Adding fry to tank with mature fish

[Michael Welber]

For various reasons, some of which I'm unsure of, I have ended up with 15 tilapia that are about 4 inches long. I started with 50. The others have died. I think I may have stabilized the system though the next few days will tell the tale. My question is that I would like to add 15 or so fry to the tank so I can get the population back up to where I want it to be. It's a 300 gallon tank. Can I do this? Do I have to segregate the fry (though I'm not sure how I would to that)? And, how would I do that?

[TCLynx]

How much grow bed do you have and what kind?  (Just because the tank is 300 gallons that doesn't tell us if you have enough filtration to support 50 fish.)

[Michael Welber]

Right. The ratio is correct based upon my initial calculations (from the numbers in Sylvia's book).I have three IBC beds, flood and drain with a sump that pumps the water back to the fish tank. 

[TCLynx]

Well in that case, it depends on how big the big ones are and how small the new ones will be.  You might get away with just putting them in together but you could have some losses.  Or you might put one group or the other into a floating "cage" in the tank to keep them separate but that can sometimes make feeding tricky.

[Michael Welber]

The "big" ones right now are about 4 inches long. I wondered if I segregated them they would actually never catch up so the size difference would always be a problem. Yes? What happens when one of the tilapia spawns fry. Do they survive in the tank with the more mature fish? 

[TCLynx]

If you are wanting to breed lots and do a hatchery operation, then you will need to have lots of separate tanks to keep the sizes separated.

If you have a big mixed population and you let them have access to the bottom of the tank, some will breed and some fry will survive but the older fingerlings and fry tend to eat the smaller ones more than the adults do.  And if you let your population boom out of control you may wind up with a stunted fish population with lots of kinda small fish.

That's the great thing about tilapia, they breed, it is also the problem with tilapia.

[Michael Welber]

No breeding or hatcheries. I do want to have a sustainable situation in which, if I can ever bring these fish to maturity (!), I won't have to keep buying fish but they will breed and I'll have a constant supply of fish and fertilizer. If I get too many (certainly not a problem right now) I'll just remove the excess. There's a wild bird rescue center near here that would love to have live fish. 

Well, the bottom line is, based upon what you are saying, that if no more die then I will buy more fry and take the losses in stride and hope that some of them survive to grow to maturity and reproduce. 

[TCLynx]

Putting a cage in your tank so that the fish can't access the bottom of the tank is a handy way to control breeding since the females need to pick the eggs back up to brood them.  Then have another tank later to keep your breeders in to let them breed when you need more stock.

[Chip Pilkington]

Hi Michael,

There are a couple of ways to build an easy "floating cage" to house the little guys if you want to take that path. The simplest design I've used came from a plastic laundry basket with a fine enough mesh that the fry couldn't escape. You'll also need a cover as they will jump out when spooked. I used large foam fishing floats to stabilize the basket in the tank - you can secure it or let it float freely in the tank. Baskets in Asia come in hundreds of shapes and sizes (they love plastic here). You might be limited in the States a bit as to the mesh options. This is no problem as you can use the basket as a frame and line it with net material. I would go as coarse as you can while not allowing escape. I've learned from commercial breeders here that fine net can greatly restrict water flow and DO. Another great method is to build a simple frame from 1/2" PVC and sew a "box" from net material to size (the netting is very easy to sew by hand or preferably by machine).

As to the Tilapia breeding freely, they will and many fry will become food, though some with good instincts will stay out of the way until they get a little bigger. You can assist this by adding some hiding places for the little guys. I use short, 4" to 8" pieces of PVC in various diameters. Some people put tree branches and all sorts of more natural cover...the problem is that it becomes really difficult to net fish with all of the obstacles. PVC pieces work pretty well.

You might think about breeding intentially at some point if you've got a use for the fish - it's easy and fun. If you provide Tilapia a few basics, they will take care of the rest. You don't need a glass aquarium or anything special (though it is fun to watch the process and the fry develop).

Hope that helps. 

 

[Michael Welber]

Thanks Chip. It surprises me that tilapia eat the fry since they are vegetarians. I avoided bluegill because I knew that would be a problem with them. If I raise them in a cage such as you describe, when are they ready to join the larger fish? They will always be smaller. 

[Chip Pilkington]

That's a good question....the rule of thumb is if a fish can get it in it's mouth, it can be eaten, That said, I've had 1" fish in with 6" fish with no problems, well, let me qualify that...no problems other than the little guys seem to miss out on meals. You will also have issues as to pellet size. The little guys can't eat standard size food. I chop it up in an old coffee grinder, starting very fine when they're small and then going coarser as they grow up. You just need to figure out a way to get the food to the smaller fish. Multiple tanks or segregating a larger tank makes sense. It's always amazed me how much a batch of Tilapia will vary in size over the course of a few months. I try to group them by size, with the faster growing fish graduating to the next big-fish tank. It's kind of rediculous - I can never seem to have enough tanks.

Also, like you, I always read or been told that Tilapia are mostly vegetarians, however, I think 99.9% of fish are opportunists. Eggs and small fry are always fair game. My Koi will even eat new born guppies, and they all love brine shimp and mosquito larvae. Most of the mature Tilapia (2 to 3 lbs) I catch in lakes and ponds here in Thailand are caught on live freshwater shrimp or worms.

Once again, I believe cover for the smaller fish is important, as is finding a way to insure the little guys can eat without the big fish driving them away.

[TCLynx]

tilapia are definitely not vegetarians, they are omnivores.  The fact that adult tilapia can filter feed on algae just gives them an additional food source, it doesn't mean they won't eat higher protein sources when it is available.  The juvenile tilapia that don't have the filter feeding ability yet tend to be very hungry and they have a protein hunger that beats out many more carnivorous fish when they are young (higher protein feed is actually recommended for tilapia than for channel catfish) and very young tilapia probably do even better on really high protein flake, powder, or crumble.

And though bluegill have been known to eat the little ones, I know people who have had bluegill breed for them just fine in pools and ponds.  And bluegill keep eating in much cooler water than tilapia.