Open Journal of Applied Sciences, 2012, 2, 168-176
doi:10.4236/ojapps.2012.23024 Published Online September 2012 (http://www.SciRP.org/journal/ojapps)
Shifts in Prey Selection and Growth of Juvenile Pikeperch
(Sander lucioperca) over Half a Century in a Changing
Lake Võrtsjärv
Kai Ginter*, Külli Kangur, Andu Kangur, Peeter Kangur, Marina Haldna
Centre for Limnology, Estonian University of Life Sciences, Tartu, Estonia
Received July 7, 2012; revised August 5, 2012; accepted August 15, 2012
ABSTRACT
Analysis of historical and recent data is essential to understand how eutrophication and/or climate change might trigger
shifts in the feeding mode of fish and trophic dynamics of shallow lakes. To assess long-term changes in the diet and
growth of juvenile pikeperch (Sander lucioperca), the prey selection and growth of pikeperch fry from Lake Võrtsjärv
was investigated in 2007-2010 and compared with data from 1920 to 1970. Over the observed period, larger cladocer-
ans have become less frequent in the diet as eutrophication has altered the zooplankton community. Furthermore, cli-
mate change has triggered a mismatch between the predator and its prey: the smelt population has collapsed but other
fish fries are too large for YOY (young-of-the-year) pikeperch. However, the mean length of fish has decreased mainly
due to the postponed diet shift.
Keywords: Climate Change; Eutrophication; YOY Pikeperch; Long-Term Changes; Diet Shift; Stomach Content
Analysis
1. Introduction
Predator-prey interactions play a major role in aquatic
ecosystems and thus can affect the whole biological
community [1]. In this respect, the diet and ontogenetic
dietary shift of juvenile pikeperch, Sander lucioperca
(L.), have been studied quite extensively in many north
temperate waters [2-9] as the size and structure of pike-
perch populations are strongly dependent on their success
at the juvenile stage [5,10]. However, due to eutrophica-
tion and climate change complex modifications in the
feeding modes of fish are expected to take place [11].
Thus, there is a heightened need to research into long-
term and developmental changes in diet, as pikeperch
have a quite rigid ontogenetic feeding pattern [10] that is
very sensitive to environmental changes, particularly to
those influencing food web components. Nevertheless,
there are only a few long-term data series on pikeperch
fry diet and prey community that could be used as basis
to study how eutrophication and/or climate change di-
rectly or indirectly might influence the diet, diet shift and
growth of juvenile pikeperch.
The diet and growth of juvenile pikeperch have been
studied several times since 1920 in a large shallow Lake
Võrtsjärv [12-17]. Moreover, the probability of major
shifts in the feeding modes of fish is especially high in
shallow lakes [11] such as Lake Võrtsjärv. Therefore,
Lake Võrtsjärv can be taken as a model for other shallow
north temperate lakes under high anthropogenic and
natural pressures to analyse the possible factors influ-
encing the diet, diet shift and growth of juvenile pike-
perch and hence the success of pikeperch populations.
Lake Võrstjärv has suffered a series of dramatic changes
since 1950s, including human induced eutrophication,
overfishing, warming and drastic water level fluctuations
due to climate changes. As a consequence, its biota has
altered and, therefore, also alterations in the diet and
growth of pikeperch in the lake can be assumed. Ac-
cording to Tuvikene et al. [18], Lake Võrtsjärv has been
under high anthropogenic pressure, primarily eutrophica-
tion, since the 1960s. In the 1980s, the discharge of nu-
trients into the lake by rivers increased steadily. However,
from the 1990s onwards the loadings of nitrogen and
phosphorus into the lake have decreased substantially. In
consequence, changes in total phosphorus and total ni-
trogen concentration in water and water transparency
have changed since the 1950s (Table 1). Additionally,
the water temperature in Lake Võrtsjärv has increased
significantly over the past half a century [19,20]. One
result of these changes is the great difference in the food
resources for the zooplanktivorous pikeperch in the lake.
*Corresponding author.

K. GINTER ET AL. 169
From the 1950s many large zooplankton species favored
by oligo-mesotrophic conditions such as Bythotrepes
longimanus Leydig, Leptodora kindti (Focke), Diaphano-
soma brachyurum (Lieven), Bosmina berolinensis Imhof,
Cyclops kolensis Lilljeborg and Eudiaptomus gracilis
(Sars) completely or nearly disappeared from this water
body [21,22]. Currently only small zooplankton species
favored by eutrophic conditions like Bosmina longi-
rostris (Müller), Chydorus sphaericus Müller, Meso-
cyclops leuckarti (Claus) and rotifers co-dominate [22-
24]. Moreover, during recent decades the fish fauna of
Lake Võrtsjärv has undergone several changes [25,26].
