Question

What does the word sustain mean in this sentence:
What level of net fishing can the model reef sustain?

Answer 1

The sentence is talking about the effects of fishing vs. the state of a reef. A reef can only support a certain level of fishing (too much fishing will destroy the fish population and negatively impact the rest of the ecosystem).

So in this sentence, sustain means something like "support" or "supply"

see definitions 1 and 2

https://www.merriam-webster.com/dictionary/sustain