The decline can be very rapid and dramatic, fishermen have to work harder and harder to maintain their catch levels (my hometown was one of the ones that suffered with the collapse of the northern cod fishery):
https://en.wikipedia.org/wiki/Collapse_of_the_Atlantic_north...
In the 1990s the Northern Cod populations collapsed to less than 1% and after a 20+ year moratorium on fishing has only increased to 10% of its historical stock.
Also striking is the lack of photos - I suspect it wasn't literally "like sailing through a garbage tip" (as opposed to occasional debris, much more than usual).