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Growth of whitefish ecotypes
A comparison of individual growth rates in
monomorphic and polymorphic populations
Fredrik Olajos
Degree Thesis in Biology 15 ECTS
Bachelor’s Level
Report passed: 2013-08-30
Supervisor: Göran Englund
Growth of whitefish ecotypes
A comparison of individual growth rates in
monomorphic and polymorphic populations
Fredrik Olajos
Abstract
In resource polymorphism, ecological opportunity and selective predatory pressure can be
considered key factors in phenotypic divergence. In post-glacial lakes of Scandinavia, the
European whitefish (Coregonus lavaretus L.) is a common species and has repeatedly
diverged along the benthic - pelagic resource axis. Recent studies suggest that predation by
northern pike (Esox lucius L.) induces rapid divergence in whitefish, leading to two
reproductively isolated ecotypes: a dwarf planktivore and a giant benthivore. In lakes where
pike is absent, whitefish are only found as monomorphic populations. In this study I
estimated growth rates in two monomorphic and two polymorphic populations having giant
and dwarf ecotypes. The aim was to use growth rates as a tool to distinguish between juvenile
giants and dwarfs, but also to find out if a population's resource use was reflected in the
growth rate. Scales were used to calculate growth rate, where like trees, variations in seasonal
growth could be observed in a ring-like structure. Growth rates differed between the morphs,
and mirrored their use of resources. The two monomorphic populations had the highest
average growth rate the first six years (40.1 and 35.5 mm/year), and quickly reached
maximum size. Dwarfs and giants in the dimorphic systems had equal growth the first two
years, after which giants grew at a substantially higher rate. Categorization between juvenile
giants and dwarfs could be done if an individual had passed its third growth season.
Key words: whitefish, growth rate, scales, pike, polymorphism
Table of Contents
1. Introduction ........................................................................................ 1
2. Methods .............................................................................................. 2
2.1 - Study area and sampling method ............................................................. 2
2.2 - Growth assessment and statistical model ................................................ 3
2.3 - Back calculating growth from scale measurements ................................. 5
2.4 - Categorizing morph based on growth ...................................................... 5
3. Results ................................................................................................ 6
4. Discussion ........................................................................................... 8
5. Acknowledgements ............................................................................. 9
3. References ......................................................................................... 10
1 - Introduction
Resource polymorphism is the evolutionary process of phenotypic adaptation and
specialization to different foraging resources or niches (Schulter 1996, Smith & Skúlason
1996). Divergence from a common ancestor in a non-allopatric environment yield different
morphs or ecotypes , which differ in physiology, morphology and behavior, and are adapted
at utilizing specific habitats and resources (Kahilainen & Østbye 2006). Resource
polymorphism can develop rapidly in young species poor systems, where unsaturated niches
and low levels of inter-specific competition promote diversification. Examples of such
environments are newly formed islands and lakes (Schulter 1996, Smith & Skúlason 1996).
Studies of resource polymorphism have often found strong morphology-environment
correlations related to foraging habitat. A textbook example is the Darwin finches, where
beak size and shape are strongly associated with different food resources (Lack 1947. Price et
al. 1984). Salmonid fish populations in post-glacial lakes on the northern hemisphere show
morphological diversification along the benthic-pelagic resource axis, where a smaller
planktivorous pelagic morph coexists with a larger benthivorous morph. Divergence is not
strictly dimorphic; up to five coexisting morphs can be found in large lakes (Smith &
Skúlason 1995, Wilson et al. 2004, Østbye et al. 2006, Siwertsson et al. 2010, Öhlund 2012).
