I'd wager much of the "where they went" can be attributed to the Chinese fishing fleet. Imagine you're a whale or fish swimming about in International waters. Then an armada of 300+ fishing vessels park above and take tonnes of things that swim or crawl around you. When they leave, some species will do well because the things that eat them are sparse, while other species suffer because the things they feed on are few.

​ https://www.youtube.com/watch?v=C9t5MC7WXNg

>China harvests 20% of the world's entire annual catch (15.2 million tonnes). China reports it's distant ocean fishing vessels numbers 2500 but the real estimate is 17,000. In comparison the US has around 300 vessels.

2020 video/data. Chances are that the Chinese fleet has grown since then.