Download Master Thesis How does predation from fish influence the benthic Phragmites Ernest Aigbavbiere

Department of Physics, Chemistry and Biology
Master Thesis
How does predation from fish influence the benthic
invertebrates’ species composition in the Phragmites
australis and Chara vegetation of Lake Takern?
Ernest Aigbavbiere
LITH-IFM-A-EX--10/2310—SE
Supervisor: Anders Hargeby, Linköpings universitet
Examiner: Karin Tonderski, Linköpings universitet
Department of Physics, Chemistry and Biology
Linköpings universitet
SE-581 83 Linköping, Sweden
1
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Date
2010-05-28
IFM, Linkoping University, Linkoping , Sweden.
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Svenska/Swedish
x Engelska/English
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Licentiatavhandling
x Examensarbete
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LITH-IFM-A-Ex—10/2310-SE
Titel
Title
How does predation from fish influence the benthic invertebrates’ species composition in the
Phragmites australis and Chara vegetation of Lake Takern?
Författare
Author Ernest Aigbavbiere
Sammanfattning
Abstract
Predation is one of the important selective factors that regulate the species composition of benthic invertebrate
communities. The study objective was to investigate the invertebrate distribution in two contrasting habitats in Lake
Takern, southern Sweden, submerged Chara vegetation and emergent Phragmites australis vegetation, and to investigate
the influence of predation from fish on certain invertebrates. Laboratory studies were used to estimate handling time and
the intake rate (mg/s) by the fish based on the optimal foraging model. In the field, fish and invertebrates were collected
with gill nets and hand nets respectively and the fish gut content was analyzed. In total, sixteen invertebrates’ taxa were
collected from the two habitats. The proportion of the invertebrate’s overlaps from each of the habitat was calculated by
Renkonen index and with Sorensen similarity index. Both indices showed a similarity larger than 65%, indicating that
there was no significant difference in the invertebrates’ distribution in the reed and the Chara habitat. The fish caught
with the gill nets were: roach (Rutilus rutilus), perch (Perca fluviatilis),tench (Tinca tinca), and rudd (Scardinius
erythrophthalmus). The caught perch had eaten: Asellus aquaticus, Gammarus lacustris, Corixidae, and the larvae of
Chironomidae and Zygoptera.A comparison was made on the invertebrates found in the field and the ones observed from
the gut of the perch and the findings were that the invertebrates that had more occurrence in the gut were less in
proportion in the benthic samples. In the laboratory experiment perch ( Perca fluviatilis)) was used as the predator fish
and the prey organisms were Asellus aquaticus,Gammarus pulex, and Corixidae of three size categories. The results
showed that perch handling time for A.aquaticus of the different size categories, was not significantly different (p>0.05);
and the same results were valid for Corixidae and G. pulex. However, the intake rate of perch across the prey and their
size categories were significantly different. The handling time was not significantly different which means that the
predator fish will gain more in terms of intake rate as it feeds on larger size prey items, thus harmonizing with the
optimum foraging theory.
Nyckelord
2
Keyword: Handling time, intake rate, predation, macro invertebrates.
Content
1
Abstract.....................................................................................................
4
2
Introduction..…..……….………………………………………… .........
6
3
Materials and Methods.. .…..….………………………………………...
7
2.1 Description of the study site..............................................................
2.2 Sampling of fish and invertebrates...................................................
2.3 Experimental design...........................................................................
2.4 Data and data analysis.........................................................................
7
8
9
9
4
Results.….…….…..……………………………………………………...
3.1 Fish sample.....................................................................................
3.2 The benthic invertebrates samples..................................................
3.3 Fish gut content .............................................................................
3.4 Fish predation experiment..............................................................
10
10
11
12
12
5
Discussion.….….….………………………………………………..........
4.1 Sampling type ................................................................................
4.2 Fish predation on invertebrates.......................................................
4.3 The results and the research questions.............................................
4.4 Summary and conclusion.................................................................
17
17
18
18
20
6
Acknowledgement......................................................................................