In the 19th century smelt (Osmerus eperlanus eper-
lanus m. spirinchus (Pallas)), which is considered to be
the primary first prey fish for pikeperch [10,27], was
rather common in Lake Võrtsjärv [28]. Before the mid-
20th century, as a result of heavy fishing pressure [29],
smelt disappeared from the lake. It was reintroduced
from the nearby Lake Peipsi in 1950-1954 [16], how-
ever, according to Kangur et al. [25,30], climate change
has continued to trigger a decrease in smelt populations.
On the other hand, the abundance of ruffe (Gymno-
cephalus cernuus (L.)) and roach (Rutilus rutilus (L.)) is
high in Lake Võrtsjärv [26] and their abundance in-
creases consistently along a gradient of increasing pro-
ductivity [31].
The purpose of this study was to explore if there has
been a shift in prey selection and growth of juvenile
pikeperch in a changing Lake Võrtsjlärv. In addition, an
attempt was made to identify possible factors influencing
the success of the pikeperch population in large shallow
lakes. Therefore, studies since 1920 [12-17] on the diet,
ontogenetic diet shift and growth of juvenile pikeperch
were analysed to understand the long-term nature of
pikeperch fry diet. This investigation concentrated on
four different time periods: the 1920s, 1950s, 1960s-
1970s and 2007-2010, as there have been significant
shifts in living conditions and food supplies of juvenile
pikeperch during those time periods. Furthermore, as
there are no data on prey-predator length relationship
since the 1950s, an additional investigation was carried
out to obtain vital information for analysing diet shifts.
2. Materilas and Methods
2.1. Study Site
Lake Võrtsjärv, situated in the central part of Estonia
(Figure 1), has a surface area of 270 km2 and is the sec-
ond largest lake in the Baltic region. It is a very shallow,
turbid water body with a mean depth of only 2.8 m and a
maximum depth of 6 m. Its water level is not regulated
and fluctuates on average by 1.4 m each year [32]. The
mean total phosphorus (TP) concentration in the lake
water is 50 µg P·l–3 and the mean concentration of total
nitrogen (TN) has been approximately 1.4 mg N·l–3 for
the last decade [20]. Based on nutrient concentrations,
the central part of the lake can be considered to be eu-
trophic, whereas the narrow and sheltered southern part
is hypertrophic [33].
Figure 1. Location of Lake Võrtsjärv.
Table 1. Changes in total phosphorus (TP), total nitrogen (TN) concentration in water and water transparency (Secchi depth)
[18,20] and shifts in zooplankton [21,22] as well as in fish community [13,16,26] in Lake Võrtsjärv since 1950s.
1950s 1980s 2007-2010
TP, µg·l–3 Undetectable 53 50
TN, mg·l–3 0.1 1.6 1.4
Secchi depth, m 1.29 1.1 <1
Dominant zooplankton species
by biomass
D. cucullata, B. berolinensis, E.
gracilis, L. kindti, B. c. coreconi B. c. coreconi, C. sphaericus C. sphaericus, B. longirostris, B. c. coreconi, M. leuckarti
Dominant fish species Perch, ruffe, roach, bream Ruffe, perch, bream, smelt, vendace Roach, ruffe, bream, pikeperch
OECD classification [33] mesotrophic eutrophic eutrophic

K. GINTER ET AL.
170
2.2. Data Set
The historical data set on the diet and growth of juvenile
pikeperch in Lake Võrtsjärv comprise three different
time periods: the 1920s [12], 1950s [13] and 1960s-
1970s [14-16]. The publications by Erm [13,14] and
Haberman et al. [16] give an overview of the diet of YOY
pikeperch in Lake Võrtsjärv in summer and autumn. The
autumn length data of pikeperch fry in Lake Võrtsjärv
were published by Schneider [12], Erm [13,14], Kangur
[15] and Haberman et al. [16]. Altogether, the historical
data set includes information on the diet of 551 and
growth of 410 juvenile pikeperch. The samples for the
growth and diet analysis were gathered with similar
methods—monthly during the ice-free period from the
pelagic zone of the lake using a bottom trawl. For growth
analysis the standard length (SL) and the total mass (W)
were measured. For the diet analysis only qualitative
methods were used as the index of frequency of occur-
rence (FO) of different prey items was calculated:
%100 i
p
FO p 
 (1)
In this Equation (1) ni represents the number of fish
with i prey species consumed and n is the total number of
fish examined [34].
