Are Goldfish Cold Water Fish?

Are goldfish (Carassius auratus) cold-water fish? James Kopco PhD BCE

What is the optimum temperature range for goldfish, and is temperature a valid reason to segregate goldfish from conventionally “tropical” ornamental fish? To some extent, the answer to this question depends on exactly how we define “optimum”. Does the optimum temperature yield the most vibrant colors, the strongest immune response, or the greatest growth rate? Is it determined by the behavioral preferences of the fish? Or is there some other determinant of “optimum”? In this brief review of the available scientific literature dealing with goldfish thermoregulation and a variety of variables that are (or are not) temperature-dependent, I am going to try to tackle this question systematically, thoroughly, and objectively.

Volpato et al. make the argument that any physiological measure (e.g. growth rate, reproductive output, etc.) may increase output in aquaculture settings but not be universally “good” for the fish. For example, Nile tilapia develop better in an environment illuminated by blue light, but in preference tests they chose to occupy environments illuminated by yellow light instead. They further argue that many aquaculture practices maximize outputs but are clearly not beneficial for the animals. To quote, “These conditions include increased stocking densities, use of antibiotics to avoid diseases in sub-optimal environments, and a strong genetic selection for certain productivity traits (which may weaken other biological traits; see also Grandin & Deesing 1998). To this end, they argue that welfare should be determined by the preferences of the animal, not by an arbitrarily selected physiological response.

Another relevant consideration is the point posited by Martin and Huey based on the shapes of response curves of typical ectotherms to different temperatures. In many cases, temperatures above the optimal are more harmful than are temperatures below the optimal, and ectotherms are often imperfect temperature regulators. Because of the potential for error and the consequences of overshooting being greater than the consequences for undershooting, Martin and Huey argue that the ectotherms should seek temperatures slightly below their optimal temperature to account for error and the risk of self-harm from overly high temperature. Focusing specifically on goldfish, the potential temperature range spans a lower lethal temperature of 0.3°C to an upper lethal temperature of 43.6°C (Ford & Beitinger 2005).
For the sake of argument, for right now let’s accept this premise that the preferences of the fish should determine what is optimal. So what do we know about temperature preferences in goldfish? Two articles provide an excellent overview of the overall methods and thought processes involved in measuring fish preferences for temperature: Golovanov and Reynolds & Casterlin. These reviews discuss the roles of evolutionary/ecological history, acclimation temperature, feeding status, life stage, disease pressure, and other factors that all play into a fish’s final temperature preferendum, or the temperature range (usually spanning 2-6°C) in which the fish will eventually settle if constantly allowed to self-select its temperature. Acclimation temperature can have a strong effect on temperature preferences but can be fully artificial, so the final temperature preferendum is the measurement most commonly used to determine true preferences.
Coutant compiled a review of temperature preference data for a wide range of fish, and across four references reported a range of final temperature preferenda of 24-30°C, with the lowest preferenda

