Supersized Sharks Full Episode | @natgeokids

Key Concepts:

• Tiger Sharks: Large, adaptable predators found in tropical and subtropical waters.
• Norfolk Island: Remote island in the South Pacific with a unique tiger shark population.
• Headstone Bay: Specific location on Norfolk Island where cow carcasses were historically disposed of.
• Stable Isotope Analysis: Technique used to determine the diet of the tiger sharks.
• Acoustic Tagging: Method to track the short-term movements of sharks around Norfolk Island.
• Satellite Tagging: Method to track the long-range movements of sharks.
• Shearwaters: Pelagic seabirds that form a significant part of the tiger sharks' diet on Norfolk Island.
• New Caledonia: Island north of Norfolk Island where the tiger sharks migrate.
• Fin Cam: Camera attached to a shark's fin to record its behavior and environment.
• Ultrasound: Used to determine if female sharks are pregnant.

Norfolk Island Tiger Sharks: An Enigma

The video explores the mystery of why Norfolk Island is home to an unusually large population of big tiger sharks. The investigation focuses on two primary theories:

1. Diet: The historical practice of disposing of cattle carcasses at sea, specifically at Headstone Bay, may have led to tiger sharks congregating there for an easy food source.
2. Breeding/Pupping: The island may serve as a breeding or pupping ground for large female tiger sharks.

Investigating the Cow Carcass Theory

• Historical Context: Norfolk Island was once a penal colony, and later, people from Pitcairn Island moved there and started farming cattle. The disposal of dead cattle at sea became a common practice due to concerns about groundwater contamination.
• Experiment: A deceased cow (from natural causes) was placed in Headstone Bay to observe the tiger sharks' feeding behavior.
• Observations: At least 15-20 tiger sharks were observed feeding on the cow, with most sharks being 13-15 feet long, significantly larger than tiger sharks in other regions.
• Stable Isotope Analysis Results: Surprisingly, the analysis revealed that seabirds, specifically shearwaters, constitute 52% of the tiger sharks' diet, while cow meat only accounts for 10%. This finding challenges the initial hypothesis that cow carcasses are the primary driver of the sharks' large size.

Unveiling the Shearwater Connection

• Shearwater Rafts: The investigation shifts to understanding how tiger sharks are able to prey on shearwaters, which are fast and agile birds.
• Local Expertise: Margaret Christian, a park ranger with 40 years of experience on Norfolk Island, provides insights into shearwater behavior.
• Drone Surveys: Drone footage reveals large rafts of shearwaters gathering near the island. Disturbances in these rafts suggest potential predation attempts by tiger sharks.
• Bird Cam Experiment (Shaza): A custom-built "bird cam" (Shaza) was deployed to infiltrate a shearwater raft and capture underwater footage of potential predation events. The first attempt failed when Shaza crashed on a cliff, but was recovered by the local rescue squad. The second deployment resulted in the loss of the bird cam, but provided valuable insights.
• Conclusion: The researchers hypothesize that tiger sharks target shearwater rafts, particularly at dusk, when the birds are resting on the surface. This consistent and readily available food source may explain the presence of large tiger sharks on Norfolk Island.

Migration to New Caledonia: A New Chapter

• Tracking Data: Satellite tagging reveals that the tiger sharks migrate north to New Caledonia during the winter months.
• New Caledonia Lagoon: The team travels to New Caledonia to investigate why the sharks migrate there. The southern lagoon is one of the largest in the world with a huge diversity of habitats.
• Initial Observations: Diving in the lagoon reveals a variety of potential prey items, including sea snakes, turtles, and fish, but no large concentrations of any particular food source.
• Fin Cam Deployment: A fin cam is attached to a large female tiger shark to observe its behavior and environment.
• Fin Cam Results: The fin cam footage shows the shark engaging in a "yo-yo" diving pattern, potentially for energy conservation or navigation. The shark is observed interacting with other fish, but no predation attempts are recorded.
• Male Sharks: A large male shark was caught, which is rare in Norfolk Island.
• Ultrasound Attempts: Ultrasounds are performed on several female tiger sharks to determine if they are pregnant.
• Pregnant Female Discovery: A large female tiger shark is found to be pregnant, with palpable pups inside her. This is the first confirmed pregnant tiger shark the researchers have encountered.
• Satellite Tagging of Pregnant Shark: A satellite tag is attached to the pregnant female, allowing researchers to track her movements and potentially identify pupping grounds.

Conclusion and Future Research

The expedition provides valuable insights into the behavior and ecology of tiger sharks in the South Pacific. While the historical practice of disposing of cow carcasses may have initially attracted tiger sharks to Norfolk Island, the primary driver of their large size appears to be the consistent availability of shearwaters. The migration to New Caledonia suggests that the sharks may be breeding or pupping in the warmer waters of the tropical islands. The tagging of a pregnant female offers the potential to identify critical pupping grounds, which is essential for the conservation of this globally threatened species.

Notable Quotes:

• "This place is like a lost world."
• "Are tiger sharks using Headstone Bay as a drive-thru hamburger joint?"
• "Don't go to Mackers. Go to Kentucky Fried Chicken."
• "We know nothing about popping grounds of tiger sharks."

Technical Terms and Concepts:

• SR Range: (Likely refers to "Species Richness" range) A measure of the diversity of species in a given area.
• Isotope Results: Analysis of stable isotopes in tissue samples to determine an animal's diet.
• Acoustic Receivers: Underwater devices that detect and record the presence of acoustically tagged animals.
• Pelagic Birds: Seabirds that live primarily in the open ocean.
• Burlly Slick: A trail of oily or bloody bait used to attract sharks.
• Gon Nesting: Nesting of gonads, referring to the reproductive organs of fish.

Logical Connections:

The video follows a logical progression, starting with the initial mystery of the large tiger sharks on Norfolk Island, exploring potential explanations, conducting experiments and analyses, and ultimately uncovering new insights into their diet, behavior, and migration patterns. The investigation builds upon each finding, leading to a more comprehensive understanding of these fascinating creatures.