For example, I'd like to get confirmation of the problem from NOAA[1], particularly the NMFS[2]. Alternatively, Green Peace[3] would no doubt be one of the first organizations to raise an alarm - they care about the ocean, and presumably gather signals from many sources. I want confirmation because the simple fact is that it's impossible to get a comprehensive picture of the Pacific's health from one voyage - there are too many variables at play.
The skeptic in me questions the tacit assumption (the one that makes the OP's observations so troubling) that the entire Pacific is normally teaming with fish. However, I assume that fish travel in schools, and that fish density is wildly variable. E.g. there are normally vast swaths of the Pacific that are "dead". Indeed, anyone going out to fish for sport knows that fish aren't uniformly distributed, and they don't stay in one place. In the open ocean, these schools are vast, but so are the dead zones when there are no schools.
That said, not being in the field, I'm not sure how we gauge the health of an entire ocean from a "supported biomass" perspective. Nor do I know anything about who would be responsible for officially raising an alarm about global catastrophe. Or if they did, what we could do about it. It seems like the only thing we really could do is reduce the rate at which we harm the ocean, rather than dumping into it.
[1] https://en.wikipedia.org/wiki/National_Oceanic_and_Atmospher..., http://www.noaa.gov/
[2] https://en.wikipedia.org/wiki/National_Marine_Fisheries_Serv..., http://www.nmfs.noaa.gov/