The data set of the recent period (2007-2010) com-
prises results of the diet analysis in 2007-2008 [17] and
of an investigation conducted in 2009-2010. To enable
comparison of earlier data with the recent period, similar
methods were used in sampling as described in publica-
tions since the 1950s. Pikeperch samples were collected
in the summers and autumns of the years from 2007 to
2010 (Table 2) using a bottom trawl (height 2 m, width
12 m, 10 - 12 mm knot-to-knot mesh size at the cod-end).
The trawl was towed by a ship for 15 min per haul at a
speed of 5.5 - 6.2 km·h–1. Trawl catches were carried out
at noon, every sampling day one haul was carried through.
Sampling was conducted in the pelagic zone of the lake,
always in the same location. After capture, the pikeperch
samples were frozen. In the laboratory, the SL and the W
of each pikeperch individual were measured. The age
was estimated from the length frequency distribution of
the YOY and YAO (year-and-older) age groups (Figure
2). Each fish was dissected, the stomach contents were
analysed under microscope and the food items were
identified and counted.
Table 2. The number, age, standard length (S L) and weight (W) (± standard deviation) of examined juvenile pikepe rch from
Lake Võrtsjärv.
Sampling date Number of individuals (n) Age SL (cm) W (g)
8 Aug 2007 18 YOY 5.34 ± 0.61 1.74 ± 1.69
15 Aug 2007 37 YOY 5.47 ± 0.94 1.97 ± 2.56
3 Sept 2007 19 YOY 5.68 ± 0.31 1.9 ± 2.89
6 Nov 2007 11 YOY 6.18 ± 1.07 2.59 ± 2.23
30 June 2008 38 YAO 8.36 ± 0.41 6.49 ± 1.19
5 Aug 2008 4 YOY 4.2 ± 0.41 0.68 ± 0.31
29 Sept 2008 5 YOY 5.63 ± 1.39 2.19 ± 0.49
20 Oct 2008 6 YOY 6.39 ± 1.43 3.45 ± 1.43
13 Aug 2009 113 YOY 5.17 ± 0.38 1.46 ± 0.35
11 Sept 2009 30 YOY 5.29 ± 0.77 1.54 ± 0.32
20 Oct 2009 28 YOY 5.76 ± 1.49 1.87 ± 0.74
10 Nov 2009 35 YOY 5.20 ± 0.24 1.45 ± 0.15
4 May 2010 5 YAO 5.5 ± 0.31 1.27 ± 0.96
7 June 2010 5 YAO 9.32 ± 2.54 12.7 ± 9.21
13 July 2010 17 YOY 3.83 ± 0.31 0.71 ± 0.17
7 Sept 2010 5 YOY 5.83 ± 1.04 2.29 ± 1.41
27 Sept 2010 5 YOY 5.33 ± 0.29 1.54 ± 0.32

K. GINTER ET AL. 171
Figure 2. The Catch per unit effort (CPUE individuals per
trawl hour) of juvenile pikeperch in Lake Võrtsjärv in
2007-2009 in autumn months.
For the diet analyses the index of frequency of occurrence
(FO) was used. This index in itself is not a robust indica-
tor of dietary changes, as the most common prey does not
mean being the dominant. However, FO was the only
basis on which diet comparison of historical and recent
data could be conducted as it was the only parameter that
was used to describe the diet of juvenile pikeperch in the
1950s [13]. To assess the potential prey community, 1673
juvenile fish, including bream (Abramis brama (L.)), ruffe,
roach and perch (Perca fluviatilis (L.)) collected with
the same trawl as pikeperch fries, were measured and
weighed in 2009 and 2010 in the current investigation.
Additionally, the prey-predator length relationship was
calculated.
2.3. Data Analysis
Firstly, a logistic analysis was used to compare the diet of
zooplanktivorous pikeperch in two investigated periods:
the 1950s and the present (2007-2010). With the logistic
analysis odd ratio (the ratio of the probability of occurrence)
was found. Odds ratio shows the estimated difference in
probability of a certain prey item occurring in the pikeperch
diet in two investigated periods. Secondly, the ANOVA
model was used to test the effect of different investigation
periods—periods with abundant smelt population (1920s,
1960s-1970s) and periods with no smelt (1950s and from
1980s up to now)—on the standard length of the pike-
perch fry. In the statistical tests, the level of significance
(α) was set to 0.05. For statistical analysis, the pro-
gramme R, version 2.11.0 was used [35].