exhibited by adult goldfish in the fall and winter. Reynolds et al. studied the pattern of temperature preferences over the course of a day in goldfish that could select temperature from a gradient across the tank, and found that they preferentially occupy 26-30°C, with a peak 4 hours before dawn. Reynolds and Casterlin (1979b) showed that goldfish exhibit decreased activity at their preferred temperature (28°C) compared to other temperatures within a range of 24-32°C, which they suggest is an indication that this behavior preference reflects a physiological optimum where metabolism is most efficient. Furthermore, Rozin and Mayer ran an experiment where goldfish were placed in a tank that gradually warmed up to a temperature of 41°C. The fish were trained to press a lever that caused a squirt of cold water into the tank to lower the temperature by 0.3°C. When the fish were initially put into the tank at 38oC, they quickly drove the tank temperature down to 33.5-36.5°C. When they were initially put into a cooler tank, they allowed the tank temperature to increase, then maintained it within the previously mentioned range.
In addition to these three studies that measured thermoregulation in unmanipulated goldfish, a number of studies have explored the affects of various drugs, surgical operations, infections, etc. on goldfish thermoregulatory behavior, and the untreated “control” goldfish serve as additional measures of unmanipulated goldfish temperature preference. Nelson & Prosser (1979) found that goldfish normally selected temperatures near their acclimation temperature (available range 2-30°C, acclimation temperatures 5°C, 15°, and 25°C, full text unavailable). Rausch et al. studied the affects of alcohol and anoxia on goldfish thermoregulation, and found that fish in control groups (“normal” oxygen and alcohol concentrations) that were acclimated to 25°C selected a mean temperature of 26.5±0.3°C, while fish in treatment groups typically selected lower temperatures. Crawshaw et al. conducted a similar series of experiments with goldfish acclimated to 10°C, and control fish selected 17.5±0.5°C, which is consistent with Nelson and Prosser’ss assessment that acclimation temperature has strong short-term affects on temperature preference but probably fails to account for final temperature preferendum. Zdanovich measured the effect of food satiation on goldfish, sterlet, and Siberian sturgeon, and found that satiated goldfish originally acclimated to 20°C selected 29.1±0.3°C after four days in a temperature selection trough, while continuous food deprivation caused temperature preferences to dip to 26.5±0.3°C after four days. Reynolds et al.  showed that goldfish final temperature preferenda shifted from 27.9°C in uninfected fish to 32.7°C in fish injected with Aeromonas hydrophila or E. coli endotoxin, and the ensuing “behavioral fever” (Kluger) increased resistance against the pathogens. Cabanac & Laberge (1998) demonstrated similar results with very young (2.5-4 g) goldfish subjected to Salmonella typhosa injection or chasing and given a choice between a 34°C chamber and a 37°C chamber; they found that pathogen-challenged goldfish exhibited behavior fever (spent more time in 37°C chamber) whereas control and chased goldfish spent most of their time in the 34°C chamber. Crawshaw & Wollmuth  and Wollmuth et al. (1988) examined the effects of acetylcholine injection and adrenaline, respectively, into different parts of the brain on temperature preferences of goldfish, and showed that pre-injection the goldfish spent most of their time in a 26-30°C temperature range.

Taken together, these results suggest that 26+°C is the preferred temperature of goldfish, but that they will not respond aversively to temperatures until they climb to 36.5°C (13 studies). By comparison, the preferred temperature for angelfish (Pterophyllum scalare) is 29-31.1°C (Perez et al.), for guppies (Poecilia reticulata) is 24.5-28.2°C (Johansen and Cross 1980), and for mollies (Poecilia sphenops) is 25.5-29.6°C (Hernandez et al.). These tropical fish have temperature preference ranges that broadly overlap with those of goldfish.

