SHORT COMMUNICATION
MORPHOMETRIC AND MERISTIC
CHARACTERISTICS ANALYSES OF TWO
WESTERN IRISH POPULATIONS OF
ARCTIC CHAR, SALVELINUS ALPINUS (L.)
D. Doherty and T.K. McCarthy
ABSTRACT
Multivariate morphometric and meristic analyses of Irish Arctic char Salvelinus alpinus (L.) from
Lough Eske, Co. Donegal, and Lough Mask, Co. Mayo, are presented. The analysis illustrated the
monomorphic character of the two populations despite differences in growth and size between the
‘stunted’, slower-growing Lough Eske fish and the ‘normal’, faster-growing Lough Mask fish. The
results are discussed in the context of other systems where sympatric morphs have been described.
Differences in body size and growth rate appear to reflect the trophic status and the productivity
of the two lakes. The results confirm earlier findings, which were based on dietary analysis and
analysis of metazoan parasites of both Irish populations of Arctic char.
D. Doherty
(corresponding author;
e-mail:
denis.doherty@mail.
esb.ie) and T.K.
McCarthy, Zoology
Department, National
University of Ireland,
Galway.
Received 5 March 2002.
Read 2 October 2003.
Published 31 May 2004.
BIOLOGY
AND
INTRODUCTION
The Arctic char Salvelinus alpinus (L.) is circumpolar in distribution, being found in many subarctic, alpine and northern temperate freshwater,
estuarine and marine regions. In Ireland, Arctic
char mainly occur in oligotrophic freshwater lakes
along the western seaboard (Fig. 1). Arctic char in
Ireland are considered landlocked glacial relicts of
an otherwise anadromous circumpolar species. In a
description of Irish char populations by Tate
Regan (1911) they were divided into six ‘species’,
with both Lough Mask and Lough Eske char being
classed as ‘Coles’ char Salvelinus colii Gunther.
More recent biochemical taxonomic studies,
though confirming the existence of considerable
heterogeneity between populations, did not
suggest that recognition of distinct Irish subspecies
was appropriate. Ferguson (1981) suggested that all
the populations examined were conspecific and
descended from a common ancestor.
Worldwide, Arctic char have been widely
noted for their potential adaptive phenotypic
plasticity, and throughout the area of distribution
the species has shown extensive phenotypic and
ecological variation both within and between
localities (Johnson 1980). In monomorphic local
populations, adult fish exhibiting different habitat
uses appeared not to differ morphologically,
whereas in polymorphic local populations, the
ENVIRONMENT: PROCEEDINGS
OF THE
groups using the two main habitats of the lake (the
benthic and pelagic zones) often differed in terms
of characteristics such as body coloration and
trophic morphology (Hindar and Johnson 1982;
Sandlund et al. 1992). Up to four morphotypes
have been identified from lacustrine and estuarine
systems, mainly on the basis of their coloration,
meristic characteristics, growth rate, size and age at
sexual maturity, place of spawning, food and
habitat choice and associated parasites, with some
sympatric morphs being reproductively isolated
(Frost 1965).
This paper represents the results of a
quantitative examination of two lacustrine
populations of Irish char to test for both inter- and
infra-lacustrine population differences in morphology. The Lough Eske population represents a
typical ‘stunted’ slow-growing population, being
primarily planktivorous, whereas the Lough Mask
population represents a so-called ‘normal’
population, being typically faster-growing and
feeding to a greater extent on macroinvertebrates
(Doherty and McCarthy 2000).
MATERIAL AND METHODS
SAMPLES
As part of a large-scale gill-netting programme
undertaken by the Irish Central Fisheries Board
ROYAL IRISH ACADEMY, VOL. 104B, NO. 1, 75– 85 (2004). © ROYAL IRISH ACADEMY
75
BIOLOGY
AND
ENVIRONMENT
Fig. 1— The distribution of Arctic char in Ireland, indicating the status of the populations, past and present (Whilde
1993; Quigley and Flannery 1997).
(CFB) and the Western Regional Fisheries Board,
a sample of 61 spawning char were obtained from
Lough Mask in November 1996 (Fig. 1). Details of
this oligotrophic lake (area=80km2, maximum
depth=58m, mean total phosphorous=8.4mg
l − 1, mean chlorophyll a =5.9mg l − 1) are given
by McCarthy et al. (2000). Samples of char from
other western lakes sampled during the CFB
76
gill-netting programme, which are described in
Igoe et al. (2001), were not available for this
analysis.
