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WALTHAM Centre for Pet Nutrition, Waltham on the Wolds, Melton Mowbray, Leicestershire, UK
3 To whom correspondence should be addressed. E-mail: abigail.stevenson{at}eu.effem.com.
KEY WORDS: • fish • stocking density • goldfish • black widow tetra • aquarium
The health and welfare of a fish can be influenced by a variety of physical factors ranging from water quality to territorial defense, unnatural habitats, stocking densities and available nutrition. Stocking density is well publicized as a factor that can influence fish health and growth rate insofar as overstocking a tank can negatively affect water quality and food availability (1). However, most of the available literature is directly related to commercially cultured food fish, and not ornamental fish species, that are kept in an aquarium setting. The limited information that does exist is for the production of common captive bred ornamental species, which are shipped worldwide, predominantly from Asian countries. Because of the economic nature of rearing fish for the ornamental fish trade, stocking densities are often at their peak under such conditions and therefore not applicable to housed aquaria set-ups. Typical stocking densities for species such as Goldfish, Koi, Barbs, Swordtails, and Cichlids are estimated at 0.13 kg/m3 (2). In addition, research shows that Goldfish fry appear to show negative effects of overstocking at lower densities than Koi fry (3).
Often, the stocking density alone or in combination with feeding rate is adjusted by fish farmers to obtain a particular size of fish at harvest, thus indicating that fish growth may be a function of available water space (4). However, low stocking density (for example, keeping a shoaling fish alone) is a factor often overlooked by the hobbyist that may also compromise welfare, particularly in shoaling fish species, where the primary purpose of the shoal is to provide increased protection from predation. Removing this protection through solitary housing may inherently compromise the welfare of the individual concerned. Recent research in African catfish (Clarius lazera) (5) found that fish housed individually grew more slowly than those housed in pairs or groups, with social interaction providing an important motivation for feeding in this species.
Using growth rate as a marker for health, these studies monitored the growth of Black Widow Tetras (Gymnocorymbus ternetzi) and Common Goldfish (Carassius auratus) when divided into different group sizes. These 2 fish species were chosen because of the differences in their natural behavior. The Black Widow Tetra is a peaceful shoaling tropical freshwater species native to the rivers of southern Brazil. In contrast, the Common Goldfish is a domesticated nonshoaling temperate species thought to be derived from the Crucian carp (Carassius carassius).
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MATERIALS AND METHODS |
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 TOP MATERIALS AND METHODS RESULTSDISCUSSIONLITERATURE CITED |
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Experimental trials were conducted at the WALTHAM Aquacentre, Birstall, Yorkshire, England. A total of 75 Common Goldfish (C. auratus) weighing 36.8 ± 6.4 g and 87 Black Widow Tetras (G. ternetzi) weighing 2.4 ± 0.6 g obtained from JMC Aquatics, Derbyshire, England, participated in these studies. Fish were housed in 50-L glass tanks set up in parallel as part of an integral tropical or coldwater recirculation system appropriate for each species. Fish were housed singularly (Group 1) or randomly allocated to a shoaling group of 4 (Group 2), 8 (Group 3), 12 Goldfish or 16 Black Widow Tetras (Group 4), with 3 replicate tanks utilized for each group. Fish were offered either standard flake food (Black Widow Tetras were fed AQUARIAN Tropical flake food and Goldfish were fed AQUARIAN Goldfish flake food, both by Masterfoods) for 8 wk (Goldfish) or 12 wk (Black Widow Tetras) at a food allowance of 2% body weight (g:g), split into 4 feeds daily. All fish were maintained on their respective diet for a minimum of 28 d before the start of the analysis. Over the duration of the study, water quality parameters were maintained within safe limits (tropical system: NH3 and NO2 undetectable, NO3 2.64–12.32 mg/L, temperature 25.1–25.4°C, and a pH of 7.60–8.09; coldwater system: NH3 and NO2 undetectable, NO3 3.08–5.76 mg/L, temperature 15.0–15.9°C, and a pH of 6.70–7.20).
Weighing
To calculate and monitor various growth parameters and predict a daily food ration, fish were individually weighed and zoometric measurements were taken before the start of the trial and then again every 2 wk. There were no significant differences in measurements between treatment groups for either fish species at the start of the study. To carry out the measurements, fish were removed from the tank using a net and placed into a holding bucket containing tank water. Fish were removed from the bucket individually and placed on laminated graph paper. Length, from the mouth to caudal peduncle, and depth, from the deepest point of the body to base of dorsal fin, were measured (mm). Fish were then placed in a tared beaker of tank water on a weighing balance for body weight measurements, before being returned to their original tank. Total time spent out of the water was <5 s.
