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

The number of large fish will increase.

Explanation:

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.

The number of big fish would increase

Explanation:

The amount of food for the big fish in the lake would increase, allowing the amount of big fish to increase until another factor (such as space or disease)  would stop that increase in population.