Rod-caught char were sampled (n= 35) at
Lough Eske (Fig. 1) during the 1996 spawning
season. Lough Eske was classified by Flanagan and
Toner (1975) as oligotrophic (area= 3.64km2,
maximum depth= 21m, mean total phos-
CHARACTERISTICS ANALYSES
OF
TWO WESTERN IRISH POPULATIONS
phorous =0.12– 0.31mg l − 1, mean chlorophyll
a =2.4mg l − 1). All fish were labelled and deepfrozen within eight hours of landing.
OF
ARCTIC CHAR
were removed and stored to determine fish age
(Frost 1978; Frost and Kipling 1980).
Comparisons of body shape variation using
multivariate analyses between two differently
sampled char samples, i.e. gill-netted (Lough Mask)
and rod-captured specimens (Lough Eske), are
acceptable because these ‘shape’ variables occur
irrespective of size, particularly when the data is
corrected for length (or size), i.e. transformed. It
was also important that both samples were similar
in age.
Prior to multivariate analysis, to allow for an
overall effect of fish size, all the morphometric
measurements, with the exception of the index of
snout bluntness, were standardised for fish size
Morphometric and meristic characteristics
A total of 25 morphometric measurements were
recorded for each fish, using the same
methodology as Boulva (1972). The body
characteristics measured are listed in Table 1. The
head characteristics are similar to those listed by
Adams et al. (1998) with the exception of the
index of snout curvature, which was not used in
the present study. Meristic characteristics recorded
included gill raker number, branchiostegal ray
number and an index of snout bluntness. Scales
Table 1a —A summary of morphometric characteristics measured in millimetres (to the nearest 0.01mm) for two
samples of char from Lough Mask and Lough Eske.
Lough Mask (n= 61)
Mean
Total length
Standard length
Head length
Head width at nostrils
Head width at eyes
Head depth at eyes
Head depth at edge of operculae
Eye diameter
Lower jaw length
Maxillary bone length
Jaw width on ventral surface
Average gill raker length
Average distance between gill rakers
Gape
Mouth height
Mouth width
Body height at insertation of pectoral fin
Caudal peduncle height
Caudal peduncle width
Caudal peduncle length
Insertation of pelvic fin to insertation of
anal fin length
Insertation of dorsal fin to insertation of
ventral fin length
Insertation of dorsal fin to insertation of
ventral fin width
Pectoral fin length
Perpendicular distance between eye and
snout
Min.
Max.
SD
Lough Eske (n= 35)
SE
Mean
Min.
Max.
SD
SE
266.93 233.99 327.13 20.16 2.58 175.28 148.75 195.55 11.23 1.87
240.22 153.50 294.45 21.06 2.70 155.18 11.30 186.48 27.09 4.58
53.14 29.58 64.31 5.35 0.68 38.20 34.57 44.01 2.12 0.35
14.14 11.16 18.59 1.73 0.22
9.48
7.65 11.30 0.88 0.15
19.05 14.96 25.59 2.55 0.33 13.61 10.58 16.24 1.56 0.26
28.72 25.17 34.95 2.23 0.29 20.07 16.61 23.33 1.50 0.25
43.65 32.99 57.26 5.09 0.65 28.93 24.78 34.09 2.26 0.38
9.70
8.21 11.17 0.67 0.09
8.55
7.51
9.59 0.61 0.10
33.10 27.26 42.79 3.70 0.47 23.19 21.17 27.06 1.43 0.24
25.85 20.21 34.72 3.03 0.39 17.19 14.79 21.35 1.29 0.21
17.15 11.82 46.62 4.47 0.57
9.39
8.20 11.31 0.79 0.13
1.14
0.67
1.84 0.24 0.03
1.22
0.89
1.78 0.23 0.04
0.14
0.10
0.20 0.02 0.00
0.10
0.01
0.14 0.02 0.00
21.93 18.26 28.26 1.76 0.23 14.28 11.73 18.85 1.38 0.23
21.52 16.32 27.96 2.44 0.31 15.02 10.02 20.22 2.05 0.34
17.46 13.87 22.03 1.75 0.22 11.36
8.50 30.98 3.55 0.59
49.89 38.93 64.62 5.65 0.72 31.08 13.43 35.10 3.60 0.61
23.08 19.57 26.83 1.85 0.24 14.79
6.60 17.07 1.68 0.28
12.34
9.24 14.95 1.27 0.16
7.97
5.82 18.35 2.00 0.34
28.39 20.39 65.29 5.93 0.76 20.08 14.72 35.76 4.31 0.73
58.70 27.34 84.83 7.54 0.97 36.56 30.32 44.72 3.76 0.64
63.97
32.47
81.71
7.64 0.98
39.17
34.80
44.13
2.40 0.41
36.21
28.91
60.94
5.42 0.69
20.12
17.36
22.38
1.24 0.21
45.03
5.74
35.02
3.48
62.67
8.68
5.51 0.71
1.16 0.15
29.48
3.66
24.23
2.30
34.19
5.02
2.36 0.39
0.69 0.11
77
BIOLOGY
AND
ENVIRONMENT
using the equation of Senar et al. ( 1994), where:
Table 1b — Meristic characteristics for two
samples of char from Lough
Mask and Lough Eske, including
an index calculation.
