One reason is that overfishing of sharks and related fish has reduced the size of some shark populations.
VOA: special.2009.06.16
The British scientists had requested the pictures to learn if the shark was the same fish they had observed on their side of the Atlantic.
VOA: special.2009.06.16
Larger fish such as shark, king mackerel, swordfish, and tilefish tend to be highest in mercury because they eat smaller fish and consume the mercury in them as well.