It is very rare that a GF will grow to the same size in any aquarium that they would achieve in the wild or in a pond.
In outdoor conditions GF respond to the cyclical nature of the seasons.
Following on from a prolonged cold spell a burst of growth hormone is released in the Spring to promote rapid growth and weight gain in preparation for spawning.
As GF do not experience the change in seasons whilst in an indoor setting, this annual growth spurt rarely occurs.
However, stunted growth can and does occur in an aquarium to such an extent as can’t be explained purely by the lack of seasonal rotation. Possible reasons for this phenomenon are:
- Poor water quality. Goldfish produce a lot of waste and overcrowding will frequently lead to elevated Nitrate levels. While there is no conclusive scientific proof that high Nitrate levels lead directly to stunted growth the two have long been linked via circumstantial evidence.
- Release of growth inhibiting pheromones. Since communication between fish via chemical agents (pheromones) was first demonstrated in 1932, this process has been suggested in many aspects of fish behaviour and development including the “crowding factor”. (manifested as an adverse effect on growth, survival and fecundity in dense populations)
In the wild the release of this pheromone serves to ensure the survival of the shoal, by limiting numbers and inhibiting growth so that the shoal can continue to survive in a hostile environment.
Some populations of the Crucian Carp, for example, have adapted to live in stunted dwarf populations in areas of the wild with a limited volume of water, very little food and possibly low oxygen.
The same can occur in the artificial “hostile environments” that we create in aquaria but often to an even more extreme degree.
- Stress. Overcrowding combined with potential poor water quality will inevitably lead to some degree of stress, triggering the release of norepinephrine and epinephrine. Both of these hormones have been scientifically linked to the suppression of the growth hormone in GF.
Other chemical changes also occur when fish are stressed such as fluctuations in cortisol and glucose levels in the blood, while not proven, these changes may equally play a part in stunted growth.
- Finally, there is the consideration of physical space. In extreme cases GF may barely have enough room to turn around in a tank. This is the aquatic equivalent of Chinese Foot Binding. If the fish cannot physically grow any bigger then it will not, but skeletal and internal deformities will often occur as a result.
Main sources of reference :
Yunker, Lee, Wong, Chang (2000) Norepinephrine Regulation of Growth Hormone Release from Goldfish Pituitary Cells. II. Intracellular Sites of Action , Journal of Neuroendocrinology, Vol. 12 (4), pp. 323–333.