Y%i= log10Y%i −b(log10Li − log10X)
and Li = Lf for fish i (where Lf = fork length),
X= mean Lf (all fish), Y%i= size-corrected
morphometric variable value for fish i, and b= the
pooled regression coefficient of log10Y on log10L
for all fish combined.
We ensured that the size-corrected variables
were not themselves correlated with fork length so
that the effectiveness of standardisation could be
checked. A multivariate technique, principal
component analysis (PCA), was used to analyse the
transformed morphometric data. The results of the
analyses, which were carried out using Excel Stat
and Minitab, highlight the possible heterogeneity
between and among populations.
For the meristic characteristics, the significance of inter- and intrapopulation differences
was initially tested using a one-way analysis of
variance. The index of snout bluntness, which was
not correlated with fish length, was compared
between the two populations using univariate
analysis of variance (ANOVA). Subsequently
discriminant function analysis was also applied to
the transformed morphometric data to try to (i)
classify the fish into one of several mutually
exclusive groups; (ii) establish the most important
characteristics for distinguishing between the
groups; and (iii) calculate the percentage overlap
between each group based on the generalised
distance (the Mahalanobis squared distance, D 2).
Lough Mask (n= 61)
Branchiostegal
ray no.
Mean
Min.
Max.
SE
SD
10.95
10.00
12.00
1.06
0.14
Gill
raker
no.
Snout
bluntness
index
23.30
22.00
26.00
0.50
0.06
0.18
0.12
0.27
0.04
0.01
Lough Eske (n= 35)
Branchiostegal
ray no.
Mean
Min.
Max.
SE
SD
9.39
8.00
12.00
0.69
0.11
Gill
raker
no.
Snout
bluntness
index
23.06
21.00
26.00
0.86
0.14
0.16
0.10
0.23
0.03
0.01
Table 2 — The length (mm) at age of Arctic char from Ireland (present study), England (Frost and Kipling 1980)
and Scotland (Adams et al. 1998) and Iceland (Sandlund et al. 1992) using scale analysis.
Age (years)
1
Lake Windermere, England Normal, autumn 63
Normal, spring
54
Dwarf, autumn
63
Dwarf, spring
53
Loch Rannoch, Scotland
Pelagic
100
Benthic
69
Lough Mask, Ireland
L. Mask
Lough Eske, Ireland
L. Eske
Lake Thingvallavatn,
Small, benthic
6.8
Iceland
Large, benthic
8.1
Piscivorous
9.1
Planktonic
7.9
78
2
116
103
102
85
142
110
172
162
8.0
11.5
13.8
12.2
3
182
175
148
121
181
147
209
175
9.1
14.9
17.0
15.5
4
242
253
181
155
201
168
261
187
10.1
17.2
19.9
17.3
5
6
7
8
271
296
209
186
209
200
298
281
317
228
195
217
209
279
330
328
223
211
210
11.1
19.9
22.4
18.5
11.8
22.4
24.1
19.0
12.6
24.1
25.0
19.8
13.8
26.2
25.9
19.9
9
10
11
12
215
219
221
264
14.0
29.3
25.7
20.0
14.5
31.8
26.2
20.1
34.2
27.0
20.2
35.4
27.8
CHARACTERISTICS ANALYSES
OF
TWO WESTERN IRISH POPULATIONS
Table 3 — The results of the one-way analysis
of variance (ANOVA) between the
char samples from Lough Mask
and Lough Eske for the three
meristic measurements.
