The Density-Dependent Growth Rate of Guppies (Poecillia reticulata)
compared to the growth of Goldfish (Carassius auratus)
Under Laboratory Conditions
Marie Payne
May 9, 2007
Senior Seminar
Final Draft
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Table of Contents
Abstract……………………………………………………………………………………………2
Introduction…………..……………………………………………………………………………3
Summary of Keeley (2001)………………………………………………………………..3
Summary of Rodd and Reznick (1997)…………………………………………………...4
Summary of Grether et al. (2001)…………………………………………………………5
Summary of Robinson and Wilson (1995)………………………………………………..6
Summary of Barlow (1992)……………………………………………………………….7
Summary of Priestley et al. (2006)………………………………………………………..8
Methods..…………………………………..………………………………………………………9
Obtaining Guppies and Goldfish………………………………………………………….9
Experiment Set-up………………………………………………………………………...9
Feeding and Monitoring Guppies and Goldfish………………………………………….10
Data Analysis…………………………………………………………………………….10
Results…………….……………………………………………………………………………...12
Discussion…………………………………….………………………………………………….15
Acknowledgments…………………………….………………………………………………….15
Literature Cited…………..………………………………………………………………………16
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Abstract
This study was conducted to determine if overcrowding has an effect on the growth rate of two
species of fish, guppies, Poecilia reticulata and goldfish, Carassius auratus. As the world
becomes more crowded, scientists have been increasingly studying the consequences of
overpopulated environments. Some species are able to adapt to overcrowding by not growing as
large or moving to another area, however, other species will die because they can not adapt
rapidly enough to the overcrowding. In this study, three replicates were used for each treatment
as a precautionary measure in case of mass mortality in one of the tanks. The guppies and
goldfish each had their own 58 qt Sterilite© bucket in which all 9 treatments were housed to
ensure the same water quality and temperature. Each half gallon tank was covered by 0.5mm
aperture, fine mesh screen to keep the fry in the tank and to allow water to flow freely among the
half gallon tanks to the 58 qt. bucket. The bucket for the guppies had a bubble wand to circulate
and aerate the water, and a heater to keep the water at approximately 26º C. The goldfish tank
also had a bubble wand, but was left at room temperature which ranged from 14.3° C to 21.8° C.
I cleaned each half gallon tank, after measuring the fish each week, by siphoning the bottom and
then adding fresh water to the 58 qt. bucket; this was done to ensure the fish had good water
quality. Overcrowding can have an effect however, not very pronounced if not under extreme
conditions.
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Introduction
As the world becomes more crowded, scientists have been increasingly studying the
consequences of overpopulated environments. Some species are able to adapt to overcrowding
by not growing as large or moving to another area, however, other species will die because they
can not adapt rapidly enough to the overcrowding.
Keeley (2001) studied steelhead trout, Oncorhynchus mykiss, from the Chilliwack River
in British Columbia. He gathered eight adult males and two adult females; and then split the
eggs of the females into eight groups and fertilized each group by a different male. He built 18
channels into a staircase design with water flowing down from one step to the next. Keeley
(2001) then placed stream gravel in the bottom to mimic natural conditions. He placed a burlap
canopy over the channels to serve as protection from flying predators and as regulation for light
exposure. There were emigration boxes on the outside of each channel to which the trout could
move. The water drained from the emigration box into the channel below at a rate of about one
liter per minute, the water for the trout was maintained at 13.3°C and each channel had a flow
rate of 435 liters per minute. Keeley (2001) conducted two experiments examining the effects of
density and growth rates of trout in the channels: (1) when the trout were completely confined;
and (2) when they could move from one emigration box to the next. In the first experiment trout
were taken out only if they died, however, in the second experiment trout were marked with a
clip on the adipose fin if they were in the emigration box; if they made it into the emigration box
a second time, these fish were also removed from the experiment.
Keeley (2001) found that the highest mortality rates were present when the density was
the highest and the amount of food provided was the lowest. This means the group of trout was
the largest and they were not getting enough food, so many starved to death. This shows natural
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selection for starvation conditions because the best adapted fish to high population density and
low food resources were able to survive as the rest died. The fish mortality was the lowest with
high food abundance and low density because the trout had enough space and enough food to
survive. Trout that lived in the channel were 11.2% larger than those in the emigration boxes,
possibly because the fish in the channel were thriving, but the trout which would have died tried
to leave instead and travel to the channel below. Over time the number of trout in all treatments
decreased due to mortality and emigration. This information can be useful to show how
important population density is to fish species. The population of trout adapted, by reaching
maturity at a smaller size in confined areas and when less food was available, however, natural
selection played a large part in the mortality rate of the trout.
