Population
size, growth, and control of exotic goldfish (Crassius atratus) in a
small impoundment: implications for
managing future invasions
By
Tyler J. Winter
Undergraduate Research Opportunities Program
Fall 2005
Introduction
Exotic Species
Exotic species have been intentionally and unintentially spread around the world by human activities. Once established, an exotic species may cause a disruption or alteration in the food web (Simon and Stewart 1999). In some cases the disruption to an aquatic ecosystem may be small, in other cases, the extinction of many species may result (Crossman 1991).
The
common carp (Cyprinus Carpio Carpio), silver carp (Hypophthalmicthys Molitrix),
big head carp (H. Nobilis), and grass carp (Ctenopharyngodon Idella)
have all become established in the
The
goldfish (Carassius auratus auratus) is closely related to the common carp, and hybridization with other
carp species is not uncommon (Scott and Crossman 1973). They
are also one of the most wide spread invasive species. Humans
have transported goldfish around the world for display in aquariums, bait and
to stock as a forage fish. Goldfish are well adapted
to be an invasive species, because they are tolerant of degraded ecological
conditions, have a high fecundity, and are generalist feeders.
They have now established populations in seventy-five countries on six
continents. Goldfish now thrive across the globe from
the
Goldfish were one
of the first aquatic invasive species to reach
Remediation Effort
Because
of the threat that invasive fish pose there are ongoing efforts to eradicate,
contain, and manage invaded ecosystems. In some cases
lakes are drawn down and treated with rotenone in order to eliminate carp and
other undesirable fish. It is then necessary to re-establish
desirable fish populations through stocking. This is a
difficult and expensive procedure, and is not a realistic option in all
situations. Other management techniques can be
implemented such as intensive commercial harvest. This
technique can not remove all undesirable fish but may reduce their
abundance. Another way to reduce the
abundance of exotic fish is through manipulation of the foodweb or
“biomanipulation”. The food web of lakes experience
both bottom up controls, in the form of nutrient availability, and top down
controls, in the form of predator-prey interactions (Carpenter and Kitchell 1993). The abundance of
large piscivores is known to influence the community structure of a lake. Increasing the abundance of large piscivores
increases the predatory demand placed on exotic fishes. This
reduces the effects that exotic fishes may have on the food web. For example, increasing the size and
abundance of walleye in two lakes invaded by exotic rainbow smelt (Osmerus
mordax) decreased the abundance of rainbow smelt, and increased the
abundance of native cisco (Coregonus artedi) (Krueger 2003). In
Project Description
Goldfish are a
regulated invasive species in
The objectives of this project were to determine the goldfish population’s size structure, growth rates, and mortality rates. This information would also be used to investigate the plausibility of using predators as agents of biological control for future invasions. This analysis was undertaken to increase the understanding of goldfish invasions and the possible means to remove them. Because the potential for goldfish introductions remains high it is important that we examine all available means to limit their spread.
Methods
Study Site
Rock
Pond is located on the
Fish collection
Rock
Pond was drained in May of 2004, by the University, in order to: 1.) replace
the existing spillway; 2.) remove all of the fish. Goldfish
were collected by hand from rock pond as it was drawn down on
Age and Growth Determination
Growth
rates and size at age was determined using analysis of scales from individual
fish. As temperatures decrease each year, the growth
of fish slows and calcification of scales declines. This allows the detection of annuli on
scales, which are similar to annual growth rings in trees, of each fish and an
estimate of the age and size of an individual fish. Four or five scales were analyzed for each
fish. Length and weight at age and was calculated by
the proportional method, which solves for the size at each annuli using the
distance to each annuli against the distance between the focus and the margin
and total length when captured (DeVries and Frie
1996). Ripe females were collected on
Estimating Population Parameters
The relationship between size and age for consecutive years of growth may be used to estimate the rate of growth and maximal size of a given population (Walford 1946, Ricker 1975). This approach was developed by Walford (1946) and uses the relationship between the average size of fish in a given age class and the size of the average fish in the previous year of life. The Wallford plot for goldfish in Rock Pond was created from the average length of each year class present in May of 2004. This relationship was represented by:
where: 
is the average length of fish at a given age, 
is the average length of fish in the following year and a
and b are constants.
Information derived from a Wallford plot may also be used to estimate
the non-linear relationship between the age and size of individuals within a
population (Von Bertalanffy 1934, 1938).
A Von Bertalanffy growth curve was approximated using parameters
estimated from the Wallford plot for goldfish in Rock Pond. The Bertalanffy growth curve took the form:
Lage t = Linf (1-e-k
(t-t0))
where, Lt = fish
length at age t, Linf = the
theoretical maximum length a fish from this population may reach, k = Brody's
growth coefficient, t = the age of a fish in years,t0 = the age of a
fish when its length is zero.