20
7
References………………………………………………….. …………....
21
8 Appendix….…..…………………………………………………………...
23
3
Abstract
Predation is one of the important selective factors that regulate the species composition of
benthic invertebrate communities. The study objective was to investigate the invertebrate
distribution in two contrasting habitats in Lake Takern, southern Sweden, submerged Chara
vegetation and emergent Phragmites australis vegetation, and to investigate the influence of
predation from fish on certain invertebrates. Laboratory studies were used to estimate
handling time and the intake rate (mg/s) by the fish based on the optimal foraging model. In
the field, fish and invertebrates were collected with gill nets and hand nets respectively and
the fish gut content was analyzed. In total, sixteen invertebrates’ taxa were collected from the
two habitats. The proportion of the invertebrate’s overlaps from each of the habitat was
calculated by Renkonen index and with Sorensen similarity index. Both indices showed a
similarity larger than 65%, indicating that there was no significant difference in the
invertebrates’ distribution in the P. australis and the Chara habitat. The fish caught with the
gill nets were: roach (Rutilus rutilus), perch (Perca fluviatilis), tench (Tinca tinca), and rudd
(Scardinius erythrophthalmus). The caught perch had eaten: Asellus aquaticus, Gammarus
lacustris, Corixidae, and the larvae of Chironomidae and Zygoptera.A comparison was made
on the invertebrates found in the field and the ones observed from the gut of the perch, and the
findings were that the invertebrates that had more occurrence in the gut were less in
proportion in the benthic samples. In the laboratory experiment perch ( Perca fluviatilis) was
used as the predator fish and the prey organisms were Asellus aquaticus,Gammarus pulex, and
Corixidae of three size categories. The results showed that perch handling time for
A.aquaticus of the different size categories, was not significantly different (p>0.05); and the
same results were valid for Corixidae and G. pulex. However, the intake rate of perch across
the prey and their size categories were significantly different. The handling time was not
significantly different which means that the predator fish will gain more in terms of intake
rate as it feeds on larger size prey items, thus harmonizing with the optimum foraging theory.
Keywords: Handling time, intake rate, predation, macro invertebrates.
4
1
INTRODUCTION
Predation is attributed as a major selective force structuring the benthic invertebrate
communities (Wellborn et al.1996). It has also been shown that habitat differences and its
gradients may influence such predation and the community structure. The freshwater
community of shallow lakes include species of fish, numerous invertebrates, amphibians,
some reptiles and birds. Some invertebrates are known to be associated with some types of
submerged vegetation and have evolved some phenotypic expressions in terms of their sizes
and pigmentation. The predatory activities of the fish on invertebrates is perceived to be
limited in areas rich in aquatic plants (Gilinsky 1984; Diehl 1992; Beckett et al., 1992); and
the reverse in open areas (Tolonen et al., 2003). Fish is considered a top predator in most
permanent aquatic systems. They exhibit high preference for large invertebrates, prey that
swims actively; while predatory invertebrates may have preference for less active, small size
preys (Wellborn et al., 1996). To this ends, environments with high fish predation will in turn
promote selection (Nilsson 1981; Crowder and Cooper 1982; Bechara et al. 1993;
Blumenshine et al. 2000).
