 was identified from
photographs of its melanophore constellations (Figs. S1–S3). F13 and F45 were recaught
by non-participatory anglers. F45 was released. Both recapture events were excluded from
the data, plots and analyses.
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 5/12
Figure 2 Mass-length scatterplot for Ferox Trout Caught by Angling. The 69 initial captures are indi-
cated by black circles and the 11 inter-annual recaptures by red triangles. Consecutive recaptures of the
same individual are linked by black lines.
The Bayesian JS mark-recapture model estimated the annual survival (S) to be 0.74
(95% CRI 0.57–0.89) and the annual probability of capture by the primary author or
his companion (p) to be 0.08 (95% CRI 0.03–0.16). The inclusion parameter (ψ) was
estimated to be 0.42 (95% CRI 0.23–0.76) while the probability of recruiting at the start of
the first year (ρ1) was 0.26 (95% CRI 0.13–0.44). The number of individuals recruiting to
the population annually (B) was 21 individuals (95% CRI 11–37). The abundance estimate
was 111 individuals (95% CRI 39–248) in 1994 and 71 individuals (95% CRI 30 –148) in
2009 (Fig. 3).
The Bayesian p-value on the posterior predictive check was 0.31 which indicates that
the distribution of the number of encounters (captures and recaptures) each year was
consistent with the assumed constant capture efficiency.
The JS mark-recapture model estimated that the population of Ferox Trout in Loch
Rannoch declined from 111 individuals in 1994 to just 71 individuals in 2009. Whether
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 6/12
Figure 3 Loch Rannoch Ferox Trout abundance estimates by year. The solid line indicates the point es-
timates and the dotted lines the 95% credible intervals.
or not the abundance estimates are accurate depends in part on the extent to which the
assumption of a constant capture probability is met. The assumption can be violated
in two ways: the capture probability can vary among years or it can vary among fish.
Variation among years can introduce an artificial trend in the abundance estimates across
the course of the study while variation among fish can cause the abundance to be over or
underestimated depending on whether any individual differences are fixed (Biro, 2013) or
learnt (Askey et al., 2006), respectively. Although angling logs were not kept the posterior
predictive check, which compared the number of predicted versus observed encounters,
statistically confirmed the relative constancy of pamong years. Nevertheless, individual
Ferox Trout may still have differed in their vulnerability to capture by angling. As is the
case for many mark-recapture studies the reliance on a single capture method and the
relatively low number of encounters means it is not possible to determine the presence or
form of any individual differences (Biro, 2013).
If the individual differences in catchability are negligible, the abundance estimates are
unbiased and by 2009 the Ferox Trout were present at a density of just 0.044 fish.ha−1when
the shallow west end is excluded (Engstrom-Heg, 1986). For comparison, Johnston et al.
(2007) estimated that the density of large piscivorous Bull Trout (Salvelinus confluentus) in
the 6.5 km Lower Kananaskis Lake, Alberta, was 0.093 fish.ha−1when being overexploited.
In response to a zero-harvest regulation, the density of large Bull Trout in Lower Kannaskis
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 7/12
Lake increased to over 2.6 fish.ha−1in less than a decade. The higher density of Bull Trout in
Lower Kananaskis Lake could reflect differences in lake productivity or life-history strategy.
The annual interval mortality estimate (1−S) of 0.26 includes handling and tagging by
the primary author and his companion as well as natural mortality and fishing mortality
by all other anglers on the loch. As all fish recovered well and were only adipose clipped
and marked with a single external tag, it is likely that handling and tagging effects were
small. Furthermore, despite the offer of a reward only two fish were reported to have been
recaught by a member of the public which suggests that the exploitation rate by other
anglers on the loch was low. Consequently, if individual differences in catchability are
negligible, 26% is probably only a moderate overestimate of the natural mortality rate.
For comparison, Johnston et al. (2007) estimated the equilibrium natural mortality rate for
adult Bull Trout in Lower Kananaskis Lake to be around 27%.