Both fish and food weight data were used to calculate the specific growth rate and food conversion ratio, using the equations below:
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where ln is the natural log
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All husbandry and handling procedures were approved by the WALTHAM Ethical Review Committee.
Statistics
Data were compiled into means ± SD. A goodness of fit test was applied to check normality of the data. Because data were found to follow a normal distribution, the effect of group size on growth parameters was analyzed using the ANOVA multiple-sample comparison and multiple-range tests (least significant difference) using Statgraphics Plus, version 2.0. The level of significance was taken as P 
0.05.
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RESULTS |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Throughout the trial, all food was consumed regardless of group size, and all fish remained healthy with no mortalities. Growth data are displayed in Table 1 (Goldfish) and Table 2 (Black Widow Tetras). Over the duration of the study, in the Common Goldfish, specific growth rates ranged between 0.39 and 0.47%, feed conversion ratios ranged from 6.86 to 10.35, and the percentage of growth values were between 19.95 and 23.05%. The Black Widow Tetras produced a specific growth rate of between 0.13 and, 0.21%, a feed conversion ratio between 2.83 and 3.17, and a percentage of growth values between 10.67 and 16.49%.
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Depth:Length ratio
Depth:length (D:L) measurements were used as a marker of body condition for fish with values obtained in this study ranging from 0.39 to 0.42 for the Common Goldfish and from 0.50 to 0.52 for the Black Widow Tetra.
The D:L of Common Goldfish increased significantly over the course of the study, regardless of group size (Table 3), indicating that in this study the Goldfish increased depth at a faster rate than length. Group size did not significantly affect the D:L ratio in the Common Goldfish.
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DISCUSSION |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In addition, individually housed fish were not significantly different from those housed in groups, unlike recent studies in the African catfish (Clarius lazera) (5) where singularly housed fish grew more slowly than those in pairs or groups. It is possible that social interaction is not an important motivation for feeding in the Common Goldfish or Black Widow Tetra. However, it is more likely that the stocking density never reached the threshold at which food availability and competition among individuals impacted growth rate. In our study, fish were also fed a set amount of food according to the total biomass in the tank and therefore the same amount of food was available to each fish whether housed individually or in groups. This effectively standardized food availability across all stocking densities in this study.
The D:L ratio for Common Goldfish increased over the course of the study at all stocking densities, indicating that the fish increased in depth at a faster rate than in length. D:L for Black Widow Tetras remained relatively unchanged over the course of the study at all stocking densities, indicating Goldfish and Black Widow Tetras may have different growth patterns. However, in the absence of an optimal D:L ratio, alternative body condition scoring system, or method of assessing optimal fish health, it is not possible to state that individual housing has an effect on body condition or on the well-being of ornamental fish.
Conclusions
Group size did not significantly affect growth rate in either Common Goldfish or Black Widow Tetras when housed in groups of up to 12 (Goldfish) or 16 (Black Widow Tetras) individuals in a 50-L tank. Individual housing did not appear to impact the growth patterns of the Common Goldfish or the Black Widow Tetra.
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FOOTNOTES |
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2 Author disclosure: no relationships to disclose.
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LITERATURE CITED |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Ellis T, North B, Scott AP, Bromage NR, Porter M, Gadd D. The relationships between stocking density and welfare in farmed rainbow trout. J Fish Biol. 2002;61:493–531.
2. Ng WJ, Kho K, Ho LM, Ong SL, Sim TS, Tay SH, Goh CC, Cheong L. Water-quality within recirculating system for tropical ornamental fish culture. Aquaculture. 1992;103:122–34.
3. Feldite M, Milstein A. Effect of density and growth of cyprinid fish fry. Aquacult Int. 2000;7:399–411.
4. Stone N, McNulty E, Park E. The effect of stocking and feeding rates on the production of feeder goldfish in pools. N Am J Aquac. 2003;65:82–90.
5. Martins CIM, Schrama JM, Trenovszki M, Verreth JAJ. The importance of social interactions on growth performance and feed efficiency of African catfish [abstract]. Proceedings of 11th International Symposium on Nutrition and Feeding in Fish 2004 88, 226.
6. Kristiansen T, Ferno A, Holm J, Privitera L, Bakke S, Fosseidengen J. Swimming behaviour as an indicator of low growth rate and impaired welfare in Atlantic halibut (Hippoglossus hippoglossus). Aquaculture. 2004;230:137–51.
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