The European whitefish (Coregonus lavaretus L.) is abundant and widespread in northern
Europe (Svärdson 1979). In Scandinavia, it occurs in hundreds of lakes and is subjected to
extremely high grades of polymorphism, having independently diverged into distinct morphs
in multiple instances (Østbye et al. 2005). Even though polymorphism in whitefish
populations is common, there are exceptions where only one type of whitefish is present. The
underlying mechanics of the diversification process seem to be caused by predatory effects of
the northern pike, Esox lucius (Öhlund 2012). Pike is a pronounced piscivorous predator
feeding in the littoral zone, it is set aside by its large gape size allowing it to consume larger
prey such as adult whitefish (Vollestad et al. 1986, Mittelbach & Persson 1998). Like many
other species, whitefish need to shift diet in order to maintain positive growth above a certain
body size. This means switching from a diet based on zooplankton to a high-yield resource
like benthic invertebrates (Kahilainen et al. 2005, Mittelbach & Persson 1998). Zoo benthos
is a predominant resource in the littoral zone, however, in lakes where pike is present this
niche is unavailable for small whitefish (Öhlund 2012). Thus it has been suggested that pike
induce disruptive selection favoring two different strategies: 1) Avoiding pike in the pelagic
zone, foraging on zooplankton and reaching sexual maturity at a reduced size, or: 2) avoiding
pike predation by rapidly growing out of the predatory window, feeding on invertebrates in
the profundal zone and delaying maturity (Öhlund 2012). In lakes where pike is not present,
we tend to find monomorphic whitefish populations with low levels of morphology-resource
correlation. Without the predatory threat from pike, monomorphic populations can utilize a
wider range of resources and need not specialize. (Öhlund 2012).
One problem in the study of young populations is to distinguish between the small
planktivorous morph and young individuals of the large growing benthivorous morph.
Categorization between different whitefish morphs have traditionally been based on gillraker
numbers (Svärdson 1953, 1979) because gillrakers show high heritability and are therefore a
reliable source for determining kinship(Kahilainen et al 2010, Bergstrand 1982). However,
in recently diverged whitefish morphs (< 100 years)(Gunnar Öhlund, unpublished data),
there is a significant overlap in number of gillrakers nullifying this method of categorization
in young populations. Despite this, seemingly more plastic phenotypic traits like body size,
growth rate, habitat use and resource use can diverge in young populations. (Bergstrand
1982, Kahilainen et al. 2003). This is an indication of an "eco-evolutionary feedback-loop"
(Öhlund 2012). In early divergence whitefish within a specific size-range have exceedingly
low fitness, in this case caused by disruptive pike predation. This initial divergence in space
and body size force smaller sized individuals to remain in the pelagic, and larger sized
whitefish to consume benthic resources. Once diverged, sexual and natural selection will
favor ecomorphological hereditary traits best suited to the current habitat, such as the
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number of gillrakers (Öhlund 2012). In time, this might lead to two reproductively isolated
populations with distinct phenotypical differences.
In this bachelor thesis, I investigate if differences in growth rates can be used for the purpose
of distinguishing between dwarf whitefish and juveniles of the giant morph. I will also look
for resource - life history correlations, to see if habitat use and niche utilization is reflected in
a population's growth rate. I will compare growth rates estimated from scales in two
monomorphic populations and two dimorphic populations. I hypothesize that average
population growth rate of a morph will be reflected in its use of habitat and utilization of
resources.
2 - Methods
2.1 - Study area and sampling method
The fish were taken from four lakes in northern Scandinavian (Table 1). Two lakes contained
monomorphic whitefish populations and two lakes had monophyletic, dimorphic
populations. Neither of these study systems were naturally colonized by whitefish after the
last ice age , but had whitefish introduced in the mid 1800 - hundreds and the 1930's,
respectively. The northern pike (Esox Lucius) was present in both polymorphic lakes, and
absent in both monomorphic lakes. All lakes were sampled with gillnets during the summers
of 2010 and 2011. A standardized set up including eight benthic multimesh nets with mesh
size ranging from 5 to 55 mm(Appelberg et al 1995), four nets with mesh size 33 mm and 12
with 44. The nets were left over night in both the pelagic and littoral zone. The fish were
individually ID marked and frozen in the field. In preparation for laboratory work, fish were
thawed over night. Individual length was measured; state of sexual maturity and gender was
determined by examining the gonads. Otoliths were processed for age determination and
scales used to assess growth rate were taken anterior to the anal fin.
