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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: • ornamental fish • predicting body weight • energy requirement • food allowance • aquarium
Aquarists often overlook the importance of feeding an appropriate amount of a balanced diet for fish health and maintenance of water quality in an aquarium. Although most commercial food manufactures do offer an on-package feeding guide containing statements like "feed between 2–4 times per day, sprinkling a few flakes per fish on the water until the fish stop feeding," such guidelines are vague, and may result in overfeeding. The overfeeding of fish in an aquarium results in increased pollution (ammonia and/or nitrite) and ultimately causes system failure if the tank is not maintained properly through husbandry procedures like water changes. These on-package instructions are deliberately vague mainly because of the limited information available on factors such as different species requirements for size, activity, age, environment, and water temperature. The aim of this research is to establish simple mathematical models that can predict body weight (BW) from body length in fish. By linking predicted BW with a known energy requirement and energy density of the food, it becomes possible to calculate accurate food allowances. This is particularly useful because, although BW cannot easily be determined by owners of ornamental fish, an estimate of body length is usually possible.
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MATERIALS AND METHODS |
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 TOP MATERIALS AND METHODS RESULTSDISCUSSIONLITERATURE CITED |
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Weight prediction models
Mathematical models were developed using fish weight and length data. A total of 472 Oranda Goldfish, 2612 Common Goldfish (2594 with BW between 1.5 and 60 g and 18 with BW between 150 and 450 g), and 6161 tropical fish from 15 different species were measured.
Fish weighing (g) was conducted by placing a beaker of water on a MC1 Sartorius balance, taring off the balance and placing the fish into the beaker. Length (mouth to caudal peduncle) was estimated by placing the fish on 1-mm laminated graph paper. 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 MC1 Sartorius balance scale for BW measurements before being returned to their original tank. Total time spent out of the water was <5 s.
Only the relation between weight and length was considered for model development because the predictive relation between length and weight has already been defined as a power function (1–5). The equations fitted to the data were of the form Y = a(length)b, or Y = Y0 + a(length)b, where Y is the predicted weight of the fish in g, Y0 and a are specific constants, b is the exponent relative to that species and length is measured in mm.
All models were fitted using SigmaPlot, version 8.0.2 (Systat Software). The adjusted R2 value was used as a measure of the goodness-of-fit for each model, with values of 
0.80 (80%) selected as being significant for the purposes of this study.
Energy requirements
To determine food allowances from predicted BW, energy requirements relative to BW were needed for the relevant species. Energy requirements were determined by examining the relation between growth rate and energy intake. Most fish grow continuously throughout their life, with the rate of growth decreasing with age. This presents a challenge when trying to determine a healthy feeding allowance, because there is no adult phase where growth ceases. For this reason, the aim of our study was to determine the food requirements and therefore energy requirements to achieve a specific growth rate (SGR) as close to 0.1%/d, for adult ornamental aquarium fish. This figure could then be used as a realistic and healthy growth rate for ornamental fish housed in a confined space like an aquarium where rapid growth may result in overcrowding and suboptimal husbandry conditions.
Food requirements
The food used was AQUARIAN Flakes (goldfish or tropical, as appropriate for the species), (manufactured by Masterfoods Complimentary Petcare). Food requirements were calculated using the following equation:
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where SGR is specific growth rate, calculated using the following equation:
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where ln is the natural log, J = joules, and BW= body weight in g.
Estimated metabolizable energy = (4 x protein + 9 x fat + 4 x NFE) x 4.186, where NFE is nitrogen free extract (6).
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RESULTS |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Separate models were produced for each group of goldfish and for each of the 15 species of tropical fish. Some of the equations fitted required the Y0 term, which indicates that they do not pass through the origin. For these species a restricted weight range is provided (Table 1) insofar as it is not possible for a fish of zero length to have a positive BW, or for a short fish to have negative BW.
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0.80) in 12 tropical species and the small Common Goldfish (BW 1.5–60 g). The BW of Large Goldfish (BW 150–450 g), Oranda Goldfish and 3 tropical species could not be predicted with sufficient accuracy. Energy requirements
Energy requirements were determined for Oranda Goldfish, Common Goldfish, and 8 tropical fish species (Table 1). The energy requirements of Common Goldfish were examined at 3 different temperatures to replicate the yearly cycle of a pond in temperate climates. Energy requirements have yet to be determined for the other 7 species mentioned in Table 2.
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The food requirement is determined from the BW (g) (predicted from measured body length), the energy needed to promote 0.1% specific growth rate (J/g BW/d), and the predicted metabolizable energy of food (J/mg).
For example, for a Tinfoil Barb, with a predicted BW of 78 g, and an energy requirement of 75 J/g BW/d, fed tropical flakes containing a predicted metabolizable energy of 16.7 J/mg, the food requirement would be:
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DISCUSSION |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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This research also shows that ornamental species have different energy requirements and that further research is warranted to provide the species with optimal nutrition. There are several factors that may influence energy requirements in fish. Fish species can vary in their energy requirement depending on their natural behavior. Data in this study demonstrates that highly active shoaling fish, such as the Black Widow Tetra, tend to expend more energy than sedentary species, such as the Common Goldfish and the highly territorial, solitary Midas Cichlid. Body weight may also influence energy requirements. A very small fish species, such as the Black Neon Barb and Peppered Corydoras, has a relatively high requirement compared with larger fish species, like the Tinfoil Barb and Midas Cichlid. This study also demonstrates that water temperature influences energy requirements, at least in the Common Goldfish, with increasing water temperature resulting in increased energy requirements. It is likely that fish activity level increases as water temperature rises, which results in higher energy requirements.
For foods with a known energy content, it is possible to calculate accurate species-specific food requirements for adult fish by combining the predicted BW with energy requirement to achieve 0.1% SGR. Feeding optimal amounts of food to aquarium fish will enable an owner to ensure that fish receive enough food for healthy growth while minimizing the risk of overfeeding and subsequent tank pollution.
Conclusion
It is possible to predict the BW of a range of ornamental fish species from body length. Thus, together with energy requirement data and food energy density data, it is possible to develop accurate feeding guides for aquarium fish that enable fish to grow at a healthy rate while reducing the risk of overfeeding and subsequent tank pollution. Energy requirements varied between fish species and a number of possible reasons for this were discussed including body size, natural behavior, and water temperature.
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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. Dulcic J, Kraljevic M. Weight-length relationships for 40 fish species in the eastern Adraitic (Croatian waters). Fish Res. 1996;28:243–51.
2. Goncalves JMS, Bentes L, Lino PG, Ribeiro J, Canario AVM, Erzini K. Weight-length relationships for selected fish species of the small-scale demersal fisheries of the south and south-west coast of Portugal. Fish Res. 1997;30:253–6.
3. Moutopoulos D, Stergiou K. Length-weight and length-length relationships of fish species from the Aegean Sea (Greece). J Appl Icthyol. 2002;18:200–3.
4. Santos M, Gasper P, Vasconcelos P, Monteiro CC. Weight-length relationships for 50 selected fish species of the Algarve coast (Southern Portugal). Fish Res. 2002;59:289–95.
5. Thomas J, Venu S, Kurup B. Length-weight relationship of some deep-sea fish inhabiting the continental slope beyond 250m depth along the west coast of India. NAGA, WorldFish Center Quarterly. 2003;26:17–21.
6. Catacutan MR, Coloso RM. Effect of dietary protein to energy ratios on growth, survival and body composition of juvenile Asian seabass, Lates calcifer. Aquaculture. 1995;131:125–33.
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