Many studies have been conducted in looking at the combined influences of fish and
invertebrates predators on the community structure of benthic macro invertebrates. Most of
such investigations are focused on interactions in flowing water systems. Shallow lakes or
ponds provide suitable spots to investigate more closely how the two predation pressures can
affect the community structure of the aquatic macro invertebrates. To understand the
phenomenon of predation some theories have been advanced by researchers to describe the
processes that modulate predator and prey relations. One of such theories is the optimal
foraging concept (Emlen 1966; MacArthur and Pianka, 1966; Charnov 1976; Mittelbach
1981). It’s proposed that, in selecting prey size, predators weigh the cost of handling a prey,
against the benefits derivable in terms of energy gain from the consumption of the prey; a
predator will select prey size that will generate maximal energy uptake per unit of handling
time (Smallegange et al. 2008). The adequacy of this theory was demonstrated in many
studies involving molluscivores predators (Elner and Hughes 1978; Hughes and Seed, 1995;
Mistri 2004). However, these foragers on mollusc also express a flexible approach to prey
selection, by feeding on sizes that are less energy giving in order to reduce the cost or risk
incurred during the handling, or as a response to hunger, presence/absence of a competitor;
this described another theory which is called contingency theory (Hughes 1988; Visser 1995;
Johnstone and Norris, 2000; Rutten et al. 2006). Optimal foraging theory has become an
important tool in studying feeding ecology because of its quantitative appeal in the prediction
of predator – prey relationship (Persson and Greenberg 1990; Jackson and Rundle 2008).
Essentially, the optimal foraging model is to maximize E/t; where E is the predator net energy
gain from the consumption of the prey, and t, the time taken for handling and searching its
prey. The cost also includes the risk incurred in the handling of the prey; so there is the
question of the prey profitability, when different preys are available to the predator at the
same time. Thus Jackson et al. (2004) and Jackson and Rundle (2008) adapted the net energy
optimal foraging model to describe the diet shifts of goby (Pomatoschistus microps), when a
variety of prey was available.
The mathematical expressions of the predator- prey relationship were represented as follows:
5
…………………………………………………. i
Where E (J), t (s), λi rate of predation on prey i (items s−1), A is assimilable part of energy, ei is
energetic content of prey i (J), Ch is handling cost (J s−1), Hi is handling time for preyi (s) and
Cs, the cost of searching (J s−1). A, assumed constant at 0.7, according to Elliott (1976). The
results of the study demonstrated among others that an increase in prey size for goby
correlated with an increase in the handling time and also that the predator size related directly
to the size of the prey they consumed.
1.1 RESEARCH OBJECTIVES
The objectives of the research were: (i) to identify the fish species and the macroinvertebrates present in the studied area, (ii) to find out the predator- prey relationship
between the fish and the invertebrates, (iii) to find out the differences in frequency of
invertebrates occurrence in the environment as compared to the feeding; and (iv) to relate the
findings to the optimal foraging model, more specifically relating it to the prey intake rate.
1.2 HYPOTHESIS:
Ho There is no significant differences in macro invertebrates’ species compositions and their
frequencies in the Phragmites australis and Chara vegetation in Lake Takern.
Ho There is no relationship between the proportion of prey found in the fish gut and the fish
handling time for the prey organisms.
6
2 MATERIALS AND METHODS
2.1 DESCRIPTION OF THE STUDY SITE
Lake Tåkern is located in the south of Sweden in Ostergotlands provincial district, 70km west
of Linkoping. The lake attracts numerous birds and is famous as a bird lake. It spans across an
area of 12km long and 8km wide, with depth averaging 0.8metres. The lake and its
contiguous area are a nature reserve since 1975 and it is part of Ramsar Convention list of
protected nature. The site remains an enviable resource for nature conservation, tourism and
research. The nature reserve is managed by both private ownership and the state and about
5400 hectares is the size of the reserve area. 270 bird species have been found feeding and
nesting in the area by bird watchers and other visitors. Agriculture, livestock graze land and
forest tree productions are the common land uses in the basin.
The lake habitat is structured by the type of vegetation present. At the P.australis, there are
aquatic grasses dominating the landscape and with a huge deposit of organic matter. Further,
there are clear water areas with sand and mud bottom, also areas with submerged vegetation
distinctly, stonewort (Chara spp) and water milfoil (Myriophyllum spicatum). The Chara
dominated area has also clear water, as the Chara is seen some few centimeters under the
water, while the Myriophyllum are seen up to the water surface.
Figure 1. Map of Lake Tåkern. The white dots represent the areas where sampling took place:
Saby and Glanas geographical positions of the lake. The P australis areas are represented by
two white dots while Chara by only one dot (Google earth 2009).
