If someone adds thousands of small fish to a lake how would the number or big fish change?

The number of large fish will increase.

Explanation:

If in a particular lake, thousands of small fishes are added, then the number of the larger fishes will increase. The reason for this is because in general the larger fishes are feeding on the smaller fishes, thus with the introduction of the thousands of small fishes there will be an increase in their food source. Also, the smaller fishes have much quicker reproduction, so the chances are that they will reproduce quickly enough so that they increase in numbers, thus providing constant food source that will enable the support of larger populations of the larger fishes.