3. Results
3.1. The Diet of Zooplanktivorous Pikeperch
In the 1950s the number of different species of zooplank-
ton counted in YOY pikeperch stomach was 14 [13]. By
the recent period (2007-2010), the number of prey spe-
cies had decreased to seven. By then B. longimanus,
Alona spp., Alonopsis spp., Sida crystalline (Müller),
Acroperus elongatus (Sars), Diaptomus spp., Achteres
spp. and E. gracilis had disappeared from the diet. In
2007-2010, the most common food object was M.
leuckarti. This species occurred on average in 87% of the
stomachs at the end of summer and in autumn (Figure 3).
Figure 3. The average frequency of occurrence (%) of dif-
ferent prey items in the diet of juvenile pikeperch from
August till October in different periods: the beginning of
the 1950s [13], the 1960s [14,16] and 2007-2010 (authors
data).

K. GINTER ET AL.
172
Compared to the 1950s, the probability that M. leuckarti
occurs in the diet of juvenile pikeperch had increased
significantly (Logistic analysis n = 521, P < 0.001): the
odds ratio was 34.8, meaning that the estimated probabil-
ity that M. leuckarti occurs in the diet in the recent period
is 34.8 times higher than previously. At the beginning of
the 1950s, the most common food object was L. kindti
(FO 85%), however, recently the frequency of occur-
rence of this organism was found to be on average only
52%. The difference in the levels of FO of L. kindti was
statistically significant (Logistic analysis, n = 521, P <
0.001) and the odds ratio was 7.6. In the 1950s Bosmina
spp. and Chydorus spp. occurred in 4% and 9% of the
YOY pikeperch stomachs, respectively. In 2007-2010
Bosmina spp. occurred in 25% and C. sphaericus in 15%
of the stomachs. However, the only statistically signify-
cant (Logistic analysis, n = 521, P < 0.001) increase was
for Bosmina spp., where the odds ratio was 3.6. Daphnia
cucullata Sars was one of the subdominants in the 1950s
in the diet of YOY pikeperch, when its FO was 15%. In
the recent period the FO of D. cucullata was approxi-
mately 7%, whereas the decrease was significant (Logistic
analysis, n = 521, P < 0.001).
3.2. Growth and the Ontogenetic Diet Shift
In the 1950s the standard length of pikeperch fry varied
from 5.2 to 6.2 cm (Table 3) and 4.5% of juvenile pike-
perch shifted to piscivory in their first autumn (Figure 4)
as their potential prey fish was 86% - 91% of YOY pike-
perch length (Table 4). However, the prey-predator length
relationship decreased towards autumn and pikeperch
was able to shift to piscivory in spring, being 7.6 - 8.5 cm
in length [13].
In 2007 and 2008 the average autumn length (± standard
deviation) of pikeperch fry was 6.2 ± 0.8 cm and 6.1 ±
0.9 cm, respectively [17]. In 2009 and 2010 it was 5.5 ±
1.1 cm and 5.6 ± 0.8 cm, respectively (Table 3). The
autumn length of possible prey fishes was in the autumn
of 2009 as follows: ruffe 4.2 ± 1.4 cm, roach 4.7 ± 0.3
cm, perch 5.5 ± 0.3 cm, bream 5.3 ± 0.9 cm (Figure 4)
and in the autumn of 2010 ruffe was 5.3 ± 0.7 cm, roach
6.2 ± 0.4 cm, bream 6.3 ± 0.7 cm in length. Prey fishes
were on average greater than 75% of the length of the
pikeperch standard length (Table 4). The prey to predator
length ratio increased towards autumn and did not
change significantly in the spring of 2010. The diet shift
to piscivory was not found among YOY in 2007, 2008
[17] and 2010. In the summer of 2009, 7% of YOY
pikeperch had shifted to piscivory, however, in the au-
tumn of the same year pikeperch samples did not com-
prise piscivorous individuals. In spring 2010, pikeperch
fry as small as 5.4 cm were caught. Thus, the shift to
piscivory can take place in the following summer: the
2007 year cohort shifted to piscivory at the end of June
2008, at a length of 8.4 ± 0.4 cm [17] and the 2009 year
cohort at the beginning of June 2010, at a length of 9.3 ±
2.5 cm. In the recent period, the first fishes eaten by
pikeperch were ruffe and pikeperch.
