There was a "Lost" episode where a shark was shown that had the numbers on its fin. What was that all about? What did the numbers have to do with a shark? It almost looked like a part number on a car or something...
Officially, no specific reason for the sharks has ever been revealed. Since the Swan Orientation video lists zoology as one of the research focuses of Dharma, it is assumed that the sharks were part of one of those studies and had been tagged for identification.
At one moment, a glimpse of the shark's tail underwater was seen. The Shark had an unknown DHARMA logo on its tail, indicating a connection to the DHARMA initiative. In the Swan Orientation Film, one of the topics listed as being studied through the DHARMA Initiative is zoology, which may account for the shark. It should be noted that this logo is very likely the normal DHARMA logo.