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

Kajiura SM, Tellman SL - PLoS ONE (2016)

Bottom Line: This resulted in a maximum density of 803.2 sharks km(-2).Shark abundance was inversely correlated with water temperature and large numbers of sharks were found only when water temperatures were less than 25 °C.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Florida Atlantic University, Boca Raton, Florida, United States of America.

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.

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Seasonal shark abundance and water temperature.The number of sharks (black circles) correlates inversely with water temperature (red lines). Shark abundance peaks from January-March which corresponds with the lowest water temperatures.
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pone.0150911.g004: Seasonal shark abundance and water temperature.The number of sharks (black circles) correlates inversely with water temperature (red lines). Shark abundance peaks from January-March which corresponds with the lowest water temperatures.

Mentions: The number of sharks counted per survey varied with season (Fig 4). Shark abundance was greatest in the winter months (January-March) with a peak winter seasonal abundance within the belt transect of 9925.0 individuals averaged over all years. Shark abundance declined precipitously in the spring and very few sharks were seen in the surveys during the summer and fall months (May-December). Summer and fall shark abundance averaged 111.7 individuals over all years, approximately 1.1% of the winter peak. Mean shark abundance differed among quarters (Kruskal-Wallis, χ2 = 24.640, df = 3, p<0.0001). Post-hoc analysis revealed that mean shark abundance for the first quarter (January-March) was significantly greater than for the other three quarters (April-June, χ2 = 10.447, p = 0.001; July-September, χ2 = 13.831, p<0.0001; October-December, χ2 = 11.772, p = 0.001). Shark abundance in the second quarter (April-June) was also significantly greater than in the fourth quarter (October-December, χ2 = 5.010, p = 0.025).


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

Kajiura SM, Tellman SL - PLoS ONE (2016)

Seasonal shark abundance and water temperature.The number of sharks (black circles) correlates inversely with water temperature (red lines). Shark abundance peaks from January-March which corresponds with the lowest water temperatures.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4814085&req=5

pone.0150911.g004: Seasonal shark abundance and water temperature.The number of sharks (black circles) correlates inversely with water temperature (red lines). Shark abundance peaks from January-March which corresponds with the lowest water temperatures.
Mentions: The number of sharks counted per survey varied with season (Fig 4). Shark abundance was greatest in the winter months (January-March) with a peak winter seasonal abundance within the belt transect of 9925.0 individuals averaged over all years. Shark abundance declined precipitously in the spring and very few sharks were seen in the surveys during the summer and fall months (May-December). Summer and fall shark abundance averaged 111.7 individuals over all years, approximately 1.1% of the winter peak. Mean shark abundance differed among quarters (Kruskal-Wallis, χ2 = 24.640, df = 3, p<0.0001). Post-hoc analysis revealed that mean shark abundance for the first quarter (January-March) was significantly greater than for the other three quarters (April-June, χ2 = 10.447, p = 0.001; July-September, χ2 = 13.831, p<0.0001; October-December, χ2 = 11.772, p = 0.001). Shark abundance in the second quarter (April-June) was also significantly greater than in the fourth quarter (October-December, χ2 = 5.010, p = 0.025).

Bottom Line: This resulted in a maximum density of 803.2 sharks km(-2).Shark abundance was inversely correlated with water temperature and large numbers of sharks were found only when water temperatures were less than 25 °C.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Florida Atlantic University, Boca Raton, Florida, United States of America.

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.

Show MeSH
Related in: MedlinePlus