Different approaches had been used by researchers in a bid to understand the complex nature
of interaction in an ecological community. Some seems to narrow investigation to a particular
population, while others may prefer a mix of populations. In shallow lakes, the use of cages or
similar barriers to partition experimental block or units is well accepted (Bronmark 1994).
However, such block or partitioning may occur in nature as exemplified by the vegetative
landscape of the studied area. Also, common is the use of core sampling device to get the
invertebrate community in the mud bottom in the studied area. There is also the possibility of
making experimental set up with plastic bowl or aquaria and having different substrates
bottom and introducing the predator and the prey.
7
2.2 SAMPLING OF FISH AND INVERTEBRATE
The approach I adopted in the study was a combination of both field sampling and a
laboratory investigation. At Lake Takern, the benthic invertebrates were sampled with a
standardized 1 meter diameter wooden handle sweep net of 35cm diameter. Two samples
were taken both in the P. australis and in Chara of the Glanas and Saby areas of the lake. The
samples were sieved and taken in plastic buckets with a lid. In the laboratory, the benthic
samples were sorted in a flat bowl containing clean water and the invertebrates were picked
into glass cylindrical tubes and preserved in ethanol. The invertebrates were then identified
into their various families with the aid of a microscope and an invertebrate manual. The
length of the invertebrates was measured in (mm) with the aid of square paper mark placed
under a Petri disc and observed through the microscope. Two similarity indexes were used to
compare the proportion of invertebrates in the two habitats; these were Sorensen and
Renkonen similarity index. While Renkonen consider the proportion, Sorensen uses the
number of species as in the equation ii:
S
2c
 100 …….........................................................................................................ii
ab
where c is the number of species that were found in both habitats, a is the number of species
in location 1, b is the number of species in location 2.
Fish sampling was conducted in Lake Takern twice in autumn 2008 at Glanas and Saby area
of the lake, using a standard survey gill net. The gill net was set both in the P. australis and in
the Chara areas Data from another sampling occasion in autumn 2009 were also used. Gill
nets were exposed to the water for two hours.
The fish were removed from the gill net, sorted, and the total length and weight were
measured. Samples of the fish tissues were taken and subsamples of the fish were taken for
gut content examination. Pair of surgical blade and scissors was used for this purpose; the
stomach content were in turn introduced to Petri disc and then examined with the microscope
to identify invertebrates’ parts remaining.
2.3 EXPERIMENTAL DESIGN
In the laboratory, perch predation on G.pulex and Corixidae (water boatman) of size 7mm,
9mm and 10mm was investigated using 100mm- 105mm size of perch. The perch were
bought from the local fish farm, while the invertebrates prey were sampled in Lake Takern
and ponds in Linkoping University. Three size groups each of G.pulex, Corixidae and A.
aquaticus were fed to 3 perch at different feeding occasions, in an aquarium fitted with
aerators and filters. The aquarium was partitioned with a vertical slide glass attached to a
string which manually released to partition the fish and prey or pulled to remove the barrier,
thus creating fish and prey access. There was also a paper screen to minimize human
interaction with the fish activity in the aquarium. The fish activities in the aquarium were
recorded by a camera mounted on a tripod. Three replicates were made using 3 different
groups of perch. There were three holding plastic tubs in which the fish were first adapted to
the prey items and also served for holding the fish from the aquarium.
8
The handling time was defined as the time between the attack of one prey and the predator’s
readiness to pick another prey. These were estimated in each feeding occasion using a stop
watch. The handling time observed from video by Andersson (2010), who was looking at the
handling time for A.aquaticus in a similar experimental set up). In all, there were three
different prey organisms and only one predator fish species. Ten individual prey were
introduced to the aquarium stocked with perch in a group of three individuals at a time and
one individual was observed and the handling time taken. The procedure was repeated 7 times
for each of the prey (Corixidae, G.pulex and A.aquaticus) and their size categories using
different fish individuals. 21 individual fish were used to estimate the average handling time.