The intersection of the Wallford line with a 45°
diagonal from the origin was used to estimate Linf (Ricker 1975). The log of the slope of the Wallford line is equal to
k. The Von Bertalanffy growth curve can
be solved for age (A), and used to estimate age from length (L).
A = LN (1-L/Linf)/-k
Age Structure and Mortality
Length and age data from 60 scale samples were analyzed to determine the average length for each year class present. Length data was compiled and presented in a graph to show the frequency of size classes.
Goldfish mortality was estimated using a catch curve plot (Ricker 1975). The relationship between log abundance of each year class and age was estimated using simple linear regression. The slope of this line, b, is an estimate of the annual survivability(S). The annual mortality rate (A), is calculated as 1-S (Ricker 1975). The instantaneous mortality rate (Z), was calculated as Z = -ln S (Ricker 1975) Z approximates the percentage of the population that dies during a given time period—in this case one year of life.
Bioenergetics analysis
Bioenergetics modeling was used to examine
plausible scenarios for the elimination of the Rock Pond goldfish population by
various potential predatory species. Application
of the
where B = somatic growth, R = respiration, F = egestion, U = excretion, and G = gonad production. Piscivores were selected for modeling based on their ability to consume goldfish and their potential availability from local sources. The initial and final weights, estimated consumption, and P-values, of one individual, were reported for each size of each piscivore. All estimates are for the 120 days after June 1st, this represents one growing season in Rock Pond. The P-value represents the ratio of estimated consumption to maximum possible consumption. A P-value of 1 would indicate that an individual fish is consuming as much as is physically possible. To determine the number of each piscivore that would be needed to eliminate the goldfish, the consumption of one individual predator was calculated on a weekly basis. I also assumed that the goldfish would be recruiting and growing during this time and that their production to biomass was approximately two. I then calculated the number of predators of each species that would be needed to eliminate the goldfish within 120 days. The stocking rate, minimum time and maximum time needed to eliminate goldfish were calculated. The largemouth bass (Micropterus salmoides), northern pike (Esox lucius), muskellunge (Esox masquinongy), and walleye (Sander vitreus) were considered as possible species based on their availability from local population sources and that each was a species native to the area. The reported stocking rate is the minimum number of predators needed at the lowest estimated consumption. The minimum time to extirpation was estimated as the number of weeks it would take the stocked fish to eliminate goldfish at the highest estimated consumption; maximum time was estimated as number of weeks to eliminate goldfish at the lowest estimated consumption.
Results
Fish Assemblage
Black bullheads (Ameiurus melas), fathead minnows (Pimephales promelus), brook sticklebacks (Gasterosteus aculatus), white suckers (Catostomus commersoni) and goldfish were collected from Rock Pond. No koi, which had been reported to exist in the pond, or piscivores were found. The average length of bullheads was 129mm (N=72), the average length of fatheads was 60mm (N=71), and the average length of white suckers was 425mm (N=4). Only four white suckers and three brook sticklebacks were collected.
Size Structure/Growth Rates/Mortality
The
128 goldfish ranged in size from 70mm to 290mm and were from
Bioenergetics
The bioenergetics analysis results, for individual piscivores, are summarized in table 1. Large mouth bass, northern pike, muskellunge, and walleye were considered as viable possibilities for stocking owing to their local availability and native status. Two sizes of large mouth bass and walleye were evaluated for a total of six piscivore options. The smaller size of large mouth bass, initial weight 240g required 30 individuals to remove goldfish at the highest rate of consumption, with 40 being needed at the lowest estimated consumption rate. The largest size largemouth bass, initial weight 1330g, would eliminate goldfish in 13 weeks with only 20 individuals at either consumption rate. This indicated that as the size of large mouth bass increases they consume more prey and that larger predators are more effective control agents. One size of northern pike was evaluated, 200g, at two different consumption rates, 0.29 and 0.45 of maximum. At the lower consumption rate 40 pike were necessary to completely remove the goldfish but at the higher consumption rate 20 individuals would eliminate goldfish in only 11 weeks. Muskellunge are a popular sport fish and grown by private and state hatcheries. 30cm muskellunge are readily available from the private sector. These 30cm fish weigh approximately 480g, much more than a similar sized northern pike. At the lowest estimated consumption 20 muskies would eliminate goldfish in only 10 weeks, at the higher consumption rate it would only take 7 weeks. Walleye are also readily available from state and private sources. A 480g walleye is approximately 32cm long. This is a size has been stocked by management agencies in the past. Bioenergetics simulations indicate that 30 walleyes, at 480g, could eliminate goldfish in 11 weeks at the lowest consumption, but at the highest consumption only 10 are necessary. A larger size of walleye, 1000g, was also evaluated. The larger size walleye consumed more than the smaller walleye. Under these simulations, 20 large walleye could eliminate goldfish in 10 weeks at the lowest consumption, but 10 could do it in the same time at the highest consumption.
