The general "fish deaths" number has never been calculated.
To calculate the number it would be necessary to determine:
- Number of fish of the species alive at any given time.
- Total mortality rate from all causes.
- Percentage attributable to pollution.
This is far more than a life's work. Additionally, the data would be changing as fast as you could collect it. And this is precisely the problem with assessment of the pollution of our oceans. Pollution is massive (consider the North Pacific Gyre), and there isn't a good way to "get a handle on it" in an attempt to affect changes. The problem is just "out there" and bringing it home takes massive effort.
Fish deaths and that of other sea creatures are being magnified by the current oil spill catastrophe in the Gulf of Mexico. No telling how much time will be needed for the waters to become clean enough to sustain and reproduce marine life.