2.4 DATA AND STATISTICAL ANALYSIS
Primary data were obtained both from the field sampling and the experimental set up. Data
from the field include fish length and weight relationship, the benthic invertebrates’
distribution, fish gut content among others. The percentage distribution of the benthic
invertebrates in the water as compared to the percentage found in the fish gut was estimated
using a Chi square test. In the experimental set up, data on perch handling time for the three
different invertebrate categories were obtained and recorded in an Excel work sheet. The
mean, standard deviation and the covariance of the samples were calculated. The rate of
biomass consumption by the predator was also estimated using length / body mass
relationship sourced from literature (Haas et al.2007).
Table 1. Equations for body length (L) transformation to body dry weight (BDW) and
calculation of handling rate
Handling rate =
body weight
wt
…………....................................................... iii

handling time
t
BDW  a  L  exp b …................................................................................................ iv
where a and b are constants taken from1,2 ;
1
1
1
  b  ………........................................................................................... v
BDW a
L
A. aquaticus1 :
Corixidae1:
1
= -4212; b =2268.......................................................................... vi
a
1
= -4286; b = 2785.......................................................................... vii
a
1
= -4.783; b= 3.258..................................................................... ....... viii
a
1
Haas et al. 2007, 2 Meyer, 1989
G. pulex2:
Test of results were carried out using similarity indexes and test of paired proportions (t- test)
for invertebrates distribution in the two contrasting habitats. Also, an ANOVA analysis for the
mean handling time for perch predation on G.pulex, Corixidae and A.aquaticus was carried
out.
9
3 RESULTS
3.1. FISH SAMPLES
The samples of the fish taken from the gill net were mainly roach (Rutilus rutilus) and perch
(Perca fluviatilis), whereas tench (Tinca tinca), and rudd (Scardinius erythrophthalmus)
(Table 2) were only found in small numbers. The most common was roach and there were
more fish in the P. australis vegetation (Fig. 2). The size of perch caught in the P. australis
habitat was 98mm average and STD of 25, while in Chara perch size was 128mm average,
with a STD of 36.
Table 2. Number of fish individuals sampled in Lake Takern at two occasions in 2008.
Date
Location
roach
perch
Tench
rudd
CPUE
Glänås
09/19/08
Chara
18
3
0
6
27
P.austalis
54
1
0
0
55
Chara
67
2
2
0
71
P.australis
221
56
0
0
277
Säby
09/24/08
Figure 2. Proportion of fish species from the pooled data from the P. australis and Chara
habitats in Lake Takern in 2008.
10
3.2 THE BENTHIC INVERTEBRATES SAMPLES
14 families of invertebrates were identified from the samples (Fig. 3). There were more
invertebrate taxa represented in the P. australis than in the Chara habitat.
Figure 3. The proportion of total number of sampled invertebrates belonging to different taxa
in Chara and P. australis habitats in Lake Takern in 2008.
The results from the calculated Renkonen and Sorensen similarity indexes show that the two
habitats were similar in the distribution of invertebrate’s species the index was 65%.
Further, a paired t-test was made between the proportions of taxa in the two habitats and the
results showed no significant differences between the frequency distribution (Table 3).
Table 3. Paired Samples t-test of the invertebrates sampled from Chara and P. australis
habitats in lake Takern in 2008.
Paired differences
Chara invert freq. - P. australis invert freq.
Mean
-.00073
Std. dev
.06837
Std. Error
mean
.01827
95 % Confidence.
interval of the
difference
-.04020
.03874
11
t
Df
-0.040
13
Sig.
(2-tailed)
0.969
3.3 FISH GUT CONTENT
Four different invertebrates were found to be eaten by the perch (Fig.4).
Figure 4. The proportion of individuals among the four most frequently found invertebrates in
the gut of Perch, the fish caught by the gillnet, and in the environment.
The chi square was estimated at 762 (Appendix 1), which was greater than the critical value of 7.8 at a
degree of freedom 3 that is (n-1), n , number of individuals and at 0.05 error level. Thus the results
indicate a significant difference between the distributions.
