Monday, September 5, 2011

Balancing an Aquatic Ecosystem - Animals

Lately I have been planning out how to balance the aquatic ecosystem in my future aquaculture greenhouse. It is a tricky concept to try to produce a significant amount of food out of a tank that is somewhere between 1100 and 2500 gallons and still have a self-regulating system. The important part is to set up a stable food web.

The thing to remember about food webs is that the base, your plants (in this case phytoplankton and duckweed), is limited by the input of nutrients and the size of the system. It can only get so big. From there on up the food web, each successive level gets smaller . If you have too many levels and you eat from the top, you don’t get much food, so the ideal food fish is capable of filter feeding on the phytoplankton. It just so happens that tilapia are just such a fish. However, I never can leave well enough alone, so I don’t want a simple system. I want an elegant, interesting system.

Currently I am considering a combination of three larger species in the tank: mosquitofish, crawdads (or as some people call them, crayfish), and tilapia. I am thinking that the three will probably work pretty well together in there.

The tilapia will obviously be the star of the show. Blue tilapia are omnivores and eat a mixture of plant and animal material. They will primarily feed on the algae and duckweed or other plant material I drop in there from the garden, but are also known to occasionally eat small fish (like mosquitofish), small invertebrates (perhaps juvenile crawdads), mosquito larvae, and other zooplankton. Since the whole system is designed to support these fish, I obviously don’t want to seriously endanger them with predators. However, considering how prolific tilapia are as breeders, many commercial producers introduce predators to keep the population at a manageable level. Crawdads are known to occasionally take small fish and often eat eggs. However, the tilapia brood their eggs in their mouths for protection, so I am not too worried about the eggs. I think that a few fry lost to crawdads wouldn’t be a problem.

Crawdads will be the cleanup crew. They mostly eat rotting vegetation and animal matter. So if a fish dies, the crawdads will probably eat it, meaning I won’t have to clean it up. I still need to do a little more research on that one to make sure it is the case. Plus, crawdads are edible, so they make another, smaller food source. Many crawdads climb up on banks to reproduce and I don’t have banks, only walls. That means they not reproduce, which may be a problem. If it is the case, I have an ample supply of invasive crawdads in the lakes and streams in my area. I’ll just need a trap. The biggest problem with crawdads, in my mind, is getting rid of them if I find out they are harmful to the ecosystem I have set up. Even if I drain the whole pond and leave it dry for a few weeks and start over, it may not completely get rid of them. The pond isn’t designed to drain completely and the plants above will be constantly leaking water into the pond area as I water them.

The mosquitofish are relatives of guppies, only much, much tougher. They do very well surviving high water temperatures, low oxygen content, and poor water quality. They are also prolific breeders and will probably fill the pond pretty quickly. Their diet consists mainly of insects, insect larvae (like mosquito larvae), and zooplankton. There is a bit of overlap between the diets of the tilapia and the mosquitofish, so I am a bit worried that there will be more competition for food going on there than I want. However, the baby mosquitofish, which are born live, will serve as a food source for the tilapias, so it probably won’t be a problem. I am not sure if the tilapias will eat the adult mosquitofish or not. The crawdads almost certainly will, though, and that means that a lot of the little fish that the crawdads take from the water would actually be mosquitofish rather than baby tilapia, which will help give a little school protection to the baby tilapia. Ultimately, the mosquitofish would serve more as another food source for the tilapia than mosquito protection, though that benefit would be good as well.

The mosquitofish and crawdads are probably pretty unnecessary to the operation of the greenhouse, overall. The only necessary function they perform is as cleanup crew of dead fish. That and an opportunity to allow me to watch and tune a functioning aquatic ecosystem right inside my greenhouse.


  1. Wow! That sounds like it will be awesome when you get everything working.

  2. Those wild crayfish might bring parasites in with them.

  3. Thanks, David. That is something I will need to look into.

  4. In my experience crayfish is pretty hard to breed successfully in closed ecosystems. They can be very choosy breeders, the also prey on their own young and are territorial, so I wouldn't worry too much about them being difficult to get rid off should you chose not to keep them anymore. They also require nests and some breeders cut lengths of plastic tubes and stack them to make sure the crayfish doesn't fight for space.

    If your ecosystem is functioning properly and you keep it disease free (a must) I don't think you'll be seeing a lot dead fish either.

    I really hope you try this complicated system out and give us the full monty: photos and detailed reports!