Several studies, which discuss the growth rate of guppies, describe the difference
predator regimes play in the growth process. Guppies with large predators will not grow to be as
large and colorful, and guppies with smaller predators will grow to an average size (Rodd and
Reznick, 1997). They collected samples of guppy, P. reticulata, populations in Trinidad on the
south facing slopes of the Northern Range Mountains and then compared their size based on the
predators they faced. Rodd and Reznick (1997) collected guppies from eight sites; four with
Rivulus, a fish found in brackish water such as small streams, as the smaller predator and four
with Crenicichla, a member of the Cichlidae family, as the larger predator. Four collections
were taken from each site for a total of 32 samples. The samples were then killed with 3aminobenzoic acid ethyl ester and preserved in alcohol or formalin. This study only considered
female growth because when a male becomes an adult, he stops growing. The scientists found
the guppies with Crenicichla, the larger predator, on average, were smaller and not full grown;
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those guppies with Rivulus, the smaller predator, were larger and mature. Crenicichla preyed
upon larger guppies, which could help explain why none of the guppies were full grown.
This study did not take into account other possibilities for growth rate, such as density or
fecundity; however, it was useful to show as the larger guppies died off due to natural selection
or predation, the guppies reached maturity at smaller sizes to escape predation. This study shows
the reason guppies were chosen for my experiment; P. reticulata populations can adjust their
growth rate very quickly in response to their environment.
Four years later Grether et al. (2001) studied guppy, P. reticulata, populations in the
same streams and on the other side of the Northern Range Mountains. However, these scientists
conducted their study on the resources available to the guppies and examined how the canopy
cover, inadvertently affected guppy growth. To determine if the growth rate was due to genetics
rather than environmental differences, Grether et al. (2001) also conducted a laboratory
experiment and studied the growth rate in the controlled environment.
The amount of light available to the area determined how much algae was available to be
consumed; if there was no sun, the algae would not grow and the guppies did not eat. Therefore,
the guppies in the streams with high levels of light grew faster than those in the low light streams
due to more available food. When the experiment was tested in the laboratory rather than in
nature, the difference was dramatic; in the field the guppies from the high light stream exceeded
the growth from the low light streams by 319%, however, in the laboratory it was only 10%. By
conducting the study in a laboratory, Grether et al. (2001) learned genetics must also play a part
in determining growth rate because males varied in the same stream and across streams. This
shows guppies under different conditions could have the same growth rate regardless of
conditions. However, most variation in growth rate in this experiment was due to environmental
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differences rather than genetics. This study was useful because it showed genetics could play a
role in the growth of guppies.
The ability of the guppy to change phenotypes in one generation was shown very well by
Robinson and Wilson (1995) with their study on different feeding treatments. A sample of 100
guppies, P. reticulata, was taken from the Turure River in Trinidad in 1988 and kept at
Binghamton University in New York. In 1995, all guppies younger than one week old were split
into five groups to undergo various feeding treatments. The first three groups were fed flake
food, which was ground into a powder and mixed with water and gelatin. The gel was then put
on glass slides and allowed to dry. These were placed on the top of the water upside down in
one tank, vertically in another and on the bottom, food side up, in the third. The fourth group
was fed the same processed food, but it was scraped from the slides and left floating on the
water. The fifth group was a rotation of the other four treatments in four day cycles. The
guppies were fed like this for three months, which was when two-thirds of the guppies had
reached maturity. The body shape and skull morphology of each treatment was then studied for
a variation in phenotypes.
Robinson and Wilson (1995) found the growth rate did not change between the feeding
styles, because they all had the same nutritional value; however, the body shapes from each
treatment were different. Guppies with the floating food had longer bodies, smaller pectoral fins
and large pelvic fins. Guppies fed with the top or bottom style had the greatest body depth and
the guppies from the floating treatment had the shallowest body depth. Guppies which were
rotated between feeding styles were intermediate in all aspects. P. reticulata fed by floating food
had narrow skulls and short teeth; guppies that fed on the bottom had large heads and long teeth.
There were no different body shapes in any one tank, because the P. reticulata not suited for
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each given treatment died. The guppies grew into whatever shape was best suited for them to
feed in each given treatment, or did not survive. This article shows in more detail the ability of
the guppy to evolve in a three month period, from fry to maturity, within one generation. P.
reticulata of all the same size and location became differently shaped based on how they were
fed. The head changed shape and their bodies became different sizes and shapes; this made it
easier for them to eat the way they had been raised under each treatment. This is important
because if a guppy is able to adapt to different feeding treatments in one generation, due to what
is best for the fish, then it may be able to change its growth rate based on the amount of area
available.