Gill raker number
Branchiostegal ray
number
Index of snout
bluntness
F1, 94
Significance
1.054
159.09
0.3072
0.0001
7.18
0.0087
OF
ARCTIC CHAR
RESULTS
The mean, standard deviation, standard error and
range of each morphometric and meristic
characteristic are shown in Table 1. The Lough
Mask char were generally larger than the Lough
Eske char due to the presence of larger, older,
gravid females. The growth rates of Lough Mask
and Lough Eske char are presented in Table 2
together with other European char populations for
which several lacustrine morphotypes have been
reported (Frost and Kipling 1980; Sandlund et al.
1992; Adams et al. 1998). The mean age and
standard deviation of the Lough Mask and Lough
Eske samples were 4.19 0.5 and 3.190.8 years
Table 4 — Principal component analysis of untransformed morphometric characteristics of
Lough Mask (n= 61) and Lough Eske (n= 35) Arctic char. The first three principal
components accounted for 86.2% of the variance. Values in the body of the table are
component loadings.
Variable
Total length
Standard length
Head length
Head width at nostrils
Head width at eyes
Head depth at eyes
Head depth at edge of operculae
Eye diameter
Lower jaw length
Maxillary bone length
Jaw width on ventral surface
Average gill raker length
Average distance between gill rakers
Gape
Mouth height
Mouth width
Body height at insertation of pectoral fin
Caudal peduncle height
Caudal peduncle width
Caudal peduncle length
Insertation of pelvic fin to insertation of anal fin length
Insertation of dorsal fin to insertation of ventral fin length
Insertation of dorsal fin to insertation of ventral fin width
Pectoral fin length
Perpendicular distance between eye and snout
Eigen value
% of variance
Cumulative % variance
Principal
component 1
Principal
component 2
Principal
component 3
0.21
0.21
0.22
0.21
0.20
0.20
0.20
0.18
0.21
0.21
0.20
0.01
0.18
0.19
0.18
0.21
0.20
0.21
0.20
0.18
0.20
0.21
0.20
0.21
0.17
0.06
0.08
0.06
0.14
0.18
−0.02
−0.16
0.25
−0.03
0.00
−0.04
0.84
0.09
−0.15
−0.17
−0.01
−0.05
0.03
−0.02
−0.25
0.02
−0.05
−0.04
0.00
0.02
−0.12
−0.14
−0.03
0.12
0.13
0.09
0.02
−0.17
−0.02
−0.03
0.05
−0.17
0.46
−0.12
−0.18
−0.15
0.00
−0.10
0.13
−0.27
−0.11
−0.12
−0.08
0.01
0.66
20.57
79.13
79.13
1.27
4.87
83.99
0.56
2.16
86.20
79
BIOLOGY
AND
ENVIRONMENT
respectively. A total of 68.9% of the Lough Mask
sample were four years old, the Lough Eske fish
being generally younger, with 40.0% of the sample
being three years old. The sex ratio of both
populations differed, with females predominant, at
59.0% (n = 36), for Lough Mask and males
predominant, at 80.0% (n =28), for the Lough
Eske sample. The significance of both meristic and
morphometric differences between these samples is
dealt with in the statistical analyses below.
STATISTICAL ANALYSES
One-way analysis of variance
The F-values and significance levels for all meristic
measurements and the index of snout bluntness for
the two samples are shown in Table 3. Highly
significant differences in the mean branchiostegal
ray number, gill raker number and index of snout
bluntness were noted between the Lough Eske and
Lough Mask samples: all were higher in the Lough
Mask sample. Sexual dimorphism did not occur in
either of the two samples, and there were no
significant differences in length between male and
female char.
Principal component analysis
Principal component analysis was initially applied
to untransformed morphometric characteristics, i.e.
measurements not standardised for fish length.
Three principal components (Table 4) were
extracted from the 25 morphometric characteristics. An analysis of the correlation matrix
shows that all of the variables were highly
correlated with fish length, the exception being
average gill raker length (r value = 0.0802;
P \ 0.05). The component loadings (Table 4)
were also very high for most of the variables
accounted for by the first principal component,
which described 79.13% of the variance within the
samples. The second and third principal
component accounted for 4.87% and 2.16% of
the total variance respectively. Jolicoeur and
Mosimann (1960) demonstrated that any
component having all coefficients of the same sign
was indicative of size variation, whereas any
component having both positive and negative
coefficients was indicative of shape variation. All of
the coefficients (component loadings) were of the
same sign and magnitude. Therefore it can be
assumed that principal component 1 accounted for
size variation between Lough Mask and Lough
Eske char. Individual fish from each sample are
plotted (axis 1 vs. axis 2) in Fig. 2. Although it
appeared from the analysis that the Lough Mask
char were separating into two groups, a significant
Spearman rank correlation between the component loadings on principal component 1 and fish
80
Fig. 2 — Principal component analysis of untransformed
morphometric data taken from Arctic char samples from
Lough Mask (n =61) and Lough Eske (n= 35). The scatter
plot shows individual fish scores for axis 1 vs. axis 2.
