Ken Buesseler at Woods Hole Oceanographic Institution has been researching the marine life and how it is affected by the Fukushima I Nuclear Power Plant accident that has released significant amount of radioactive materials into the Pacific Ocean.
He has a page with questions that he gets from people concerned about the radiation in marine life and his answers.
From Q&A by Ken Buesseler at Woods Hole Oceanographic Institution (emphasis in the answers is mine):
What is the state of fisheries off Japan and along U.S. West Coast?
The coastal fisheries remain closed in Japan near Fukushima, where there is a concern for some species, especially the bottom dwelling ones, which are being tested and many have been found to be above the Japanese government's strict limits for cesium in seafood. These contaminated fish are not being sold internally in Japan or exported. Because of the dilution that occurs even a short distance from Fukushima, we do not have a concern about the levels of cesium and other radionuclides in fish off the West Coast of the U.S.
More about the state of Japanese fisheries (pdf).
Are fish such as tuna that might have been exposed to radiation from Fukushima safe to eat?
Seawater everywhere contains many naturally occurring radionuclides, the most common being polonium-210. As a result, fish caught in the Pacific and elsewhere already have measurable quantities of these substances. Most fish do not migrate far from home, which is why fisheries off Fukushima remain closed. But some species, such as the Pacific bluefin tuna, can swim long distances and could pick up cesium in their feeding grounds off Japan. However, cesium is a salt taken up by the flesh that will begin to flush out of an exposed fish soon after they enter waters less affected by Fukushima. By the time tuna are caught in the eastern Pacific, cesium levels in their flesh are 10-20 times lower than when they were off Fukushima. Moreover, the dose from Fukushima cesium is considered insignificant relative to the dose from naturally occurring polonium-210, which was 1000 times higher in fish samples studied, and both of these are much lower relative to other, more common sources, such as dental x-rays.
More about the dose and associated risk (pdf) of radiation from Fukushima to marine life and humans.
Is there concern about other radionuclides, such as strontium-90?
The continued release of radionuclides from groundwater and leaking tanks at Fukushima nuclear power plants site needs to be watched closely, as the character or mix of radionuclides is changing. One example is the higher levels of strontium-90 contained in groundwater and storage tanks that are leaking into the ocean. Because strontium-90 mimics calcium, it is taken up by and concentrated in bones, where it remains for long periods of time (it has a half-life of 30 years and calcium/strontium is not replaced as quickly in the body as cesium). If leaks of strontium-90 continue, this radionuclide could become a larger concern in small fish such as sardines, which are often eaten whole. So far, however, evidence suggests that levels in fish of strontium-90 remains much lower than that of cesium-137.
Is radiation exposure still a concern?
I stood on a ship two miles from the Fukushima reactors in June 2011 and as recently as May 2013, and it was safe to be there (I carry radiation detectors with me) and collect samples of all kinds (water, sediment, biota). Although radioactive isotopes in the samples and on the ship were measurable back in our lab, it was low enough to be safe to handle samples without any precautions. In fact, our biggest problem is filtering out natural radionuclides in our samples so we can measure the trace levels of cesium and other radionuclides that we know came from Fukushima.
Where does radiation from Fukushima go once it enters the ocean?
The spread of cesium once it enters the ocean can be understood by the analogy of mixing cream into coffee. At first, they are separate and distinguishable, but just as we start to stir the cream forms long, narrow filaments or streaks in the water. The streaks became longer and narrower as they moved off shore, where diffusive processes began to homogenize and dilute the radionuclides. In the ocean, diffusion is helped along by ocean eddies, squirts, and jets that broaden, mix, and continue to dilute the cesium as it travels across the ocean. With distance and time, radionuclide concentrations become much lower in the ocean, something that our measurements confirm.
More information about our oceanographic studies off Fukushima (pdf).
Are the continued sources of radiation from the nuclear power plants of concern?
The site of the Fukushima Dai-ichi nuclear power plant is an ongoing source of radionuclides (pdf) in to the ocean—something I've seen evidence of in my data and published about since 2011. Although the numbers sound large (300,000 gallons of water leaked or 20 trillion bequerels [sic] per liter), we calculated in 2011 when radiation levels were much higher than today that the dose to someone on a ship or in the ocean was not of concern. For the workers at the site, direct exposure from leaking storage tanks is of greater health concern because exposure from these concentrated sources is much higher. For the general public, it is not our direct exposure, but uptake by the food web and, hence, the potential for human consumption of contaminated fish that is the main health concern.
Will radiation be of concern along U.S. and Canadian coasts?
Levels of any Fukushima contaminants in the ocean will be many thousands of times lower after they mix across the Pacific and arrive on the West Coast of North America some time in late 2013 or 2014. This is not to say that we should not be concerned about additional sources of radioactivity in the ocean above the natural sources, but at the levels expected even short distances from Japan, the Pacific will be safe for boating, swimming, etc.
Is debris washing ashore on the US/Canadian West Coast of concern?
Debris washed out to sea by the tsunami does not carry Fukushima radioactive contamination—I’ve measured several samples in my lab. It does, however, carry invasive species, which will be of serious concern to coastal ecosystems on the West Coast.
Have there been increased deaths as a result of radiation from Fukushima?
Reports of increased deaths are simply not true. Read this reasoned response in Scientific American to the most often-cited "scientific" paper about erroneously linking deaths to radiation from Fukushima. That article ends “This is not to say that the radiation from Fukushima is not dangerous (it is), nor that we shouldn’t closely monitor its potential to spread (we should).” I agree with that statement.
Where can people go for reliable information?
Here are some other links I have passed to others.
Fukushima's Radioactive Water Leak: What You Should Know
Latest Radioactive Leak at Fukushima: How Is It Different?
See also following article from the Woods Hole Oceanographic Institution (w/ links to many others)
From the special issue of Oceanus Magazine devoted to the cause and impacts of Fukushima:
Consider supporting our new Center for Marine and Environmental Radioactivity and check out CMER public education links, such as ABCs of radioactivity
(I wish he didn't mention the dental x-rays...)