Now let’s presume that you don’t accept that the fish knows what is best for it, and you want to look into the physiological responses. To simplify this discussion, I’m going to focus on constant temperatures. Heat shocks and sudden temperature changes aren’t good for most ectotherms, but that would open a whole new can of worms that I don’t want to get into here.
Let’s start with the old “heat speeds up their metabolism and shortens their life span” argument. I’ve searched high and low for any evidence to support this for goldfish, and as far as I can tell there is no empirical evidence to back it up. It is a logical extension of the degree day model that suggests that any ectotherm will live for a set number of “degree days” above a baseline temperature. If the baseline temperature is 50 degrees, then a 70 degree day would use up 20 degree days, while a 80 degree day would use up 30 degree days. This model is useful for predicting the emergence of seasonal crops and insect pests, but the evidence that it is directly applicable to aging and dying is limited, especially for long-lived species that reproduce throughout their adult lives, such as goldfish. We could also reasonably argue that a fish that lives for 24 years, with 3 months dormant under ice each year, is no better off than a fish that lives for 18 years but that is active for the entirety of its life because both fish are alert and active for a total of 18 years. I did however find one potentially relevant study on temperature effects on fish longevity that focused on sticklebacks (Lee et al.). The researchers reared young sticklebacks during their first spring in warm, medium, or cold water, then allowed them all to grow at the same temperature through the summer and treated them the same for the remainder of their lives. They found that the cold-water sticklebacks’ growth was inhibited during their initial cold period while the warmwater sticklebacks’ growth was accelerated, but that over the first summer all of the fish modified their growth rates so that they attained the same size by the end of the summer. The cold-water fish that had to catch up reduced their lifespans by 10% compared to the medium- temperature fish, while the warmwater fish that slowed their growth later on had their lifespans extended by 30% compared to the medium-temperature fish. If the same trends apply to goldfish, then rearing them in relatively warm temperatures that allow for efficient growth may actually extend their lifespans, despite the degree-day effect. An additional relevant reference to mention is Belk & Houston (), who conducted a review to determine whether Bergmann’s Rule applies to freshwater fish. Bergmann’s rule states that animals from greater latitudes, higher elevations, or otherwise colder environments tend to be larger than other members of their species from lower latitudes, lower elevations, or warmer environments, and it has considerable support from mammals. The three variables they considered in their review were length-at-age, maximum length, and lifespan. Of the thirteen species they investigated for lifespan, only five showed significant positive correlations between latitude and lifespan. Goldfish was not among the species reviewed, but common carp (Cyprinus carpio) was reviewed and showed no positive correlation between latitude and longevity. Because habitats at higher latitude tend to be colder and the common carp is sufficiently closely related and similar to goldfish to serve as a potential proxy, this hints to a lack of correlation between temperature and longevity in goldfish.
Now let’s jump into some other physiological responses. Kestemont (1995) showed that goldfish grow most rapidly and have the greatest feed conversion ratio (the ratio of body weight gain to weight of food consumed) at 28oC, the highest temperature tested in the experiment. Gouveia and Rema (2005) showed that ornamental goldfish develop the most vibrant coloration at 26-30oC, also peaking at the highest temperatures that the researchers tested. Brezden et al. (1975) and Riege and Cherkin (1972) demonstrated positive correlations between water temperature and the speed with which goldfish learned and the amount of time goldfish remembered what they learned, respectively, over temperature ranges from 10-28oC and 10-30oC, respectively. Rahman et al. (2001) tested the immune response and survival of goldfish at a range of temperatures to an experimentally administered