3.4 FISH PREDATION EXPERIMENT
In the fish predation experiment, there was no significant differences in the perch handling
time for the different prey sizes, neither for Corixidae, G. pulex nor A. aquaticus (Fig. 5) and
(Table 4-6). There was a significant difference in the intake rate for A.aquaticus, G.pulex and
Corixidae (fig.6).
12
A
Handling
time
S
5mm
7mm
9mm
B
Handling
time
S
7mm
Handling
time
S
9mm
10mm
9mm
10mm
C
7mm
Figure 5. Perch predation on A) A.aquaticus, B) G. pulex, and C) Corixidae of three different
sizes.
13
Table 4 Results from ANOVA analysis of experimental data on handling time and intake rate
of perch predating on Corixidae
Sum of df
Mean
F
Sig.
Squares
Square
Intake rate
Between Groups
.006
2
.003
.441
.663
mg/sec
Within Groups
.042
6
.007
Total
.048
8
Handling time Between Groups
.434
2
.217
.219
.809
Within Groups
5.941
6
.990
Total
6.375
8
Table 5 Results from ANOVA analysis of experimental data on handling time and intake rate
of perch predating on G.pulex
Sum of df
Mean
F
Sig.
Squares
Square
Intake rate
Between Groups 0.057
2
0.028
8.175
0.019
Within Groups
0.021
6
0.003
Total
0.078
8
Handling
Between Groups 1.267
2
0.633
2.253
0.186
time
Within Groups
1.686
6
0.281
Total
2.953
8
Table 6 Results from ANOVA analysis of experimental data on handling time and intake rate
of perch predating on A.aqauticus
Sum
of df
Mean
F
Sig.
Squares
Square
Intake rate
Between Groups .249
2
.125
13.548 .000
Within Groups
.166
18
.009
Total
.415
20
Handling
Between Groups .761
2
.381
1.196
.325
time
Within Groups
5.727
18
.318
Total
6.488
20
14
Intake rate
mg/s
A
5mm
7mm
9mm
7mm
9mm
10mm
B
Intake rate
mg/s
C
Intake rate
mg/s
7mm
9mm
10mm
Figure 6. Perch intake rate when feeding on A) A.aquaticus, B) G. pulex, and C Corixidae of
three size classes.
15
The data for the handling time and the respective intake rate of perch across the three prey
taxa of sizes 7mm and 9mm were compared (Fig 7 and 8). The results from a two way
ANOVA indicated that there were significant effects of both the species and their sizes on the
intake rate (Table 7); and that the size of the prey contributed in the same way in the three
species ( Species* Size p>0.05).
Handling
time
S
Figure 7. Perch handling time for 7mm and 9mm size of A.aquaticus, G.pulex and Corixidae
in a laboratory experiment.
Figure 8. :Perch intake rate for the three taxa of A.aquaticus, G.pulex and Corixidae in a
laboratory experiment
16
Table 7: Results from a two way ANOVA of the effects of species and size of the prey on perch
intake rate. Data were log-transformed in order to eliminate differences in variance.
Sources
DF Sum of
squares
Mean
square
F -Value
P- value
Species
Size
Species* Size
Residual
2
1
2
12
0.264
0.465
0.003
0.001
192.873
339.873
2.401
<0.0001
<0.0001
0.1327
0.527
0.465
0.007
0.016
17
4 DISCUSSION
4.1 SAMPLING TYPES
In the investigation of the research questions, sampling were carried out in the field and in the
experimental set up. In the fish sampling the roach were more in number but their gut content
analysis revealed so little of their prey organisms but for the perch it was possible to identify
different prey organisms. Thus using perch as the benthic predator in the predator and prey
experiment was an adequate choice.
Also, hand net samples of benthic invertebrates samples were taken in both P. australis and
Chara macrophytes vegetation area of the lake. However, there were no differences in the
benthic invertebrate taxa distribution; there was the general habitat complexity in the two
areas.