Table 1: Overview of the studied lakes, including lake names, coordinates, area, maximum depth, year of
whitefish introduction, presence of pike and number of whitefish morphs.
Lake
Coordinates
(RT - 90)
Area
(Ha)
Maximum
depth (m)
Year of
introduction
Pike
Number of
Morphs
Gråsjön
X: 1355220
Y: 7068860
401
20
1933
Absent
1
Fyresvattnet
X: 1062067
Y: 6578805
4968
377
1855
Absent
1
Hökvattnet
X: 1452520
Y: 7086590
348
22
1865
Present
2
Stor Arasjön
X: 1585960
Y: 7167170
713
21.5
1937
Present
2
2
2.2 - Growth assessment and statistical model
In fish, calcified parts such as scales, vertebra, cleithra, opercula and otoliths produce annual
or daily augmentations which reflect rate of individual growth (Casselman 1990). As seasons
change, so does growth rate, indicated by dense and tightly packed augmentation during the
winter period and sparse augmentations during summer. Much like trees, this kind of
development gives a ring-like structure where the darker patches indicate slow growth.
Therefore, the space in between each "ring" represent one year of growth (Picture 1).
However, in older fish where growth rate is slowed down or halted, scale development is also
greatly reduced, where as otolith growth is constant (Casselman 1990). This is why otoliths
are used to determine age. Correct age determination is a necessity when calculating growth;
As scales cease to develop once an individual growth is halted, otoliths will reveal how long a
individual lived at maximum size. In this study, age determination by otoliths had already
been carried out and was included in the dataset together with IDs and fish lengths.
Picture 1. A scale taken from a 15 year old individual in Lake Stor-Arasjön. The black markings indicate winter
periods, where growth is reduced.
For the growth rate analysis, multiple scales from each individual were pressed onto a piece
of plastic using a manual scale press. The scale imprints were then examined in a microfiche
reader with a magnification of 42 times. The radius of every "ring" was measured from the
center of the scale, as well as the radius of the entire scale using a ruler. In lake Hökvattnet
and Stor-Arasjön where two morphs were present, individuals were divided and categorized
based on size: sexually mature individuals larger than 300 mm together with intermediate
and small sized juveniles were categorized as a "large" morph. Sexually mature individuals up
to the size of 240 mm together with juveniles smaller than 120 mm, and no older than two
years were categorized as "dwarf" morphs.
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Table 2. The amount of variation explained (R2), number of samples (N) and the slope (b) for power models fitted
to data on scale size and body length.
Population
R2
N
Slope
(b)
Gråsjön
0.997
24
1.575
Fyresvattnet
0.950
36
1.500
Hökvattnet (Dwarf)
0.944
45
1.414
Hökvattnet (Large)
0.989
33
1.532
Stor-Arasjön
(Dwarf)
0.869
68
1,607
Stor-Arasjön
(Large)
0.996
21
1.647
Figure 1. An example of a calibrating graph of scale radius and fish length. This is data for the Gråsjön population.
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2.3 - Back calculating growth from scale measurements
The following procedure was used to estimate whole body growth rate from the scale
measurements. The slope of the relationship between scale-radius and body size, (b), was
estimated for each population by fitting a power function plot of the form Sm = aLmb where a
and b are fitted parameters. Sm is the observed radius of the whole scale, and Lm is the
observed body length (Table 2, Fig. 1)
An individual intercept, ai, was then calculated for each fish using: ai = Sm / Lmb. Finally, the
body length at different ages (Ly) were calculated using: Ly = (Sy/ai)(1/b), where Sy is the
observed radius for year 1, 2, 3 etc. These calculations provided individual growth trajectories
from which the average growth rate for each population could be calculated.
