I am able to find Reports about death/ die off of Sea Mammals
since the Eighties (at least)
Source:
[link to www.spiegel.de]
[link to blog.nz-online.de]
[link to www.welt.de]
[link to www.t-online.de]
[link to www.ecomare.nl]

All this is Pre-Fukushima and the main-reason why i think
it is stupid to be satisfied with the explanation:
"All is made by and because of Fukushima Daiichi"!