Sorensen similarity index was calculated as 65%. This indicates that the similarity of the two
habitats is above 65% by any standard procedure. Then what can one say about the dissimilarity, and the test of significance of such differences? The test of significant differences
between pairs of frequencies using standard test such as a t- test shows p value of 0.969 which
indicates no significant differences between the pairs.
4.2 FISH PREDATION ON INVERTEBRATES
The invertebrates’ distribution in an aquatic environment is a noted factor that affects the fish
carrying capacity (Wetzel 2001). The fish depends on a wide variety of invertebrates as prey
at different stages of their life cycle. Fish has been described as the dominant habitat regulator,
a top predator in an aquatic system (Vanni 1986, Hansen and Jeppesen 1992, Scheffer 1998).
The presence or absence of some species of invertebrates in a lake is used as an indicator of
fish or fishless lakes in some recent work (Schilling et al. 2009). In the fish predation
experiment, perch predation on A. aquaticus, G. pulex, and Corixidae was observed. There
was evidence of the perch catching, chewing and swallowing the prey organism. Some
handling time may be longer due to an incorporating search time in between actions. Handling
time also include both the time for prey manipulation and the time the prey evacuate the
stomach to the intestine (gastric evacuation), described as the total handling time (Nilsson and
Bronmark 2000). Handling time may also be due to the prey size, the predator experience and
the prey type (Bromley 1994, Andersen 1999). Preys with soft and small size body are likely to
be taken more easily than those with rough body both in the manipulative stage and gastric
evacuation stage.
In the experiment, the observed handling time defined as the manipulative period for A.
aquaticus was observed to be 4s at most, but 6s and 7s for G. pulex and Corixidae. One can
explain that the prey characteristics may have some impact on the predator handling time; G.
pulex is known to swim faster compare to A. aquaticus, while Corixidae are also fast
swimmers and with an adaptive rough body texture. Considering the gut content analysis,
Corixidae was more common as a prey item compared to other prey. This might also be a
factor of the nature of the body surface affecting the gastric evacuation time (Nilsson and
Bronmark 2000). Also, Corixidae is often seen dangling between patches of submerged
vegetation and the water surface, thus likely a more vulnerable prey to predators. A. aquaticus
might be less vulnerable because of its association with the vegetation and the bottom
substrates (Rooker et al 1998). This type of behaviour is likely a factor that can predict what
types of prey items that will occupy the niche of the predatory fish species. The number of
18
perch individuals that were caught in the Chara vegetation was also more, which could be due
to the favourable environment for predation on invertebrates and other small fishes.
4.3 RESULTS AND THE RESEARCH QUESTION
The research questions constructed from the hypotheses include: Are there any significant
difference in the macro invertebrate species composition and their frequency in the two
habitats? Secondly, are there any differences in the handling time for G. pulex and other
invertebrates of the same size? The statistical evaluations of these two questions were as
follows.
The benthic invertebrate distribution in the P. australis and Chara dominated areas showed
no significant difference using a t- test, p>0.05. While the two habitats were similar, with a
similarity index above 65%, there were some differences. The P. australis habitat had more
taxa than the Chara habitat but the overlaps showed that the two habitats were indeed similar.
The reason for the higher number of invertebrates taxa in the reed could be the complexity of
the vegetation, that attract different types of invertebrate’s organisms. The Chara area is well
dominated by Chara vegetation, which can be seen as a large cluster of Chara plants just
underneath the water. Predation also is another source of community structuring. One can
expect predation from fish to be more in Chara vegetation than in the P. australis because of
the differences in their vegetative structures and physical characteristics which could affect
the predator efficiency.
A. aquaticus is a common invertebrate prey organism in freshwater. In the experiment it was
preyed upon with the shortest handling time. It could be because of the soft body texture and
the small size. The handling time is also related to the gastric evacuation rate (total handling
time), which is directly related to the temperature, prey type and prey size. Perch had been
reported to have a similar gastric evacuation rate across some invertebrates including: G.
pulex, Chaoborus, chironomids and zooplankton (Persson 1979, Singh-Renton and Bromley
2005).
