Difference between a shark and a tuna or a tilapia? Well, sharks don't reproduce as quickly. Still, the tiger shark population seems to be ok. Most fish are placed into a creel or thrown on ice to suffocate slowly to death. I would say this shark died relatively quickly in comparison with the norm for fish. Most would consider it impractical to kill every fish netted in commercial fishing operations...I mean, how long would that actually take? They would suffocate before you managed to kill them all.
As for mercury, people are welcome to take their own risks.