oof, raw fish? most fish have parasites (which is why sushi/sashimi has to be frozen or chilled to a certain temperature). no, you’re right of course but those folks wont last long enough for it to matter.
and if, just spitballing here but lets say 75% of the population succumbs to this hypothetical disaster, then the remaining population will have significantly lower impact on the environment, giving it a chance to recover. if anything, it’d be a net gain as there’s a lot of intentional dumping of toxins right now.