Bob,
If it is of any help, where I live tilapia live and breed year round. Tilapia are difficult to catch with the only real preditors being catfish and man and they are the cheapest fish to buy at the hatcheries so they wind up in every waterhole. But, like everything else they are part of the food chain and as such are kept in check by bugs, birds and big fish.
When the ponds around here start drying up during the hot season the locals clean out almost all of the fish with cast nets and their catches are always a mix.

1/4 & 3/4 acre ponds. A thousand miles from no where and there is no place I want to be...
Dwight Yoakam