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

Abstract

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.

Philopatry and Regional Connectivity of the Great Hammerhead Shark, Sphyrna mokarran in the U.S. and Bahamas

“…For aquatic ectotherms, seasonal rises in water temperature can act as an important cue in the timing of migration (e.g., salmon [Salmo salar] smolts Jonsson and Ruud-Hansen, 1985;squid [Loliginid forbesi] Sims et al, 2001). All three great hammerheads that exhibited long-distance migrations departed Bimini April to early May when the water temperature began to rise above 26 • C. In the same region, water temperature was found to be a key predictor for the presence and absence of lemon and blacktip shark seasonal aggregations in Florida (Kessel et al, 2014a;Reyier et al, 2014;Kajiura and Tellman, 2016). Future studies on great hammerhead sharks should explore the timing, repeatability, and motivations for such long-distance movements, particularly as their travel routes cross various management and political boundaries.…”

“…Dasyatid stingrays dominated the diet of great hammerheads captured in South Africa, Australia as well as in Florida, U.S. (Dodrill, 1977;Stevens and Lyle, 1989;Cliff, 1995). Similarly, in the Jupiter region spawning bait fish, as well as, migration of blacktip sharks, Carcharhinus limbatus and abundance of other small sharks and batoids, likely provide important feeding grounds for great hammerheads (Hueter and Tyminski, 2007;Wiley and Simpfendorfer, 2007;Kajiura and Tellman, 2016).…”

“…Preliminary analyses indicated that like other coastal species (Kessel et al, 2014a;Kajiura and Tellman, 2016; e.g., lemon and blacktip sharks) water temperature was a major driver of seasonal occurrence, however, future research should include other abiotic and biotic factors to better understand the causal relationship between environmental factors responsible for migratory behavior in this species (Schlaff et al, 2014). Like other coastal shark species (e.g., bull sharks, Espinoza et al, 2016), FIGURE 6 | Regional movements and migrations of great hammerhead sharks, Sphyrna mokarran tracked using satellite telemetry and photo ID (insert), main map acoustic telemetry and tracks of great hammerhead # 12 (black dashed line) and great hammerhead # 13 (green dashed line) and multiple individuals (blue dashed line).…”

“…Great hammerheads are thought to be predators of blacktip sharks and have been documented via aerial surveys along the Florida coastline among mass aggregations of this species, suggesting this could be an important seasonal food source (Kajiura and Tellman, 2016). Interestingly, great hammerhead # 15, with its tag popping off in a similar location to the individual tagged in Hammerschlag et al (2011), showed a distinct narrow temperature use (22.54 ± 0.34 • C) after departing Bimini matching the water temperatures of the Gulf Stream.…”

Using unmanned aerial vehicle (UAV) surveys and image analysis in the study of large surface‐associated marine species: a case study on reef sharksCarcharhinus melanopterusshoaling behaviour

“…This is particularly true for elasmobranchs and large marine mammals for which measuring variation in densities over various spatial and temporal scales is necessary to gather the information necessary for developing sound conservation and management plans. Over the years, aerial surveys have been extensively used in studies of marine wildlife, particularly to assess their distribution and abundance (Bayliss, ; Hodgson et al , ; Kajiura & Tellman, ; Marsh & Sinclair, ; Pollock et al , ; Rowat et al , ). Recent studies have demonstrated the potential of using unmanned aerial vehicles (UAV or drones) equipped with a high‐resolution video camera to provide density estimates (or at least encounter rates) of marine fauna with no or limited disturbance (Hodgson et al , , ; Kiszka et al , ).…”