Tiger Sharks of Bimini
Figure 1: Location of Vemco acoustic receiver arrays
Tiger sharks are found circumglobally in tropical and warm-water coastal and oceanic ecosystems (Randall 1992). As the largest apex predator in many of these ecosystems, tiger sharks (Galeocerdo cuvier) regulate lower trophic levels through both direct consumption and behavioural modification of prey (Heithaus 2012). Loss of large bodied, top predators is linked to large-scale changes to ecosystems and strong trophic cascades (Estes 2011). Tiger sharks are a highly mobile species, capable of 7,500 km migrations in a year, but have also shown philopatry to distinct areas (Lea et al. 2015). Due to this highly mobile nature, tiger sharks are likely to inhabit and regulate several ecosystems (Heupel et al. 2014).
During long migrations sharks cross-jurisdictional boundaries and enter waters with varying harvest regulations. For example, The Bahamas has banned all shark harvest within its territorial waters, but once sharks leave this area they are not afforded the same protection. Understanding dispersal patterns of tiger sharks is important to regional conservation efforts and effective fisheries management. Over the past few decades Sharklab staff have consistently caught tiger sharks in the waters around Bimini. The high frequency of mature females and juveniles led to the hypothesis that Bimini is serving as a nursery grounds for this species. Tiger sharks prefer shallow water habitats (such as those found to the south, east, and north of Bimini) as juveniles and progressively move to deeper oceanic waters (such as to the west of Bimini) as they grow larger (Afonso and Hazin 2015).
To better understand movement patterns, residency time, and potential returns to Bimini acoustic tags will be implanted in both young-of-year/juvenile and mature tiger sharks. These tags will be recorded anytime a shark comes within approximately 500m of a receiver. The array of Sharklab receivers around Bimini will be used in conjunction with other collaborative receiver arrays (Figure 1).
Long lines and deep line surveys will be used to assess the influence of season and location on the catch rates of juvenile and mature tiger sharks. Stable isotope analysis will be used to assess possible ontogentic shifts in prey selection. Ultrasound will be used to determine if mature females are pregnant when captured around Bimini.
Genetic analysis will help to determine if females are continually returning to pup in the waters around Bimini and the potential of natal philopatry. Videos will be used to assess shark interactions with longlines and how size impacts likelihood of being caught. Comparing the movements of tiger sharks to the availability of prey will help to determine if residency patterns is driven primarily by food or habitat preferences.
How long are juvenile and mature tiger sharks remaining around Bimini, where do they go when they leave, and how often do they return?
Are females returning over multiple years to pup in the waters around Bimini and is there natal philopatry?
What is the ecological role of tiger sharks around Bimini and does prey selection change as they grow?
How are sharks interacting with the longlines?
Establishing what areas sharks are using during key life stages is important for conservation and management efforts. Understanding dispersal patterns is essential for creating accurate population models, creating effective regulations, and determining the rebound potential of species. Observing how sharks are interacting with the longlines will help to establish is particular size classes and species are being underrepresented in these surveys. Additionally it may be possible to determine how to intentionally reduce shark capture on commercial longlines.
Matthew Smukall—Bimini Biological Field Station/University of Alaska Fairbanks
At Lab Since: July 2015 - First came to the lab in January 2012 and volunteered over 3 winters during leave from work
From - Orlando, Florida / Kenai, Alaska
Education/Work Experience - PhD student, University of Alaska Fairbanks, School of Fisheries and Ocean Sciences
BSc University of Florida, Wildlife Ecology and Conservation
Masters University of Florida, Fisheries and Aquatic Sciences
2009 - Florida Keys, Caribbean spiny lobster research project, University of Florida
2009 to 2015 - Fisheries Technician/ Biologist, Cook Inlet Aquaculture Association, Kenai Alaska. Studying salmon populations in Southcentral Alaska, movement patterns of invasive northern pike, and the impact of pike on native salmonids.
Research Interests - Movement patterns of sharks. In particular the residency time, dispersal patterns, and philopatry of tiger sharks around Bimini. I am also interested in the use of longlines to assess shark populations and how sharks are interacting with the longlines.