Tropical Fish – Growth, Age, and Size

In common with most living things, fishes grow most rapidly when they are young and gradually slacken off as they become older. Al-though there is an approximate maximum or fully grown size in most species, this maximum is really only the top of the growth curve, which does not flatten out entirely under favorable conditions, with plenty of food, oxygen, and room.

A starved or semi-starved growing mammal is likely to die and will certainly be stunted in growth. However, if it survives at all, it will not be much smaller than normal, although it will be thinner and wretched looking. A semi-starved fish may die, but if it lives it will grow very little or even not at all, and it may reach breeding age at a tenth of the normal weight. There are often tremendous differences among members of the same batch of fry growing under apparently identical conditions in the same tank. Some become so much larger than others that they can eat them, and so get larger still. This difference is usually seen when the baby fishes are underfed, so that the

lucky ones happen to swallow most of the available food in the early stages or are hatched before the others and are then progressively better ter able to grab whatever comes, at the expense of their smaller breth-
ren. When there is plenty of suitable food, much smaller differences are normally seen, and an even batch may frequently result in wildtype fishes such as barbs or characins. Fancy fishes, such as some of the goldfishes, usually throw a fair proportion of runts even under good conditions, which would indicate a genetic rather than a nutritional cause.

Thus fishes of the same species may differ enormously in size at the same age, depending on the food and possibly the room available to them in their earlier stages. The exact extent to which swimming space really matters has not been determined, but it is suspected that, as long as adequate food and air are available, swimming space makes little difference and the apparent effects of crowding are usually due to insufficient food and air.

Starved fishes may catch up to the normal if given the chance, and it is not clear whether they can be permanently stunted within, say, a year or so of hatching. If kept on short rations for a longer period, the
average aquarium fish is said to be permanently affected, but there seem to be no records substantiating this claim. The usual effect of aquarium life in itself is to stunt in comparison with the size found in the wild, with one or two very curious exceptions, such as Copeina guttata, which grows to 4 inches in the aquarium but has never been caught in Nature longer than 3 inches. It has been found, however, that in very large tanks with everything as nearly perfect as possible, including much live food, fishes grow as large as in the wild. The
limiting factor is probably oxygen and carbon dioxide exchange and adequate food, as noted above.