Aeromonas infection, and found that the immune response and survival of goldfish is highest at 10oC or 32oC, and lowest at 17oC and 25oC.
So far, we’ve seen a lot of alliance between goldfish preferences and physiological responses – optimal growth, feed conversion, coloration, learning, memory, and immune responses all occur within the preferred temperature range of 26-30oC. However, things get a little different if we focus on reproduction. Wiegand et al. tested goldfish egg development at 17oC, 22oC, and 27oC, and found the lowest rate of abnormal larvae at hatching and highest larval viability at 22oC. Consistent with this, Gillet and Billard  showed that gonad development was inhibited in goldfish at high temperatures (30oC compared to 20oC), though this effect had a strong interaction with photoperiod.
It bears mentioning that there are a lot of in-depth physiology studies that show differences between goldfish kept in different temperatures where it is very difficult to consider one result “better” than another. Pang et al. () showed that goldfish increase their metabolic rate, as measured by oxygen consumption, by 143% at 25oC compared to 15oC. Beamish and Mookherjii  showed a linear increase in O2 consumption from 10oC to 35oC. The specific increase depends on whether the fish is fed or fasted, and a comparison of the two revealed that in fed fish the oxygen consumption for digestion and processing of food dominates this effect over an increase in activity. You could argue that substantially increased oxygen demands, coupled with warmer water that holds less oxygen, is a major problem. This oxygen paradox is accounted for by the findings of Sollid et al. and Tzavena et al. who showed that as goldfish acclimate to water with lower oxygen content, they can alter the morphology of their gill lamellae to increase gill surface area by ~7.5-fold and reduce the covering of an interlamellar cell mass that is thought to reduce energetic costs of maintaining osmotic homeostatis in the gills. This morphological plasticity in the gills allows a goldfish to adjust to low oxygen concentrations despite high oxygen demand. Acclimation temperatures of 5oC and 25oC yielded chemical differences in the phospholipids of membranes in goldfish gills and liver (Anderson). Acclimation temperature also effects the phospholipid structure in the outer and inner mitochondria of the brain (Chang and Roots). Cytochrome oxidase activity (an enzyme involved in respiration) was higher in goldfish acclimated to lower temperatures, suggesting that they can compensate for low temperatures by increasing enzyme activity (Caldwell). I could go on and describe several more physiological changes that occur in goldfish in response to acclimation temperature, but again the “best” state is unclear. The salient point to take from the body of evidence that goldfish undergo physiological changes at different acclimation temperatures is that goldfish are highly adaptable and can survive, or even thrive, over a wide range of temperatures, seemingly from a basal temperature of around 10oC to an upper lethal limit of about 40oC.
Finally, I want to circle back on the close phylogenetic relationship and ecological similarity between goldfish and common carp to draw on some additional comparisons between the two. Jha et al. showed that goldfish and koi (a domesticated relative of common carp) are actually poor cohabitants in captivity because their diets are so similar, but koi are more competitive for food than are goldfish. Both common carp and goldfish are notorious invasive species with similar invasion patterns and harmful effects (competition with native fish species, uprooting/consumption of macrophyte plants, increasing turbidity and nutrients by rooting up the bottom, enhancing algae blooms, etc.). Bajer & Sorensen provide compelling evidence that seasonal hypoxia events that wipe out competitors and predators are a key ingredient to successful invasion by common carp. Because goldfish have comparable hypoxia tolerance to common carp (Dhillon et al.), it is likely that the same factor is involved in their successful invasions. Hypoxia events most commonly occur during the winter, when ice cover over the water’s surface prevents gas exchange with the air, photosynthesis slows drastically or stops, but respiration from plants, animals, and microbes continues, though hypoxia events can also occur during periods of extreme heat. If goldfish follow similar trends as common carp, it could create a situation where populations are most robust in waterways that experience very long, very cold winters that create seasonal hypoxia events that eliminate predators and competitors. However, this should not be confused with a preference for low temperatures given all of the behavioral and physiological data presented here. Rather, the goldfish are simply more tolerant of cold and oxygen-poor conditions than many other fish and capitalize on the ecological vacancies created by hypoxic fish-kill events. These seasonal environments still provide goldfish with periodically-available warm, suitably oxygen-rich water that abundant research has shown provides the most favorable growing environment.
To summarize this research, thirteen studies showed that goldfish exhibit behavioral preferences for 26-30°C in most situations, with potentially higher preferences if they are pathogen- challenged and the lowest preference (24°C) exhibited only by adult goldfish during winter. Five studies showed clear physiological advantages to occupying a 26-30°C temperature range. Two studies show advantages in egg development and viability at a lower temperature range (e.g. 20-22°C). There’s an overwhelming convergence of data indicating that the optimal temperature for juvenile or non- reproductive adult goldfish is 26-30oC, though they can more-or-less do well over a range of about 10- 36oC and can tolerate temperatures down to 0.3°C. If you want to breed them and maximize brood sizes, you may be better served providing your fish with seasonal abiotic shifts, with a focus on both temperature and photoperiod, dipping to a winter low of 10°C and gradually warming to a summer high of 26-30°C. If you want to keep them as pets and are not concerned with breeding them, then housing them in temperatures suitable for many tropical fish is ideal, though lower or higher temperatures are also acceptable. Goldfish are also distinctly tolerant of the harsh conditions posed by winters in small, shallow ponds, and are likely to benefit from the extra space and natural forage available in a pond during the summer compared to tank-raised counterparts, so pond-rearing is also a perfectly acceptable method of raising goldfish. The common argument that goldfish cannot be housed with tropical fish because of differences in temperature requirements is simply not supported by empirical data, and providing them with a heater to maintain an elevated temperature can yield a number of significant physiological benefits.
interconnected lakes.

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