There wasn't even enough radiation washed into the ocean off the coast of Japan to cause major fish kills right after the incident; after 5 years of dilution across the entire ocean, it's unlikely there is anything more than a background level of radiation in those fish.
The fish could have been killed by some pollutant. Or they could have been killed by an algae bloom. Or some microorganism might be causing a fatal disease. It is impossible to know without testing both the water and the fish.
For the past 5 years, tons of radioactive waste have been washing into the Pacific daily. The radioactive cores of 3 nuclear powerplants melted through their containment (and some fuel was simply blown out there during the explosions). The radioactive cores are being “cooled” with water but water has to go somewhere. No one is sure exactly where the cores are except they are not contained but are “somewhere” under the nuclear power plant structures and that wells in the vicinity have spiked upwards with high amounts of radioactive waste.
Now, for the rest of my life, and the lives of others, the groundwater under Fukushima will be washing nuclear fuel into the pacific. The concentration is increasing on a daily basis. No one can stop it. That’s why insurance companies won’t insure nuclear power plants and instead governments do - when it gets out of control, there’s no cleanup possible. And then, because governments have no intention of spending a dime on damages, there are false assurances that all is well, situation normal etc.
It’s not logical to think that the “whole ocean” is diluting the radiation - the material is heavy and is carried on currents and trapped in the bodies of marine life so it’s not evenly distributed but becomes concentrated in various areas.
It takes awhile for it to become concentrated in the food web. The first level might be seaweed which is eaten by urchins (which concentrate the radioactive waste) which are eaten by other animals - each level of the food web, the waste is more concentrated.
There have been reports of animal die offs that defy historical precedence all along the west coast of the US.
What was thought to be known about currents has been updated by the detection of radiation moving at rates and routes not previously anticipated.
I don’t know if the Vietnam problem is related - but I do think it’s likely. It has taken time for currents to carry materials, food webs to concentrate it, and ground water to continue to pour toxins into the ocean.
Some of the illnesses that can cause die offs (algal blooms or viral diseases) may be caused by radioactive wastes. Living organisms exposed to radiation can experience suppressed immune systems and therefore die of diseases they would otherwise resist and recover from.