A food web is shown above. What would most likely happen in this ecosystem if the shark population significantly increased?

The food web is not shown. However, a significant increase in the shark population would likely lead to a decrease in populations of the shark's prey species. That decrease could lead to increases in other populations, which could lead to decreases in others. But without actually seeing the food web that you forgot to post, I can't give you specifics.

The fish would decrease because both dolphin and Shark eat fish.