In 1992, Barlow studied the ability of the guppy, P. reticulata, to alter its growth rate
based on available space. Barlow started fifteen populations of guppies and fed them all the
same amount of food to be sure changes were due to space. Every two weeks he removed a
percentage of the guppies to determine the growth rate. The guppies were then preserved in
ethanol and measured for length, fecundity, and mass. To determine the mass to the nearest
1mg, 206 guppies were dried in an oven for six hours at 60° C. Barlow (1992) found density can
greatly change the growth rate of guppies. He found per capita growth rate as a linear function
of population biomass. This means that the number of fish in a tank was inversely proportional
to the growth rate of the fish in the tank. As the population went up, the growth rate went down.
This study was helpful because it brought forward questions I had not thought about and is a
very similar study because I also studied the ability of P. reticulata to adapt to a large population
biomass. However, Barlow’s (1992) study was conducted over several generations, so the
growth rate was based on several different guppy groups.
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In 2006, Priestley et al. conducted a study on the common goldfish, Carassius auratus.
The study was conducted in a lab with 75 goldfish split into 12 different groups with three
replicate tanks each of 1, 4, 8, and 12 goldfish. The fish were all fed the same ratio of food four
times a day. Every two weeks for eight weeks measurements of the fish were measured to
determine depth: length ratio and placed in a tared beaker to determine weight. Priestley et al.
(2006) determined, although the growth rate changes based on the number of fish in the tank, it
was not a significant level of difference.
My hypothesis was that the percentage of change in the growth rate of P. reticulata,
guppies, would be greater than the percentage of change in the growth rate of Carassius auratus,
goldfish. Due to the background information, I believed as the number of fish in the tank
increased, the guppies would have a slower growth rate; and the goldfish would show almost no
difference with a higher density. To test my assumption I took a length and depth measurement
of each fish from the population, and weighed each whole group to make the possible differences
in growth more noticeable.
I compared the effects of overpopulation of Carassius auratus, goldfish, and Poecilia
reticulata, guppies, on their growth rates. By placing different numbers of fry from both species
in half gallon tanks for 4 weeks I was able to measure the rate at which the fish grew and then
determined the effects of population density on growth rates for each species. The growth rate of
other fish besides goldfish and guppies have been studied and shown to be density dependent as
well.
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Methods
Obtaining Guppies and Goldfish
I began my experiment with eighteen 0.5 gallon tanks. Half gallon tanks were chosen
because the fry were so small that a larger tank might not show density dependence in just four
weeks. It would take longer to show a difference in growth rate if the tank were larger; the fish
would be less crowded and therefore possible changes would be less noticeable in just four
weeks. I had three tanks containing 1, 3, or 5 fish for each species; guppies, Poecilia reticulata
and goldfish, Carassius auratus for a sample size of three for each species. This allowed me to
measure the density dependence without killing the fish. I bought the guppies from PetSmart®
and the goldfish from WalMart®. I to purchased fish as close to the same size as possible to
eliminate the possibility of initial size affecting the growth rate. I chose P. reticulata because
they have been shown to rapidly adjust to their environments (Robinson and Wilson, 1995) and
C. auratus because they have been shown to have the same growth rate regardless of
environmental conditions (Priestley et al., 2006).
Experiment Set-up
Three replicates were used for each treatment as a precautionary measure in case of mass
mortality in one of the tanks. The guppies and goldfish each had their own 58 qt Sterilite©
bucket in which all 9 treatments were housed to ensure the same water quality and temperature.
The water was Lacey tap water which was left overnight to acclimate. No chemicals were used
to obtain proper environmental conditions. Each half gallon tank was covered by a 0.5mm
aperture, fine mesh screen to keep the fry in the tank and to allow water to flow freely among the
half gallon tanks to the 58 qt. bucket. The bucket for the guppies had a bubble wand to circulate
11
and aerate the water, and a heater to keep the water at approximately 26º C. The goldfish tank
also had a bubble wand, but was left at room temperature which ranged from 14.3° C to 21.8° C.
I cleaned each half gallon tank after measuring the fish each week, by siphoning the bottom and
then adding fresh water to the 58 qt. bucket; this was done to ensure the fish had good water
quality.
Feeding and Monitoring Guppies and Goldfish
Each tank was fed TetraColor Tropical Flakes™. They were fed as much as they would
eat in 7 minutes because by this time they had all stopped eating, then the excess food was
removed from each tank using a brine shrimp net. This eliminated the possibility of differences
in growth due to feeding. Once a week, for four weeks, the fish were removed from their tanks
and their standard lengths were measured to the nearest 1mm using a ruler. Then, the entire
population of the tank was weighed together, to the nearest 0.01g. This was done by placing a
beaker of tank water on a scale; the beaker was tared before placing the fish in the water. Any
dead guppies and goldfish were recorded, removed from the tank and replaced by a guppy or
goldfish of similar size to decrease the impact on the tank. The replacement fish were kept in the
same size tanks in the larger buckets with the rest of the fish of the same species. The
experiment was then continued with the new additions to the tank.
