1 + 2 are opinions and 3, 4 are written as facts.
- is an opinion because of the use of the word most. The idea of most is subjective. Other people may very well have a different opinion on what is the most significant threat.
Is an opinion because the use of the phase little ground for optimism. How is the level of optimism measured. Other people might think that alternations to fishing practices are a major ground for optimism.
Is written as a fact 77% is a number that can be measured. The criteria for what constitutes over exploited can be debated and clearly needs to be defined. Despite the problems with the statement it is written as a fact.
Is written as fact. The number of fish being caught can be measured. That the number of fish being caught can be measured and compared. Others may dispute the facts in the statement but it is written as fact