The possible stocking densities and estimated time, for each piscivore, are reported in table 2. Each of the six possible piscivore options is able to eliminate goldfish with 40 or fewer individuals in less than 15 weeks.
Discussion
The Rock Pond goldfish population provided an excellent opportunity to evaluate the population dynamics of an invasive species and examine the possibility of using a biological control agent. The results show that introducing piscivores into an invaded system may represent an effective means to control potentially harmful exotic species.
The Minnesota Department of Natural Resources
has collected goldfish from six separate lakes in
The consumption rates of suggested piscivores alone provided a feasible mechanism for the complete removal of all exotic goldfish from Rock Pond. The pond had no piscivores at the time of introduction, which may have allowed goldfish to establish a viable population. The goldfish in Rock Pond already experienced a mortality rate of almost 80%, with no piscivores present. The high mortality rate can probably be attributed to winterkill, kingfishers and the green herons seen foraging in the pond. Adding another predator into this environment would only increase the predation pressure on any surviving goldfish.
Any of the
evaluated piscivores would be capable of eliminating the goldfish in Rock Pond
if stocked in high enough numbers at large enough sizes. However, two of the predatory species would
be more effective than the others. Large
mouth bass and muskellunge are two very effective native piscivores; and both
are readily available from private hatcheries.
Both are also ambush predators that prefer to forage in or near weed
beds, they are well adapted to the habitat available in Rock Pond. However, muskellunge seem to be the more effective of the
two and require fewer individuals to eradicate goldfish. Muskies represent an excellent and cost
effective method to remove goldfish. One example of a
private hatchery that offers 30cm muskellunge is “Minnesota Musky Farm” in
Stocking predatory
game fish is a common practice in
These results provide insight into the possibility of managing food webs, and predator populations, for the control of goldfish. In situations where other control methods are prohibitively expensive, ineffective, or unpopular, increasing the size and abundance of predatory fish may be the most prudent option. Food web alteration has the added benefit of increasing fishing opportunities, instead of eliminating them. Establishing a balanced and functional food web in a lake should also reduce the likelihood of future introductions.
To eliminate
exotic goldfish in
References
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Revision Date:
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Tables
and Figures
Table 1. Piscivores that could be used to control goldfish through top-down processes. Their initial and final weights, their estimated consumption and P-value are listed in sequential columns.
|
species |
Initial Weight |
Final Weight |
Consumption |
P-Value |
|
Large Mouth Bass |
240 |
414 |
460.42 |
0.4394 |
|
Large Mouth Bass |
240 |
550 |
653.24 |
0.5595 |
|
Large Mouth Bass |
1330 |
1550 |
918.44 |
0.3193 |
|
Large Mouth Bass |
1330 |
1850 |
1320.6 |
0.4316 |
|
Northern Pike |
200 |
300 |
483.38 |
0.2915 |
|
Northern Pike |
200 |
700 |
1132.29 |
0.4589 |
|
Musky |
480 |
800 |
1166.11 |
0.3442 |
|
Musky |
480 |
1100 |
1643.83 |
0.4194 |
|
Walleye |
480 |
600 |
702.92 |
0.2973 |
|
Walleye |
480 |
1100 |
1530.34 |
0.5068 |
|
Walleye |
1000 |
1200 |
1229.15 |
0.3085 |
|
Walleye |
1000 |
1900 |
2370.12 |
0.496 |
Table 2. Stocking rates for various piscivore species that would be needed to eliminate goldfish from Rock Pond. Number stocked corresponds to the number needed eliminate goldfish at lowest estimated consumption. Time is the estimated number of weeks needed to eliminate the goldfish population.
|
Piscivore |
Size at stocking
(g) |
Number stocked |
Minimum time |
Maximum time |
|
Large Mouth Bass |
240 |
40 |
10 |
14 |
|
Large Mouth Bass |
1330 |
20 |
10 |
14 |
|
Northern Pike |
200 |
40 |
6 |
13 |
|
Musky |
480 |
20 |
8 |
11 |
|
Walleye |
480 |
30 |
6 |
12 |
|
Walleye |
1000 |
20 |
6 |
11 |