length (r 2 = − 0.382, P =0.002) indicated that
these differences were due to fish size rather than
fish shape.
Principal component analysis was again
applied to transformed morphometric characteristics excluding total and standard length.
Analysis of the correlation matrix showed a mixed
number of correlations with no clear pattern
emerging. Details of the first three principal
components are listed in Table 5. Principal
component 1 (40.73%) coefficients were both
positive and negative, indicating shape variation.
Average gill raker length, dorsal fin to ventral fin
length and jaw width on the ventral surface were
among the characteristics most highly correlated
with PC1. Principal component 2 described
11.32% of the variance, with caudal peduncle
length and head length being most highly
correlated with principal component 2. The first
three principal components accounted for 60.12%
of the total variance in the three samples.
Individual fish from each sample are plotted (axis 1
vs. axis 2) in Fig. 3. Some separation of both the
Lough Mask and Lough Eske char samples was
evident along axis 1 as shown in Fig. 3.
CHARACTERISTICS ANALYSES
OF
TWO WESTERN IRISH POPULATIONS
OF
ARCTIC CHAR
Table 5 — Principal component analysis of transformed morphometric characteristics of Arctic
char in Lough Mask (n=61) and Lough Eske (n=35). The first three principal
components accounted for 60.12% of the variance. Values in the body of the table
are component loadings.
Variable
Principal
component 1
Principal
component 2
Principal
component 3
Head length
Head width at nostrils
Head width at eyes
Head depth at eyes
Head depth at edge of operculae
Eye diameter
Lower jaw length
Maxillary bone length
Jaw width on ventral surface
Average gill raker length
Average distance between gill rakers
Gape
Mouth height
Mouth width
Body height at insertation of pectoral fin
Caudal peduncle height
Caudal peduncle width
Caudal peduncle length
Insertation of pelvic fin to insertation of anal fin length
Insertation of dorsal fin to insertation of ventral fin length
Insertation of dorsal fin to insertation of ventral fin width
Pectoral fin length
Perpendicular distance between eye and snout
0.07
0.07
0.14
0.22
0.27
−0.21
0.25
0.22
0.27
−0.24
0.10
0.11
0.10
0.13
0.25
0.26
0.22
0.08
0.00
0.27
0.25
0.04
−0.12
0.45
0.37
0.34
0.13
−0.06
0.23
0.15
0.19
0.03
0.23
0.21
0.00
0.02
0.08
−0.02
−0.01
−0.02
−0.24
−0.05
−0.07
−0.11
0.29
0.28
0.19
−0.29
−0.31
0.03
0.04
0.07
0.18
0.19
−0.06
−0.02
−0.12
0.44
0.41
0.23
−0.10
−0.10
−0.19
0.25
−0.16
−0.08
−0.08
0.26
−0.13
10.59
40.73
40.73
2.94
11.32
52.05
2.10
8.07
60.12
Eigen value
% of variance
Cumulative % variance
Discriminant function analysis
Since it was possible to distinguish between the
two populations of char on the basis of length
alone, linear discriminant function analysis was
carried out without total length and standard
length using transformed morphometric data, with
fish being classified a priori to their respective
samples. The predicted group membership is
determined by assigning each char specimen to
the group whose mean is closest (under the
Mahalanobis distance) to the specimen. The overall
percentage of fish classified correctly to each group
according to the discriminating function was
100%, with a Mahalanobis squared distance (D 2)
between the two samples of 86.6 (Fishers
F = 79.89, P= 0.0001). The standardised function
coefficient for each morphometric characteristic is
given in Table 6. The large size of the dorsal fin to
ventral fin length function coefficient and the head
length coefficient compared to the function
coefficients of other characteristics indicates that
the addition of many of these characteristics was
trivial, the least relevant being the perpendicular
distance between eye and snout. However, no
consistent shape variation that could be related to a
functional use could be identified in either
population investigated.
