Quantification of Massive Seasonal Aggregations of Blacktip Sharks (Carcharhinus limbatus) in Southeast Florida

Southeast Florida witnesses an enormous seasonal influx of upper trophic level marine
predators each year as massive aggregations of migrating blacktip sharks (Carcharhinus
limbatus) overwinter in nearshore waters. The narrow shelf and close proximity of the Gulf
Stream current to the Palm Beach County shoreline drive tens of thousands of sharks to the
shallow, coastal environment. This natural bottleneck provides a unique opportunity to estimate
relative abundance. Over a four year period from 2011–2014, an aerial survey was
flown approximately biweekly along the length of Palm Beach County. A high definition
video camera and digital still camera mounted out of the airplane window provided a continuous
record of the belt transect which extended 200 m seaward from the shoreline between
Boca Raton Inlet and Jupiter Inlet. The number of sharks within the survey transect was
directly counted from the video. Shark abundance peaked in the winter (January-March)
with a maximum in 2011 of 12,128 individuals counted within the 75.6 km-2 belt transect.
This resulted in a maximum density of 803.2 sharks km-2. By the late spring (April-May),
shark abundance had sharply declined to 1.1% of its peak, where it remained until spiking
again in January of the following year. Shark abundance was inversely correlated with
water temperature and large numbers of sharks were found only when water temperatures
were less than 25°C. Shark abundance was also correlated with day of the year but not with
barometric pressure. Although shark abundance was not correlated with photoperiod, the
departure of the sharks from southeast Florida occurred around the vernal equinox. The
shark migration along the United States eastern seaboard corresponds spatially and temporally
with the spawning aggregations of various baitfish species. These baseline abundance
data can be compared to future studies to determine if shark population size is changing
and if sharks are restricting their southward migration as global water temperatures
increase.