Data Analysis
After four weeks, the final data were recorded and analyzed using Minitab (2005) and an
Analysis of Variance (ANOVA) test to compare the differences in the growth rate of the guppies
and goldfish at different density levels. I first compared the guppies and goldfish to their own
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species and then I compared the goldfish to the guppies. If a significant difference was present, a
Tukey test, (Minitab, 2005) was used to determine where there were significant differences
between treatments.
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Results
Guppies and goldfish were housed in half gallon tanks and measured for weight and
length once a week. At the final measurement the experiment had been reduced to two replicates
for the guppies and two replicates for the goldfish; all three replicates of tanks of five goldfish
died within two weeks of the start of the experiment. The average weight of the goldfish
throughout the project can be seen in Figure 1. The tanks with five fish had a continuous decline
in the size of the fish until week two when all the fish from all three replicates had died. The fish
in tanks of 1 and 3 showed no difference in growth rate because they had the same slow upward
trend in growth. The average weight of the guppies throughout the four weeks can be seen in
Figure 2. The tanks with 1 fish had a greater growth rate than the tanks of 3 and 5 fish; these
tanks began a decline at week three while the tank of one had continuous growth.
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5.0
Weight of all Goldfish in Tanks (g)
4.5
4.0
3.5
3.0
1 fish
3 fish
5 fish
2.5
2.0
1.5
1.0
0.5
0.0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weeks
Figure 1. The average percent change in weight of the Carassius auratus in tanks over four weeks at density levels
of 1, 3, and 5 fish per tank. The fish were fed TetraMin Tropical Flakes once a day for 7 minutes. Shows averages
of three replicates.
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0.45
Weight of all Guppies in Tank (g)
0.40
0.35
0.30
1 fish
3 fish
5 fish
0.25
0.20
0.15
0.10
0.05
0.00
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weeks
Figure 2. The average percent change in weight of the Poecilia reticulata in tanks over four weeks at density levels
of 1, 3, and 5 fish per tank. The fish were fed TetraMin Tropical Flakes once a day for 7 minutes. Shows averages
of three replicates.
Overall the guppies in a tanks of 1 had the greatest growth rate, approximately 50% and
it was statistically different (F= 14.26, d.f.=2, p= 0.0293) than the others, however the other
guppies and all of the gold fish were not significantly different and were similar to each other,
with growth rates between 2% and 17%. Although, there are some large differences between
groups there are also large differences within the same group in different tanks. There was a
statistical difference between the tanks with one fish of both species (F=21.344, d.f.=1,
p=0.0437). The growth rate for both species was rather sporadic, which occurred due to death of
fish and then addition of new fish. As fish died new fish were put in the tank and were a
different size than the originals.
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Discussion
The results obtained in this study suggest differences on growth rate are only present in Poecilia
reticulata when the species is alone in a tank or with others. It did not matter how many others
were in the tank but if the fish was alone it grew more rapidly. The species Carassius auratus
did not show any differences however regardless of numbers of fish in the tank. This could
mean that even with only one fish in the tank it was overcrowded because the tank was too small.
The guppies in a tank of one had an average growth rate of approximately 50%, this is much
larger than any of the other fish. This shows the adaptability of the fish in the tanks of 3 and 5 to
remain small even when fed the same amount due to space constraints. This shows the results
found by Barlow (1992) were accurate because the guppies did adapts. It also supports the
results found by Priestly et al. (2006) because the goldfish all grew at the same rate.
Acknowledgements
I would like to thank Saint Martin’s University professors Dr. Hartman and Dr. Olney. I would
also like to thank Cheryl Guglielmo for her assistance with the project. I would also like to show
my appreciation for my fellow students, Chris Blackhurst for his support and Tu Ong and Wendy
Mattson for supporting me and correcting my papers.
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Literature Cited
Barlow, J. 1992. Nonlinear and logistic growth in experimental populations of guppies. Ecology.
73: 941-950.
Grether, G.F., Millie, D.F., Bryant, M.J., Reznick, D.N., Mayea, W. 2001. Rain forest canopy
cover, resource availability, and life history evolution in guppies. Ecology. 82: 15461559.
Keeley, E.R. 2001. Demographic responses to food and space competition by juvenile steelhead
trout. Ecology. 82: 1247-1259.
Minitab, 2005
Priestley, S.M, Stevenson, A.E, Alexander, L.G. 2006. Growth rate and body condition in
relation to group size in black widow tetras (Gymnocorymbus ternetzi) and common
goldfish (Carassius auratus). The Journal of Nutrition. 136:2078s-2080s.
Robinson, B.W., Wilson, D.S. 1995. Experimentally induced morphological diversity in
Trinidadian guppies (Poecilia reticulata). Copeia. 1995: 294-305.
Rodd, F.H., Reznick, D.N. 1997. Variation in the demography of guppy populations: The
importance of predation and life histories. Ecology. 78: 405-418.
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