DISCUSSION
According to Malmquist (1992), morphotypes
described from lacustrine environments are usually
morphologically and ecologically distinct, with the
feeding of cryptically coloured benthic morphs on
slow-moving zoobenthos (such as snails) being
improved by their broad body form, relatively
81
BIOLOGY
AND
large pectoral fins and the wide, terminally
positioned mouth. The size of the mouth in these
benthic morphotypes is thought to allow the larger
benthic morphotypes to feed upon relatively larger
prey items. In comparison, the fusiform body of
the more brightly coloured, slimmer limnetic
morphotypes aids their efficiency in searching for
and feeding on dispersed and mobile open water
prey, and their narrow terminal snouts help
decrease drag and increase suction power. In
addition, their relatively longer gill rakers and the
narrower spacings between the gill rakers may
further increase the retention of zooplankton.
Malmquist (1992) also noted that larger piscivorous
char were similar to planktivorous char, with a
fusiform body but with wide powerful jaws with
pronounced teeth.
In contrast, the results of the present study
indicate the monomorphic characteristics of the
Lough Mask and Lough Eske char populations
investigated. However, significant differences in
body size between the two sites were apparent and
higher meristic values for the Lough Mask samples
were also clear (Table 1). Although the Lough
Mask fish appeared to separate into two groups in
the PCA analysis of the untransformed data (Fig.
2), this separation could be accounted for by
differences in fish size, not fish shape. However, in
the case of the transformed data (Fig. 3), there
were differences associated with a number of
morphometric characteristics (Table 3). Nevertheless, since none of these characteristics could be
attributed to a clear functional use, it is thought
that the Lough Mask population is monomorphic.
Similarly, an analysis of the diet and parasite
burdens of the Lough Mask char using PCA
analysis (Doherty and McCarthy 2000) also found
no separation of the Lough Mask samples.
Differences in char morphometrics linked to
body size, as observed in respect of the Lough Eske
and Lough Mask char populations, may be due to
differing parasite burdens, as well as opportunities
for benthic feeding (Doherty and McCarthy 2000).
Lough Mask char fed predominantly on benthic
macroinvertebrates, and the presence of three
acanthocephalan and one swim-bladder nematode
parasite species (all four of which use Gammarus
species as intermediate hosts) further substantiated
the dominance of macroinvertebrates in the diet.
In contrast, the Lough Eske char were recorded to
be feeding on planktonic copepods to a large
extent, and consequently higher burdens of three
cestode species occurred (Doherty and McCarthy
2000). The partitioning of the food and the habitat
use by sympatric morphotypes have been shown
elsewhere to be important as determinants of the
parasite community and in distinguishing between
anadromous and lacustrine char stocks (Dick 1984;
Frandsen et al. 1989; Dorucu et al. 1995).
82
ENVIRONMENT
Fig. 3 — Principal component analysis of transformed
morphometric data taken from Arctic char samples from
Lough Mask (n =61) and Lough Eske (n= 35). The
scatter plot shows individual fish scores for axis 1 vs. axis
2.
However, Doherty and McCarthy (2000)
concluded that no evidence existed for the
presence of sympatric char morphotypes in either
Lough Mask or Lough Eske based upon dietary
analysis and associated metazoan parasites.
Despite the monomorphic nature of the two
lacustrine populations investigated in the present
study, it does not preclude the existence of
sympatric morphotypes within lacustrine environments elsewhere in Ireland. Behnke (1980)
suggested that some geographical regions may have
suitable environments but no native char
populations sufficiently specialised to most
efficiently occupy the various trophic niches.