In the 1920s and in the 1960s-1970s YOY pikeperch
gained length up to 12 cm in autumn as they started to
eat fish (primarly smelt) already in their first midsummer,
being only 3.5 cm in length [13]. Comparing periods
with abundant smelt populations (1920s, 1960s-1970s)
with periods with no smelt (1950s and from 1980s up to
now) discrepancy in the autumn length of pikeperch fry
can be observed. Furthermore, the differences in those
investigation periods were statistically significant—1.5-
fold increase in pikeperch length during periods of
abundant smelt (ANOVA, n = 589, P < 0.001).
Table 3. The average standard length (SL) and total mass (W) of juvenile pikeperch for October and November in Lake
Võrtsjärv [12-17], whereas n marks the number of fishes in each group.
1920 1950 1951 1953 1954 1966 1968 1969 1970 2007 2008 2009 2010
SL (cm) 12.0 5.2 4.9 5.2 6.2 10.5 12.1 10.7 9.8 6.2 6.1 5.5 5.6
W (g) 1.46 1.25 1.3 3.4 15.9 24.2 15.6 14.0 2.15 2.7 2.1 1.91
n 40 48 50 7 186 28 7 44 30 11 64 10
Table 4. The average (± standard deviation) potential prey and pikeperch length relationship (%) in 1954 [13], 2009 and 2010
(author’s data).
Food object 1954
Aug
1954
Oct
2009
Aug
2009
Oct
2009
Nov
2010
May
2010
June
2010
Sept
Ruffe 85.9 84.6 78.2 ± 6.4 75.5 ± 5.8 86 ± 7.9 83.9 ± 5.2 55.1 ± 3.7 95.3 ± 12.2
Perch 91.3 88.6 96.8 ± 4.9 96.8 ± 10.3 100.7 ± 4.4 79.8 ± 5.4
Roach 91.3 82.9 ± 6.7 79.6 ± 5.9 90.6 ± 13.3 94.4 ± 0.7 57.9 ± 4.1 110.1 ± 6.25
Bream 82 ± 8.3 107.6 ± 18.6 90.9 ± 2.4 104.6 ± 6.5 60.5 ± 4.7 112.4 ± 14.1

K. GINTER ET AL. 173
Figure 4. The autumn length of juvenile fish in Lake Võrts-
järv in August, October and No vember 2009. The box shows
interquartile range, line inside the box indicates the median,
whiskers show the 95% range of observed data and points
are outliers, n marks the number of fish in each group.
4. Discussion
The diet, growth and recruitment to piscivory of juvenile
pikeperch in Lake Võrtsjärv in four different time peri-
ods with dissimilar living conditions and food resources
varied significantly. Comparison of the diets of juvenile
pikeperch in Lake Võrtsjärv and in other shallow eutro-
phic European lakes [2,6,8-10] indicates that the diet of
zooplanktivorous pikeperch in Lake Võrtsjärv was some-
what more similar to that in other lakes in 1950s than in
the recent period. The type of food consumed by juvenile
pikeperch is argued to depend mostly on the availability
of zooplankton species [2]. According to Haberman et al.
[23], in the course of eutrofication the food supplies of
juvenile fish decrease. In Lake Võrtsjärv feeding condi-
tions for pikeperch fry have been worsened by eutrophi-
cation as it has triggered the decrease of large zooplank-
ters like E. gracilis, B. berolinensis, Daphnia spp. and L.
kindti [23,24]. Thus, discrepancies between the trophic
status and food resources of Lake Võrtsjärv in different
time periods can evoke variations in the diet of juvenile
pikeperch. Therefore, similar shifts in the diet of juvenile
pikeperch may occur in other north temperate lakes in the
case of ongoing eutrophication. Nevertheless, despite the
variation in the diet and slight difference in growth, there
were no differences in recruitment to piscivory. This,
however, might indicate adaptation to the changed condi-
tions caused by eutrophication or there is simply a
threshold size in pikeperch growth that can be reached on
zooplankton diet.
In years when the slender-bodied cold-water species
smelt was abundant in Lake Võrtsjärv, YOY pikeperch
did start to consume fish as early as in their first mid
summer and the average length of YOY pikeperch was
1.5 times longer than in the years of postponed diet shift.
In 2009-2010 the potential prey fish were found to make
up on average more than 75% of the pikeperch’s own
length at the end of the first growing season. Furthermore,
the prey to predator length ratio increased towards au-
tumn and in the following spring; differently from the
1950s when the feeding opportunities were somewhat
better and the prey to predator length ratio decreased
towards spring. Consequently, pikeperch was not able to
switch to piscivory before the second summer.
