Population size, growth, and control of exotic goldfish (Crassius atratus) in a small impoundment:  implications for managing future invasions



Tyler J. Winter



Undergraduate Research Opportunities Program

Fall 2005


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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 United States. The silver carp and the big head carp arrived in the 1970’s (Chick and Pegg 2001), but both are currently confined to the Mississippi river drainage. The common carp was established in the 1880’s to promote fisheries for this species (Smiley 1886; Smith 1896; Cole 1905). All of these fish are large, fast growing, have high fecundity and feed at low trophic levels. Their feeding habits can directly alter their environment by affecting nutrient cycling, turbidity, dislodging and consuming macrophytes (Nico and Maynard 2005).  In some cases they can completely dominate an ecosystem, and represent a large proportion of the fish biomass within the invaded ecosystem.

            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 United States and Canada to Saudi Arabia and Pakistan. Their popularity as pets insures that there will always be the threat of goldfish being released into the wild

Goldfish were one of the first aquatic invasive species to reach North America, arriving here in the 1600’s (DeKay 1842).  Goldfish became established in ponds in New York by 1842 (DeKay 1842). Goldfish were propagated in Washington D.C. and later distributed to state hatcheries through out the country. State hatcheries raised and used them as forage for large mouth bass (Micropterus salmoides). Goldfish were also used as live bait, a practice that is illegal in Minnesota today. Many introductions of goldfish were probably done through accidental release through used as bait fish. Every state except Alaska has reported goldfish as an invasive species (Nico 2005).  They are also established in Canada and all five of the Great Lakes.  Minnesota has established goldfish populations as well.  They have been collected from Duluth Harbor, St. Louis County, Lake Minnetonka, Hennepin County.;  Minnesota River below Fort Snelling, Dakota County, Dakota-Ramsey and in the upper and lower  Mississippi River.

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 Crawling Stone Lake, Vilas County, Wisconsin, increased walleye predation decreased smelt populations to below the level of detection (Krueger 2003).  In situations where complete removal of exotic species is impractical or prohibitively expensive, increasing the size and abundance of piscivores may provide an effective mechanism for the control of exotic planktivores. 

Project Description

Goldfish are a regulated invasive species in Minnesota, which means that they can be bought, sold, transported and possessed but not introduced into the wild. Despite the laws prohibiting their release, goldfish have managed to invade many lakes and streams in Minnesota, including Rock Pond, St. Louis County.  Rock Pond is a small spring fed pond that empties directly into Tischer Creek. After the pond’s construction a fish community became established through natural and human aided dispersal of fish. In May of 2004 rock pond was drained in order to replace the outlet and to remove all the fish. At this time it was suspected that goldfish had been established by an aquarium release, and it was feared that they would soon spread to Tischer Creek, a tributary of Lake Superior and serve as a source population for future spreading of this species.

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.


Study Site

            Rock Pond is located on the University of Minnesota, Duluth campus in the Bagely Nature Center. It is approximately 0.4 ha in size and ovate in shape. The pond was constructed by the university in the 1970’s and has steep banks, a flat bottom and a maximum depth of 2.5m. The soil is heavy clay, but substantial organic deposits have accumulated in some areas. The pond is spring feed and has an outlet spillway into Tischer Creek, a tributary of Lake Superior. Rock pond has become more eutrophic with time and the amount of aquatic vegetation has steadily increased.

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 May 12, 2004. Every effort was made to collect all goldfish present. Every goldfish collected was measured for mass and length. Scale samples were taken from 60 goldfish for age and growth determination. Samples of other fish species were collected and the average lengths calculated from a sample of each species.

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 May 12, 2004, and allowed us to estimate the approximate time of spawning for gold fish in Rock Pond.   

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 Wisconsin 3.0 bioenergetics model refined by Hewett and Johnson (1991) and later made available as a computer program for Microsoft windows by Hanson et al. (1997) allowed examination of various piscivore species as potential agents that might be used for the elimination of goldfish from the pond.  The core equation of the Wisconsin 3.0 bioenergetics model is given by:



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.


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 1 to 5 years old. The lengths found in each year class, at the time of harvest, are represented in figure 1. The most common size class was 140mm to 160mm and was represented by 47 individuals.  The average goldfish was 144mm long and weighed 53.3 gm (N=128).  Figure 2 shows the number of individuals in each size class.  The total live weight of all goldfish was 6.82 kg. This translates into a density of approximately 15.33 kg/ha.   The growth rate parameters estimated from the Von Bertalanffy’s equation are k = 0.1215 and Linf = 540mm.  Annual survivorship was 0.45, and annual mortality was approximately 0.55. The instantaneous mortality rate was 0.80.


            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.


            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 Minnesota’s waters, and all of them are located in urban areas. Because goldfish are a regulated invasive species, and a common aquarium pet, more introductions are inevitable.  As long as people keep and propagate goldfish, it is likely that they will continue to be released into aquatic environments. Obviously Rock Pond is not an isolated incident and understanding and preparing for future introductions in necessary.  

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 Alexandria, MN (www.minnesotamuskiefarm.com). Their price for 25cm to 35cm muskellunge is approximately $15.00.  20 muskellunge at $15.00 is $300.00 and would have represented an effective means for the complete removal of all goldfish from Rock Pond.

Stocking predatory game fish is a common practice in Minnesota.  The Minnesota  DNR stocks 200 million walleye fry, 2 million walleye fingerlings, 54,000 walleye yearlings, 32,000 walleye adults, 32,000 muskellunge fingerlings and 7,500 large mouth bass fingerlings or adults each year (www.dnr.state.mn.us/faq/mnfacts/fishing.html).  Usually stocking takes place to provide sport fishing opportunities or to establish populations in reclaimed lakes. Stocking can also be done to control or eliminate exotic species.  Rock pond is one situation were stocking predatory fish to eliminate an exotic species would have been possible. 

            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 Minnesota we need to both increase the public’s awareness of the ecological impacts of goldfish and develop effective methods to remove goldfish from any body of water.  The public perception of goldfish is that of a small and harmless little fish, not of an ecosystem altering alien species.  Minnesota Sea Grant as a member of the Habitude project is working to educate the public about the dangers of aquarium releases.  As long as aquarium owners believe that it is acceptable and harmless to release their fish we will continue to fight invasions of exotic species.  Informing the public about the dangers of exotic species, including goldfish, is a worthy but never ending task. Secondly, I would encourage the Minnesota DNR to compile a complete list of all established wild goldfish populations and take steps to eliminate them. These wild populations also serve as possible sources for future invasions.  Understanding the full breadth of this problem is the first step in working towards a day when we have no goldfish invasions.





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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. 



Initial Weight

Final Weight



Large Mouth Bass





Large Mouth Bass





Large Mouth Bass





Large Mouth Bass





Northern Pike





Northern Pike







































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.



Size at stocking (g)

Number stocked

Minimum time

Maximum time

Large Mouth Bass





Large Mouth Bass





Northern Pike