According to Sandlund et al. (1992) and Adams
et al. (1998), possible reasons for sympatric
morphotypes in lacustrine habitats are thought to
be related to lake morphology. Factors such as lake
depth (with distinct pelagic and benthic zones) and
the substrate type available to populations of arctic
char may affect the development of multiple
morphotypes. However, Behnke (1980) was of the
CHARACTERISTICS ANALYSES
OF
TWO WESTERN IRISH POPULATIONS
opinion that sympatric stocks may have been
subject to multiple invasions of incipient species
that have developed in geographical isolation or
they may be the result of differentiation in
response to ecological conditions within a
particular waterbody. According to Langeland and
Jonsson (1990) and L’Abée-Lund et al. (1993) the
most pronounced monomorphisms are to found in
the so-called ‘stunted’ populations, which feature
low juvenile growth rates, small and homogenous
size at sexual maturation and negligible adult
growth rates. According to Bjøru and Sandlund
(1995), the occurrence of stunted or ‘dwarf’ char
populations are generally caused by good spawning
conditions yielding high recruitment relative to the
food available. This may be the case for the Lough
Eske population, where suitable known spawning
sites are sufficient for recruitment but the
oligotrophic nature of the lake may result in poor
feeding. No spawning site is known for the Lough
Mask Arctic char. Both Lough Mask and Lough
Eske char populations appear to be exclusively
autumn-spawning, in contrast to other populations
OF
ARCTIC CHAR
such as Lake Windermere in northern England,
where both autumn- and spring-spawning char are
found (Frost 1965). At least within Lough Eske,
the apparent lack of spatial and temporal segregation, which may enhance genetic segregation of
within lacustrine char stocks, may be a factor for
the monomorphic nature of the char stock.
The distribution of Arctic char populations in
Ireland (Fig. 1) has been reported by Whilde
(1993). Ireland’s Arctic char population appears to
be decreasing, despite the recent addition of some
newly reported populations by Quigley and
Flannery (1997). In Ireland, the effects of
pollution, eutrophication, climate change and
possibly competition with and predation by
introduced species such as roach (Rutilus rutilus L.)
and pike (Esox luscius L.) have led to a contraction
of the Arctic char’s range (Whilde 1993; Doherty
and McCarthy 2000; McCarthy et al. 2000, 2001;
Igoe et al. 2001). Possible threats to European
populations of freshwater fish have been
extensively covered by Cowx (2002) and Cowx
Table 6 —A summary of linear discriminant function analysis of standardised morphometric
characteristics for Arctic Char in Lough Mask and Lough Eske.
Variable
Lough Mask
(n= 61)
Lough Eske
(n= 35)
Standardised
function
coefficient
Head length
Head width at nostrils
Head width at eyes
Head depth at eyes
Head depth at edge of operculae
Eye diameter
Lower jaw length
Maxillary bone length
Jaw width on ventral surface
Average gill raker length
Average distance between gill rakers
Gape
Mouth height
Mouth width
Body height at insertation of pectoral fin
Caudal peduncle height
Caudal peduncle width
Caudal peduncle length
Insertation of pelvic fin to insertation of anal fin length
Insertation of dorsal fin to insertation of ventral fin length
Insertation of dorsal fin to insertation of ventral fin width
Pectoral fin length
Perpendicular distance between eye and snout
Constant
9678
1828
−2172
196
−8
1764
−668
581
−2074
281
−2038
816
−434
−1539
−186
496
−17
811
2652
3144
3286
2482
479
−18564
9880
1866
−2199
205
−115
1922
−727
528
−2151
327
−2061
926
−468
−1514
−29
400
4
881
2625
2897
3184
2557
479
−18591
−202
−38
27
−9
107
−158
59
53
77
−46
23
−110
34
−25
−157
96
−21
−70
27
247
102
−75
0
27
83
BIOLOGY
AND
and Collares-Pereira (2002), who concluded that
there was an immediate need to protect threatened
species from further loss through sanctuaries and
enforcement of legislation.
As expected, the growth rates (Table 2) of the
four char populations from the British Isles were
much faster-growing compared to the Icelandic
char population. Differences in growth between
the morphotypes present in these lakes were also
evident. The two monomorphic Irish char
populations were faster-growing and shorter-lived
compared to the more northerly populations from
Windermere and Lough Rannoch. Char sampled
from Lough Mask in 1996 were considerably older
and larger than those reported by Went (1945,
1946), although it appears that the Lough Mask
char growth rates have not varied in the past 50
years.
In conclusion, the results confirm earlier
findings, which were based upon dietary analysis
and metazoan parasites of both Irish populations of
Arctic char (Doherty and McCarthy 2000). Thus,
no clear evidence exists for the presence of
sympatric char morphotypes within either Lough
Mask or Lough Eske, based upon the present
morphological and meristic analyses.
ACKNOWLEDGEMENTS
This study was undertaken as part of Investigation
of Eutrophication Processes in the Littoral Zones
of Western Irish Lakes, a project funded under the
European Union Operational Programme:
Environmental Services 1994– 99. The assistance
of the Central Fisheries Board, the Western
Fishery Board and the Northern Fishery Board is
gratefully acknowledged.
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