Goldfish Growth and Reproduction

With thanks to David Padfield, the following article published previously by the NGPS, and the BAS has been added to the AMGK website for reference to our members.  The following notes were taken during the lecture by Dr Peter Miller and are reproduced as food for thought and a basis for further discussion.

Growth: is an increase in weight correlated with a corresponding increase in length.

Food: is the supply of basic chemical materials into the fish where digestion takes place and up to 80 or 90% of the food is absorbed into the blood stream as simple chemical substances.

Functions of the fish are expressed in four different uses of the chemical substances (food):

(1) Metabolism (2) Food storage (3) Reproduction (4) Growth

Metabolism is the primary priority to draw on the food supply and includes the burning up of energy used for the heartbeat, gill movement and swimming.

Food Storage is where the fish has the facility to put by energy, which is stored in the body as fat and can be drawn upon under adverse feeding conditions or more often during cold weather when feeding ceases altogether.

Reproduction – in Spring, the fish uses food to form the reproductive gonads and weight of the female fish may include as much as 25% of the total weight in the ovaries. Some food materials are also used up in repairing the body tissues. The main areas of repair are the relining of the gut or intestine which takes place every few days, and the replacement of the slime or mucous covering of the skin, which acts as a lubricant in swimming and as a protection against disease. As this protective covering is lost during normal activity, it is replaced.

Growth – it is only when the three priorities above have been satisfied that food is used for growth purposes. There are two controls on the growth of the fish: (1) Genetic (2) Environmental

Genetic control on growth: every species has a maximum size within which that species will grow due to its genetical makeup. Very small genetical differences in growth rates within this maximum will increase disproportionately due to the behavioural patterns and the establishment of a pecking order. In other words, a very small genetical difference may cause some fish in a spawning to grow very slightly faster than others kept in the same tank under the same conditions. But very quickly these fish, which are only very slightly larger at the outset, will become dominant and take precedents in the ‘pecking order’ of that group of fish.

They will gain more of the food supply and dominate the other fishes so that what was at first only a small difference in size becomes greater and greater and out of all proportion to the original genetical difference. Genetical differences between fish of the same species can now be determined by an examination and comparison of the protein tissues of the respective fishes. However, for practical purposes it seems that we should continue with our selective breeding programmes and test mating.

It is the genetical makeup which determines which type of food is best suited for a particular species.

Environmental controls on growth: A rise in temperature speeds everything up and increases growth rate but we need to be very careful in changing the environment – don’t raise the temperature too much, too quickly; the fish may die. In general, the faster grown fish will make smaller adults. Experimental work carried out by Dr Miller (but not with Goldfish) showed that the

growth rate of young fish raised at a higher temperature levelled out to a maximum size which was much less than that attained by similar fish raised under cooler conditions, although the growing period of the latter was of course much longer. This suggested that the slower the fish is grown the larger it will eventually grow. Slower growing fish use food more efficiently with a greater percentage of food producing growth, faster growing fish in higher temperatures are more active and use up more of their food in respiration and swimming, even though they do rear faster up to the point where they stop growing.

The faster the fish grows the sooner it dies. Other environmental factors for growth include the length of daylight hours, which keep fish active and feeding, and of course water quality.

In addition to the more well-known water requirements there is a slightly better growth rate in slightly brackish water. The fish in freshwater must pump out from its body under osmosis process whereas there is less pumping (and less energy used) in brackish water. In the case of adult fish, the formation of reproductive organs will have a prior claim on the food supply above growth – we should prevent our fish becoming sexually mature. Fish kept in a short-day length make better growth than those under a long day length because those ‘in the dark’ do not become sexually mature and therefore go on growing whilst the others are using their food to produce gonads, other factors being equal.

When reproductive activities are at their highest the growth rate will be at its lowest.

For our own practical purposes there are many considerations, but it is reassuring to know that it is not necessary to rear our fish under tropical conditions in order to achieve satisfactory results.

David Padfield