Sutela and Hyvärinen [36] point out that in the north-
ern edge of pikeperch’s distribution range juvenile pike-
perch does not shift to piscivory even when smelt is pre-
sent (Figure 5), as the summers are cool and pikeperch
does not hatch before early June and prey fish are given a
head start. Vice versa, in the southern part of its distribu-
tion area, pikeperch is able to predate on even YOY ruffe,
perch, roach, bleak (Alburnus alburnus (L.)), pumpkin-
seed sunfish (Lepomis gibbosus (L.)) and monkey goby
(Neogobius fluviatilis (Pallas)) in its first growing season
[4,8,37] as there seems to be no mismatch between
predator and its prey.
The mean air temperature in Estonia [39] and the water
temperature in Lake Võrtsjärv have increased signifi-
Figure 5. Possibilities for the diet shift in the first growing
season for juvenile pikeperch in different latitudes and in
changing climate [8,13,14,16,36-38].

K. GINTER ET AL.
174
cantly [19,20]. Thus, it is most likely that climate warm-
ing influences the recruitment success of juvenile pike-
perch as warmer climate has already seriously affected
the prey community structure. However, prey to predator
size ratio depends not solely on the water temperature,
but also on the rate at which water temperature increases
during spring and early summer as pikeperch are shown
to spawn later than most of their possible prey fish (smelt,
ruffe, roach) [40]. Namely, if the water temperature rises
gradually, pikeperch will have less size advantage to be
able to capture the fry of prey fishes spawning at lower
water temperatures. But if the water temperature rises
rapidly, there will be a shorter gap between the hatching
times of pikeperch compared to prey fishes. Thus, if the
temperature rises rapidly, juvenile pikeperch may be ca-
pable of starting predation on ruffe, perch, roach etc.
already in their first summer like in the southern part of
pikeperch’s distribution areas. On the contrary, when the
temperature increases gradually in spring, pikeperch
populations may seriously suffer due to shortage of suit-
able prey.
The growth of YOY pikeperch varied between periods
with different prey communities: 1) period with abundant
smelt population, 2) period with no smelt but high num-
bers of larger cladocerans and 3) period with no smelt
and low numbers of larger cladocerans. The presence of
suitable prey fish, e.g. smelt, rather than the presence of
larger cladocerans, is the main factor influencing growth
at the juvenile stage in Lake Võrtsjärv. Although it has
been claimed that pikeperch growth depends largely on
the availability of different prey items [10,41], particu-
larly on the abundance of large-bodied cladocerans [2,5,
42], the length of individual fish in Lake Võrtsjärv has
shown a reduction solely due to the postponed diet shift.
Therefore, investigating long-term processes in one cer-
tain lake enriches results that can be gained from different
study sites.
Pikeperch fries smaller than 6 cm SL do not survive
their first winter [26,43]. Owing to climate changes win-
ters have become warmer and wetter in Estonia [19,38,
44] and thus, as the current research demonstrated, juve-
nile pikeperch can survive the winter despite the growth
acceleration triggered by ontogenetic diet shift and be-
come piscivorous in their next summer. Similarly, with a
warmer climate there will be an increase in fish winter
survival [11,45]. Currently, populations of the first suit-
able prey fish, smelt, have collapsed due to the warmer
climate, but juvenile pikeperch are able to survive only
thanks to the changed environment, becoming the top-
predator in their second growing season in Lake Võrts-
järv. Hereby, pikeperch fries are now zooplanktivorus
considerably longer than previously and become pis-
civorous a year later.
In conclusion, there have been significant changes in
the diet and growth of juvenile pikeperch in Lake Võrts-
järv since the 1950s. Furthermore, these changes are
most probably related to eutrophication and climate
change as these trigger changes in food supplies and liv-
ing conditions. However, further research is needed to
assess the exact effects of shifts in the environment on
the population of pikeperch, the most important com-
mercially fished species in the lake.
5. Acknowledgements
The research was supported by the Estonian Science
Foundation (grants No. 6820 and 7643) and the Estonian
Target Financed Project SF0170006s08. Sincere thanks
to Dr. Jüri Ginter for his valuable advice and unfailing
guidance concerning the structuring of the work. Addi-
tionally, we gratefully acknowledge Dr. Renee Miller
and Tiia Kaare for language editing. Finally, sincere
thanks to the anonymous reviewer for hes valuable
comments.
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