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 Lemon shark aggregations in Jupiter, Florida

Donation code: Lemon sharks Jupiter

Fig.: Lemon sharks swimming around a VR2 sonic receiver.
© M. D. Potenski / Shark Foundation
Fig.: Lemon sharks swimming around a VR2 sonic receiver.

A unique winter aggregations of lemon sharks at Jupiter Florida was first reported to the project team in 2001. Over the next four winter seasons, 2002 to 2005, the Shark Foundation team conducted preliminary studies to assess the feasibility of investigating these aggregations with the goal of gathering information on the unknown adult life history of the lemon shark (Negaprion brevirostris). Based on these preliminary studies they formulated the following research questions:
  • Do individuals remain on a single aggregation ground for the entire winter aggregation period (method: acoustic tracking)?
  • What are the larger scale movement patters of individuals found in the aggregation; where do the sharks go after the winter aggregation disperses (method: satellite tracking; acoustic tracking)?
  • Do individuals, especially reproductively active females, return to the same aggregation grounds over the years (method: genetics and acoustic tracking)?
  • Do winter aggregations represent individuals from discrete geographic units (method: genetics and acoustic tracking)?
  • Are different habitats and locations used by aggregating individuals (method: Video monitoring, acoustic tracking)?
  • Do females attract males with pheromones (method: water sampling, biochemical analysis)?
  • What is the behavior of these aggregating adult lemon sharks? Are females philopatric and males nomadic (method: genetics, satellite and acoustic tracking)?
  • Does the aggregation represent a refugium; do sharks rest during the day and become active at night (method: acoustic-tracking and behavioral observation)?
Subsequently seasons, 2006 and 2007, saw the beginning of an organized and systematic effort to sample the aggregating lemon shark population. Shark Foundation funds allowed the expansion of our VR2 acoustic telemetry array of bottom monitors and the systematic collection of environmental parameters. Until today (March 2009), 54 adult sharks (size 210 - 281 cm) were tagged with Vemco V16H transmitters, 27 males and 27 females

Fig.: Location of VR2  receivers in the Jupiter region.
© Shark Lab / Shark Foundation
Fig.: Location of VR2 receivers in the Jupiter region.

For the analysis of shark movements an array of 18 acoustic receivers was installed on the sea floor in the Jupiter FL over an area of about 60 km area. To date the array monitors recorded a total of 232,104 "hits" from our transmittered lemon sharks and 12,714 from monitors belonging to other research partners.
 
 


Preliminary Results
Over the current two year period, there is distinct seasonality to the lemon shark aggregations in the Jupiter area. This season runs from early January almost to the day through mid April. It is noteworthy that the vast majority of monitor detections occurring April and December are from female sharks. This begs the question: "What are the environmental cues that define the seasonal trend?" The Shark Foundation team used the environmental data to try to identify the cues and immidiately found a strong correlation between ambient water temperature and seasonal periodicity.

Identification
© Shark Lab / Shark Foundation
Fig.: This figure illustrates a clear relationship between the reduction in temperature (red) over the winter months and the presence of the lemon sharks (blue) over this period.

Results to date clearly demonstrate that this population provides the first real opportunity to investigate the adult life history of lemon sharks in a natural setting. A critical temperature of 24 ° C has now been identified as the most likely cue for both local movements and seasonal residence, with an identified temperature preference of around.

Fig.: Large scale movements of the the agregating lemon sharks are represented by directional arrows.
© Shark Lab / Shark Foundation
Fig.: Large scale movements of the the agregating lemon sharks are represented by directional arrows.

Male lemon sharks at Bimini have been shown to be much more nomadic than females, but at Jupiter we were surprised to find that males have a cyclic migratory pattern. Most notably some of the mature males repetitively migrating to Georgia and the Carolinas returned regularly to Jupiter to join the aggregation in December. Based on data from satellite tags showing that a mature male tagged at Jupiter crossed Gulf Stream to Tiger Beach on the Little Bahama Bank and the fact that the many of the females at Tiger Beach display mating scars, we believe there is strong potential for Florida-Bahamas-Caribbean gene flow.

With so many potentially significant findings from our preliminary analysis, and planned long-term continuation of this study, the Shark Foundation team believes that insight into the complete life history pattern of this protected coastal species will be possible.


Project leader: Prof. Samuel "Doc" Gruber

The Foundation supports the Jupiter project since 2006.
Invest until today: CHF 53,000.


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