The number of fish needed for species "x" to be healthy and less stressed in an aquarium is a topic that enters quite a number of threads on TFF. Some of us realize how vitally important the number of fish for a particular species really is, but having scientifically-controlled studies that provide concrete evidence has been largely lacking. One such study is "The effect of group size on the behaviour and welfare of four fish species commonly kept in home aquaria," authored by Amelia Saxby, Leoni Adams, Donna Snellgrove, Rod W. Wilson, and Katherine A. Sloman in the journal Applied Animal Behaviour Science, 125 (2010), pp. 195-205. This paper is available online (free) and I will include the link at the end of my post. The authors are marine biologists or similar, based in the UK, and the majority of the following is cited directly from this paper.
"Functioning-based” approaches to animal welfare aim to ensure the animal is functioning naturally on a biological level while a “natural behaviour” approach defines good welfare as being when an animal is free to fulfil its natural behaviour (Duncan and Fraser, 1997). More recently the “five freedoms” approach to animal welfare has been put forward where in order to guarantee welfare, animals should be free from: (a) hunger and thirst; (b) discomfort (an appropriate environment including shelter should be provided); (c) pain, injury and disease; (d) restriction of normal behaviour (including lack of space); (e) fear and distress (Farm Animal Welfare Council, 2009).
The welfare of four commonly kept species of ornamental fish (neon tetras, white cloud mountain minnows, angelfish and tiger barbs) was investigated in relation to group size. Behaviours including darting, aggression, shoaling and latency to feed were found to vary with group size in a species-specific manner. Each species was kept in specific group sizes in individual tanks. For neon tetras and white cloud mountain minnows, group sizes were one, two, five and 10 fish per tank; for angelfish the group sizes were one, two, three and five fish per tank, and for tiger barbs they were initially one, five and eight fish per tank.
It is important to note that for "ethical reasons," the Tiger Barb groups of one and five were removed and only the tank with ten was used; elevated aggression between groups of two and three individuals was noted as soon as they were placed together in the tank. Therefore, as holding tiger barbs at these stocking densities was likely to result in lasting harm, these treatments were terminated immediately. [Sound familiar?]
Neon tetras and white cloud mountain minnows displayed reduced aggression and darting and spent more time shoaling in the larger groups. Both species were most aggressive when held in groups of two and five. Behavioural patterns were more variable in angelfish and tiger barbs although larger group sizes resulted in increased shoaling. The results indicate improved welfare in larger groups of neon tetras, white cloud mountain minnows and tiger barbs. In angelfish, aggression was not reduced with greater numbers, but recognizing that this species naturally requires greater shoal numbers suggests that the need to defend territories or maintain social position causes aggressive actions to persist regardless of the shoal size.
I have dealt with aggression as this is a significant aspect of keeping these fish; the study delves into the other aspects which are equally interesting. There is a lot to be learned here.
"Functioning-based” approaches to animal welfare aim to ensure the animal is functioning naturally on a biological level while a “natural behaviour” approach defines good welfare as being when an animal is free to fulfil its natural behaviour (Duncan and Fraser, 1997). More recently the “five freedoms” approach to animal welfare has been put forward where in order to guarantee welfare, animals should be free from: (a) hunger and thirst; (b) discomfort (an appropriate environment including shelter should be provided); (c) pain, injury and disease; (d) restriction of normal behaviour (including lack of space); (e) fear and distress (Farm Animal Welfare Council, 2009).
The welfare of four commonly kept species of ornamental fish (neon tetras, white cloud mountain minnows, angelfish and tiger barbs) was investigated in relation to group size. Behaviours including darting, aggression, shoaling and latency to feed were found to vary with group size in a species-specific manner. Each species was kept in specific group sizes in individual tanks. For neon tetras and white cloud mountain minnows, group sizes were one, two, five and 10 fish per tank; for angelfish the group sizes were one, two, three and five fish per tank, and for tiger barbs they were initially one, five and eight fish per tank.
It is important to note that for "ethical reasons," the Tiger Barb groups of one and five were removed and only the tank with ten was used; elevated aggression between groups of two and three individuals was noted as soon as they were placed together in the tank. Therefore, as holding tiger barbs at these stocking densities was likely to result in lasting harm, these treatments were terminated immediately. [Sound familiar?]
Neon tetras and white cloud mountain minnows displayed reduced aggression and darting and spent more time shoaling in the larger groups. Both species were most aggressive when held in groups of two and five. Behavioural patterns were more variable in angelfish and tiger barbs although larger group sizes resulted in increased shoaling. The results indicate improved welfare in larger groups of neon tetras, white cloud mountain minnows and tiger barbs. In angelfish, aggression was not reduced with greater numbers, but recognizing that this species naturally requires greater shoal numbers suggests that the need to defend territories or maintain social position causes aggressive actions to persist regardless of the shoal size.
I have dealt with aggression as this is a significant aspect of keeping these fish; the study delves into the other aspects which are equally interesting. There is a lot to be learned here.
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