BOCA RATON, Fla. – The waters off the South Florida coast have become more crowded as the annual blacktip shark migration has begun, but with lesser numbers than previous years.

Blacktip sharks are known as the "snowbirds of the seas" for their yearly trek south during the cold weather months. The sharks travel as far south as northern Miami-Dade County during their journey.

In the past, more than 15,000 sharks have been spotted on any given day off South Florida shores. However, scientists are worried about the lower numbers this season possibly due to the water temperature.

Blacktip sharks help coral reefs and sea grasses by "cleaning out" weak fish species.

"Last year, we saw a dramatic decline in the number of blacktip sharks that migrated south. In fact, it was so low that we estimated the population to be about one-third of what we have seen in previous years,” said Florida Atlantic University professor Stephen Kajiura, Ph.D. "We want to make sure that these snowbirds come back to South Florida, because if they don’t, it will have a huge ecological impact in this region."

When he started his work, Kajiura saw up to 12,000 sharks in just one fly over and said he has counted somewhere around 100,000 total.

"We're not sure what the cascading effects might be," he said. "Someone still needs to look at that, but it is troubling that there used to be so many sharks, and now there are so few."