Heh! Well yes there are chemical reactions involving the breakdown and decay of biological tissue, but the methods used in conventional functional MRI wouldn't be sensitive to that. The dead salmon result has everything to do with statistical correction for multiple comparisons, or lack thereof.
The brain is a big place, and fMRI is used to study all of it because its the best method available for non-invasive study of the human brain. Some sub-fields of fMRI are pretty solid, the vision stuff for example is generally high quality. Other areas in fMRI are rife with over-interpreted and poorly controlled studies. Social neuroscience is perhaps the worst.
The dead salmon study had nothing to do with fMRI per se, it had to do with correction for multiple comparisons. fMRI measures hemodynamic brain activity at thousands of separate locations. If you don't correct the statistics for multiple comparisons you will get false positives. Even in a dead salmon.
This study may be unimpressive (I haven't read it yet), but not because of dead salmon.
Apparently some may have missed the main point of the post, so let me summarize:
We burn the equivalent of 1/3 of all the earth's current biomass every single day.
How can that possibly be a good thing? And how can anyone believe that this could never possibly be a problem?