You are right to be skeptical in the fact that anecdotal evidence from a yachting voyage could establish a general case for mass bio-mass (or lets just say a su

You are right to be skeptical in the fact that anecdotal evidence from a yachting voyage could establish a general case for mass bio-mass (or lets just say a sub-set of bio-mass, like catchable fish) decline, and your point that large portions of the Pacific are normally dead-zones is true because of the way that anoxic regions can develop due to oceanic circulations of nutrients and dissolved oxygen concentrations. But, you might be concerned to learn that the NOAA typically only reports on mass die offs fairly close to US shores, at least within our EEZ zone- because they are only adequately funded to do so- [1] is an example of such a report. There is no well-funded transnational environmental authority that can make a reliable 'alarm' about the potential of mass common-water oceanic die-offs (think 'tragedy of the commons'); Greenpeace is more of an advocacy organization than a scientific or monitoring authority. One thing we can indirectly empirically gauge is the size of these large garbage patches at sea simply via imaging. In fact NOAA has put together a fact sheet on this [2], although it is rather simplistic. What we can glean from this is the fact that they are real, quite large, and if you look at the map it looks as though the OP's voyage passed through it at least once if not twice. Which would explain much of what he saw, but not necessarily the consistency (pervasiveness) of the dead zones observed.. Finally, just food for thought (er.. perhaps bad choice of words.. haha) that there is a sort of conflation in the article about the long-going decline of fish stocks and accumulation of garbage due to dumping and the more acute poisoning/death of fish stocks due to radioactive leakage from Fukushima in the areas off-coast Japan specifically. There is academic evidence [3] that large fish, specifically tuna, are acting as vector to transport large amounts of spoiled biomass as far afield as California. How much of the biomass die-off does this explain? I have absolutely no idea, but since both issues are quite concerning in their own right we should probably keep tabs on both.. [1] http://www.dailykos.com/story/2013/07/30/1227489/-Mass-Aquatic-Die-Offs-in-FL-NOAA-Declares-2nd-Unusual-Mortality-Event-Since-April http://www.dailykos.com/story/2013/07/30/1227489/-Mass-Aquat... [2] http://marinedebris.noaa.gov/info/pdf/patch.pdf http://marinedebris.noaa.gov/info/pdf/patch.pdf [3] http://www.pnas.org/content/109/24/9483.short http://www.pnas.org/content/109/24/9483.short