2.4 - Categorizing morph based on growth
To examine if juvenile giants and dwarfs from polymorphic systems could be categorized
based on growth rate, the following methods were used. Inspection of growth rate
trajectories from the dimorphic systems suggested that growth during the third year in lake
Hökvattnet and the third and fourth in lake Stor-Arasjön differed between the morphs. The
logarithmic quotient for the third year of growth was calculated for every fish in both lake
Hökvattnet and lake Stor-Arasjön. In lake Stor-Arasjön, this was also done for the fourth year
of growth. The formula for the logarithmic quotient was: lq = ln ( y3 / y2) where y3 is the
estimated size of a individual year 3, and y2 the size of the same individual year 2 (lq = ln( y4 /
y3 ) was used when calculating the quotient for the fourth year of growth).
Logistic regression was used to test how well growth rates could separate the morphs. Morph
(giant or dwarf) was used as a binary response variable and growth rates as predictors. The
model was then used to classify fish as giants or dwarfs based on growth rate. This
categorization was then compared with the categorization based on body size.
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3 - Results
A general difference in growth rate was observed between all three types of whitefish
populations(fig. 2,3). The two monomorphic populations showed the highest rate of initial
growth, reaching maximum body size in six years after which growth was drastically inhibited
(fig. 2). The monomorphic populations also showed the highest average growth rate the first
six years with 40.1 and 35.5 mm/year (fig. 3). In the polymorphic populations the first two
years of growth were very similar between giants and dwarfs (fig. 2). After the second year
the giant increase their growth rate substantially, where as dwarfs continue along the same
trajectory (fig. 2). The giant morph had intermediate growth rate the first 6 years (fig. 3) with
25.9 and 25.1 mm/year. However, in both Stor-Arasjön and Hökvattnet, the giant morphs
reached a larger average body size than the two monomorphic populations.
The results of the logistic regression showed that juvenile giants can be separated from small
dwarfs after a individual has passed its third growth season (Table 3). When classified by
growth rate, the majority of individuals remain assorted to the same morph as when
categorized by adult body size (Table 3).
Growth trajectories from all populations
450
400
Fish length (mm)
350
300
Fyresvattnet
250
Gråsjön
200
Hökvattnet (Large)
Stor-arasjön (Large)
150
Hökvattnet (Dwarf)
100
Stor-arasjön (Dwarf)
50
0
0
5
10
15
20
Years
Figure 2. Growth rates of all whitefish populations included in this study, lake Fyresvattnet, lake Gråsjön, lake
Hökvattnet and lake Stor-Arasjön.
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Average growth rate (First 6 years)
45
Average growth rate (mm/year)
40
35
Fyresvattnet
30
Gråsjön
25
Hökvattnet (Dwarf)
20
Hökvattnet (Large)
15
Storarasjön (Dwarf)
Storarasjön (Large
10
5
0
Populations
Figure 3. Average growth rate during the first 6 years of the studied populations. Error bars indicate 95%
confidence intervals.
Table 3. The number of individuals categorized to morph based on body size (N), (Categorized as dwarf) and
(Categorized as giant) show the categorization of the same individuals based on growth rate year 3 (and year 4 in
Stor-Arasjön). The classification is based on logistic regression models ( Hökvattnet year 3: Chi2 = 8,7, p = 0,003;
Stor-Arasjön year 3: Chi2 = 9,1. p = 0,003, year 4: Chi2 =6,4, p= 0,011).
Population
N
Categorized as
dwarf
Categorized as
giant
Hökvattnet
Dwarf
41
41
0
Hökvattnet
Giant
26
2
24
Stor-Arasjön
Dwarf
59
57
2
Stor-Arasjön
Giant
14
4
10
7
4 - Discussion
Polymorphic whitefish populations that have diverged along the benthic-pelagic resource axis
are common in lakes of northern Scandinavia (Svärdson 1979, Bergstrand 1982, Amundsen
1988, Smith & Skúlason 1996, Østbye et al. 2006, Öhlund 2012). Traditionally, differences in
number of gillrakers have been used to identify the different forms (Svärdson 1953,1979).
