Simplify the following text for a Grade 10 native English-speaking person.
Original: For most of the 20th century , Fleetwood was a prominent deep-sea fishing port , but , since the 1970s , the fishing industry has declined precipitously and the town has undergone economic difficulties .
Simplified: In the twentieth century , Fleetwood was a major deep-sea fishing port , but , since the 1970s , the fishing industry has declined .