Experiment: Growth Rates of Fish

Last updated: 2012-01-13 — Created: 2012-01-13

A comment posted yesterday reminded me of an experiment a friend of mine did once that I’ve been meaning to repeat. I will have to procure a proper apparatus, but I at least want to share the concept to get the ball rolling.

The hypothesis is that water changes are much more important to rapid fish growth than frequent feeding. I will have to cite this, but it’s said that fish excrete hormones that build up in the water and stunt growth, which is a natural defence for keeping fish small in crowded spaces. When fish have more open swimming spaces, they tend to grow faster. Have you noticed how fish at a store may appear to stay the same size for a long time, but really start to grow once you get them home into your own tanks?

My friend’s experiment involved just two tanks:

  1. heavy feeding with light water changes, and
  2. light feeding with heavy water changes.

I would propose doing the experiment and include a control group as well:

  1. heavy feeding with normal water changes
  2. normal feeding with normal water changes
  3. normal feeding with heavy water changes

Temperature can affect metabolic rate. Similar to how pond fish or frogs hibernate through the winter with their metabolic rates slowing to almost nil, organisms in warmer environments will have accelerated biological processes and life cycles – disease will be fought and food processed faster, parasites will die off more quickly and fish will become more aggressive. For this reason, identical heaters should be set to the exact same temperature, verified with (perhaps multiple) accurate thermometers.

The tanks should be exactly the same: the same volume with the same filtration rate and method, same lid and lighting (probably none), etc. Filters should be stocked with new, yet fully-cycled media and all rinsed out on the same schedule. Tanks should be located next to each other to minimise any differences due to external light sources, draught, evaporation, ambient temperature, etc. Visual barriers will be kept between tanks to ensure no macho stuff happens between tanks 1 & 2 and 2 & 3.  Fish stocking should be identical, ideally all from the same spawn, grown out for a small time, then equally divided amongst the tanks. Basically, anything and everything should be done to ensure that the only two variables are: a) the amount of food added, and b) the volume of water changed.

Food types and amounts should be consistent, so some measure of a “dose” of food would need to be established. This “dose”, as well as its frequency and the frequency and volume of water changes would have to be chosen such that “heavy feedings” do not lead to wasted food and/or toxic water parameters, while “heavy water changes” don’t lead to shocked fish. Nitrate measurements could be taken periodically to ensure these choices are reasonable.

I would propose the following definitions as a starting point:

  • heavy feeding = “a dose of food, twice daily”
  • normal feeding = “a dose of food, every other day”
  • heavy water changes = “50% of water volume, twice weekly”
  • normal water changes = “25% of water volume, bi-weekly”

This means that tank #1 gets 4 times more food and tank #3 gets 8 times more water changed than tank #2. Observation between the three tanks would show how these two factors can affect fish growth over time. It is predicted that fish in tank #1 will show only a moderate increase in growth over those in tank #2, where fish in tank #3 will show a significant one.

Updates to follow once I decide to perform the experiment.

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