It is conventional fishkeeping wisdom that schooling fishes are most comfortable in large groups, but this apparent truism has not been scientifically tested until now.
Amelia Saxby and colleagues have published a study in a recent issue of the journal Applied Animal Behaviour Science, in which they examined the effects of group size on the behaviour and well-being of four species of common aquarium fish: Angelfish (Pterophyllum scalare), Neon tetra (Paracheirodon innesi), Tiger barb (Puntius tetrazona) and White Cloud Mountain minnow (Tanichthys albonubes).
The authors tested several hypotheses:
- that increasing group size would allow the Neon tetras and White cloud mountain minnows to exhibit more natural behaviour (therefore increasing their welfare);
- that increasing group size would not have an effect on angelfishes;
- that increasing group size would reduce intraspecific aggression in tiger barbs and increase their welfare.
The authors placed the fishes in varying densities within 10 l. tanks for their experiments.
For Neon tetras and White Cloud Mountain minnows, group sizes of one, two, five and 10 fish per tank (equating to 0.1, 0.2, 0.5 and 1.0 fish per litre) were used.
For Angelfish, group sizes of one, two, three and five fish per tank (equating to 0.1, 0.2 and 0.5 fish per litre) were used and for Tiger barbs, one, five and eight fish per tank (equating to 0.1, 0.5 and 0.8 fish per litre) were used.
The authors then recorded the behaviour of the fish by filming them for 20 minutes with a video camera three times daily.
The authors then scored several different types of behaviour, e.g. darting (indicating being chased or ‘unsettled’, a sign of reduced welfare), neighbour index (to measure shoaling) and latency to feed (a greater reluctance to feed indicating decreased welfare).
After statistically analysing their data, the authors found that Neon tetras and White cloud mountain minnows displayed the lowest levels of aggression and darting in the larger group sizes.
Groups of 10 had the greatest tendency to shoal and they surmised that the reduced aggression and darting are a result of increased shoaling at higher densities.
For the Angelfish and Tiger barbs, the propensity to shoal also increased in larger group numbers, but there was no association with a decrease in aggression or darting behaviour.
The authors considered it possible that the group sizes used might have been too small for a decrease in aggression to take place.
The authors also found the latency to feed decreasing with increased group size in Neon tetras, White Cloud Mountain minnows and Angelfish, but not in Tiger barbs.
The authors concluded that their results were "…in broad agreement with recommendations in the hobbyist literature."
For more information, see the paper: Saxby, A, L Adams, D Snellgrove, RW Wilson and KA Sloman (2010) The effect of group size on the behaviour and welfare of four fish species commonly kept in home aquaria. Applied Animal Behaviour Science 125, pp. 195–205.