However, in early stages of divergence distinguishable differences in heritable traits such as
gillrakers have not yet developed, where as differences in more plastic characteristics like
body size and growth rate are pronounced (Bergstrand 1982, Kahilainen et al 2003). The
northern pike (Esox lucius), is widespread in Fennoscandia and an important predator on
coregonids (Bohn et al. 2002). Due to its utilization of the littoral zone, pike is assumed to
cause the initial and rapid divergence of whitefish, forcing individuals to either remain
planktivorous in the pelagic zone, or to switch to a more profitable benthic prey in the littoral
zone (Öhlund 2012 ). Once initialized, this diversification of the phenotype and resource use
may cause further divergence and, even speciation. Prezygotic characteristics that are
reproductively isolating will enhance this process; such as selecting mate based on equal size
or pigmentation (Seehausen et al. 1997, Nagel and Schluter 1998). As a population continues
to age, natural selection will favor, heritable traits best suited to the current environment,
leading to further divergence.
The polymorphic populations of lake Stor-Arasjön and Hökvattnet originate from the mid
1800 - hundreds and 1930's respectively. These populations are still in early stages of
divergence and have not yet developed significant differences in amount of gillrakers
(Gunnar Öhlund, unpublished data), although growth rate patterns vary significantly
between morphs . In the monomorphic populations, free from the predatory presence of pike,
the whitefish can utilize a wider range of resources including the littoral habitat and grow
very rapidly. These populations can allocate all available resources into rapid growth and
development quickly reaching adulthood, after which growth is substantially reduced,
plausibly to focus on reproduction. In the two polymorphic populations the initial two years
of growth was similar in the two morphs. As both populations are young, with an overlap in
gillrakers, juvenile dwarf whitefish have no physiological foraging advantage when feeding
on zooplankton. Despite close similarities in growth rate between the dwarf and giant morph
the first two years, giant morphed whitefish significantly increased in body size after its
second year, whereas dwarfscontinue along the same trajectory. This indicates that an
ontogenetic habitat shift (Kahilainen et al. 2003) occurs during the third growth season,
where the giant morph switch from zooplankton to a more profitable resource whilst
avoiding pike predation in the littoral zone. A dietary shift to a more energetic resource is
mandatory for whitefish to maintain positive growth above a certain body size (Kahilainen et
al. 2005, Mittelbach & Persson 1998) and can be found in both giants and monomorphic
populations. However, the initial average growth rate of monomorphic whitefish was
significantly higher than that of giant morphed whitefish, suggesting that the littoral zone is
energetically propitious (Cummings and Wuychek 1971). The small pelagic morph never
undergoes a niche shift, which is reflected in the growth rate graphs. It seems that avoiding
pike predation in space and foraging solely on zooplankton yield slow growth and small adult
size, yet high fitness. An alternative, but not exclusive, explanation for the difference in
growth rate is that dwarfs, but not giants, mature their third or fourth year and thus spend
their energy on gonad development rather than growth.
Conclusively, in recently diverged populations juvenile individuals of giant and dwarf
ecotypes are physiologically homogenous. Based on the results of this study, distinguishing a
certain morph in younger years using growth rate trajectories will work if an individual has
passed its third summer. Unfortunately, very few juveniles were available for the compilation
of the statistical models used in this study, which might leave room for minor unaccounted
size variations where few individuals get to represent an entire population. This may cause
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the back calculated growth rates to differ slightly from the actual growth rates. A
comprehensive dataset of juvenile individuals from the polymorphic populations could reveal
differences in growth rates between morphs prior to the third summer.
5 - Acknowledgements
I would like to thank Göran Englund and Gunnar Öhlund for allowing me to be a continuous
part of their whitefish research-program, and for keeping me occupied the past two summers.
I could not have asked for better supervisors. It has been an enormous pleasure even though
admittedly I really missed the field sampling this summer. I would like to thank Pia Bartles
for her positive attitude always willing to lend a helpful hand, and for cheering up the
deserted EMG summer department. Lastly, I want to thank Magnus Kokkin and Alfred
Sandström from the Department of Aquatic Resources in Drottningholm for being extremely
teaching and patient, their knowledge of fish scales is truly elite.
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