Equally important is the comparison of the proportion of the prey items found in the gut of
perch to the observed handling rate. The question is if the prey with higher proportion in the
gut had less handling time or a higher intake rate (mg/sec). From the results of the perch gut
analysis, Corixidae were the most common prey found in the fish gut but considering the
handling rate, perch intake rate for G. pulex was higher than Corixidae. Thus, the proportion
found in the gut may not necessarily mean that the particular prey organism has a proportional
optimal foraging benefit for the predator. In another set of gut content analysis carried out in
september 2009, Corixidae were also observed to be more common in the stomach of perch
and the ratio Corixidae/A.aquaticus, was 4.2 ( unpublished data). The abundance of Corixidae
in the stomach of perch also corresponded to the observed data I got in the stomach analysis.
19
4. SUMMARY AND CONCLUSION
The invertebrate distributions in the two habitats, P. australis and Chara submerged
vegetation, were not significantly different.
In the predation experiment, perch predation on G. pulex of all sizes, Corixidae of all sizes
and A.aquaticus of all sizes showed no significant differences in the handling time within the
respective groups. Considering the intake rate of prey per unit time, the results showed that
the time spent by predator fish across the three taxa was not significantly different, thus the
intake rate (mg/s) increases with an increase in the prey size. This supports the optimum
foraging theory that predators will preferably select prey items that is more beneficial. In all,
the observed handling time shows that perch predation on A. aquaticus took the shortest
handling time, followed by Corixidae and lastly G. pulex, but looking at the intake rates G.
pulex was a more optimal foraging prey for the perch than A aquaticus.
Considering the gut content analysis, perch had G. pulex, A. aquaticus and Corixidae in their
gut. Corixidae were observed to be more common in the gut of perch. Therefore, A. aquaticus
and G. pulex are just as likely a good prey organism for perch. It will be important to also
consider the search time, because most handling time is followed by a time lapse for search
planning of prey attack. In the natural water, where there is an abundance of prey refuges, the
search time will be very important in a predator and prey situation. A.aquaticus and G. pulex
are more likely to gain more protection from the submerged vegetation in the P. australis due
to their constant use of the vegetation for both feeding on attached dead leaves and predator
avoidance. Thus, perch predation on invertebrate would be higher in Chara habitat than in the
P. australis habitat due to the differences in the vegetation and turbidity of the two habitats.
Acknowledgment
I am sincerely grateful to the academic team, administrators and service personnel of the
department of biology, Linköping’s University for their relentlessness and commitment to
their work and service, thus paving way for the successful completion of the course work and
project. More so, my greatest regard to my supervisor Anders Hargeby for his constructive
criticism and objectivity; friends and family members for their generous emotional support all
the time.
20
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Appendix
1 Estimating the invertebrates’ abundance in the benthic envioronment.
A aquaticus Corixidae
Chironomidae
Zygoptera larvae
Benthic
88
samples
Fish
guts 26
samples
Benthic prop
0.578947
13
42
9
106
2
16
0.085526
0.276316
0.059211
Fish
guts 0.173333
proportion
0.706667
0.013333
0.106667
A.aquaticus
Corixidae
Chironomidae
Found in gut
Abundance in the benthic samples
26
86.84211
106
12.82895
2
41.44737
X2=sum all (found expected)2/expected
42.62635
676.6607
37.54388
X2=
762.5362
II The intake rate of perch on its prey.
Size A.aquaticus
mm mg/sec
7
0,31
7
0,27
7
0,31
9
0,68
9
0,58
9
0,63
G.pulex
mg/sec
0,78
0,758
0,687
1,63
1,95
1,59
Corixidae
mg/sec
0,47
0,525
0,417
0,89
0,89
0,85
24
Zygoptera
larvae
16 n
8.881579 150
5.705283