Management and conservation implications
A concern for any small salmonid population is that the loss of genetic variation results in
loss of adaptive potential or inbreeding depression (Wang, Hard & Utter, 2002). Although
the levels at which the low genetic variation results in population-level consequences are
difficult to predict (Vincenzi et al., 2010), the rate at which genetic variation is being lost can
be calculated from the effective population size (Ne) (Wright, 1931;Wright, 1978). Due to
their mating systems and life-histories, the Neof most salmonid populations is considered
to be around 25% of the spawning population size (Allendorf et al., 1997;McElhaney et al.,
2000). Thus, even if all the adult Ferox Trout in Loch Rannoch spawn in each year then
this suggests that if individual differences in catchability are negligible the Nein 2009 was
just eight. The low effective population size is concerning because an Ne≥50 is needed
to minimize inbreeding effects and an Ne≥500 is required to retain long-term adaptive
potential (Allendorf et al., 1997).
Whether or not the Ferox Trout in Loch Rannoch are at risk of inbreeding depression
partly depends on the extent to which they are reproductively isolated from the other
Brown Trout in the loch. If they, like the Ferox Trout in Lochs Melvin, Awe and Laggan,
are sufficiently isolated and genetically distinct to be considered a separate species (Duguid,
Ferguson & Prodohl, 2006) then inbreeding is likely occurring. Alternatively, if the Ferox
Trout in Loch Rannoch are simply Brown Trout adopting an alternative life-history
strategy, then the effective population size is a function of the total number of Brown Trout
spawners and inbreeding is not an issue.
Nonetheless, even if the Ferox Trout in Loch Rannoch are not genetically isolated, a
sustained high exploitation rate could result in adaptive change. Mangel and Abrahams’
Mangel & Abrahams (2001) individual-based model predicted that the proportion of the
population adopting the ferox life-history strategy is affected by mortality with high
size-independent mortality being associated with no or few Ferox Trout. The explanation
is straightforward; with increasing mortality the chances of benefiting from delayed
maturation diminish. The potentially high catchability suggests that in the absence of catch
and release even small amounts of angler effort could produce sufficient fishing mortality
to select against the ferox adaptation (Hard et al., 2008).
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 8/12
Given the concerns associated with a potentially high exploitation rate on a long-lived,
late-maturing population it is recommended that anglers adopt a conservative approach
and release all trout longer than 400 mm caught by trolling in Loch Rannoch. There is an
urgent need to assess the status of Ferox Trout in other lakes.
We thank the Loch Rannoch Conservation Association for approving the study; C and J
Monkton for allowing access to the loch and providing a mooring; PJ Bacon, RA Duguid,
R Greer, RL Irvine, IA Malcom and AF Youngson for feedback on earlier drafts and
S Vincenzi and two other anonymous reviewers for their helpful comments. We particularly
thank the Ferox85 group members who assisted the study.
ADDITIONAL INFORMATION AND DECLARATIONS
The study was partially funded by Poisson Consulting Ltd. in the form of a salary for Joseph
Thorley. The funders had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
The following grant information was disclosed by the authors:
Poisson Consulting Ltd.
Alastair Thorne and Alasdair MacDonald are members of Ferox85, an informal organization
dedicated to understanding Ferox Trout biology and management. Joseph Thorley is
employed by Poisson Consulting Ltd. to provide independent analytic services for a wide
variety of government agencies, corporations and conservation organizations on a range
of different species and issues.
•Alastair Thorne and Alisdair I. MacDonald conceived and designed the experiments,
performed the experiments, reviewed drafts of the paper.
•Joseph L. Thorley analyzed the data, wrote the paper, prepared figures and/or tables.
The following information was supplied relating to ethical approvals (i.e., approving body
and any reference numbers):
All the fish were caught during the angling season (March 15 to October 6) by
licensed anglers using permitted angling methods. The research was conducted within
the framework of the UK 1986 Animals Scientific Procedures Act.
Thorne et al. (2016), PeerJ, DOI 10.7717/peerj.2646 9/12
The following information was supplied regarding data availability:
ranmrdata R data package:
ranmr R analysis package:
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
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