Radiolab Episode Summary: "Mystery Bay"

Release Date: June 19, 2025
Hosts: Lulu Miller and Latif Nasser
Guest: Rachel Cusick
Episode: Mystery Bay

Introduction

In the "Mystery Bay" episode of Radiolab's week-long exploration of sharks, hosts Lulu Miller and Latif Nasser delve into the enigmatic behaviors of apex predators in South Africa's marine ecosystems. Guided by marine biologist Alison Cook and marine ecologist Dr. Neil Hammerschlag, the episode uncovers startling interactions between great white sharks, seals, and orcas at Seal Island.

Alison Cook's Discovery

Latif Nasser introduces the episode's central figure, Rachel Cusick, who spotlights Alison Cook, a marine biologist based in South Africa. Cook's journey begins unexpectedly at a modest hand-operated car wash during her student years at the University of Cape Town in the 1990s.

[01:38] Alison Cook: "I used to also love just being around water. So that was another draw card for."

While performing routine checks for valuables and managing cars, Cook's fascination with water leads to an extraordinary discovery.

Life at Seal Island

One day, as Cook inspects a vehicle's trunk, she uncovers a dead body accompanied by an astonishing photograph of a flying great white shark—an image that defies her scientific training and understanding.

[02:13] Rachel Cusick: "And so, you know, when the car owner returned to get his car, Alison asked him like, what is this photo? Is this fake?"

Eager to verify the authenticity of the photograph, Cook meets Chris Fellows and Rob, who introduce her to the reality of these aerial sharks. This encounter sets Cook on a path to Seal Island, a massive breeding colony of seals off the South African coast.

Life at Seal Island

At Seal Island, Cook and her team witness the formidable prowess of great white sharks as they prey on seals. The island teems with 60,000 seals, creating a dynamic and often perilous environment.

[04:19] Alison Cook: "So, you know, Chris kept saying to me, keep your eyes peeled."

The first-hand experience of witnessing a great white shark in flight leaves Cook in awe, compelling her to dedicate her career to studying these mysterious creatures.

[05:05] Alison Cook: "I'm in shock."

The Impact of Great White Sharks on Seals

As Cook delves deeper into the ecosystem, Dr. Neil Hammerschlag discusses the intense predation pressure sharks exert on seal populations, leading to significant behavioral and physiological changes among the seals.

[06:15] Dr. Neil Hammerschlag: "One of the days I saw 42 great white attacks."

This relentless predation not only affects individual seals but also the overall stress levels and survival strategies of the colony.

The Role of Dr. Neil Hammerschlag

Dr. Hammerschlag's research reveals that seals on Seal Island experience quadruple the stress levels compared to those on islands without such intense shark activity.

[07:32] Rachel Cusick: "And what he found was that the seals that live on Seal island have stress levels that are four times higher than the seals on all the other islands."

His studies highlight the profound impact predators have on their prey, shaping behaviors and ecosystem dynamics.

The Mystery of the Slashed Sharks

In 2015, Cook encounters dead sevengill sharks with unnervingly clean surgical-like wounds and missing livers, sparking a new mystery. Subsequent findings of similarly mutilated sharks deepen the intrigue, suggesting an unusual predator or a sinister human intervention.

[11:09] Alison Cook: "I mean, it was such a clean wound that my immediate suspicion was people that had done it."

These incidents raise questions about the forces at play beneath the ocean's surface, threatening the already fragile balance of marine life.

The Orca Intervention

The mystery takes a dramatic turn in 2017 when a drone pilot captures shocking footage of orcas preying on great white sharks. The orcas methodically kill the sharks, targeting their nutrient-rich livers.

[14:48] Latif Nasser: "Oh."

This revelation not only explains the decline of great whites at Seal Island but also introduces orcas as a formidable force reshaping the marine hierarchy.

[15:40] Alison Cook: "I was in denial for a very long time because for me, white sharks were always the apex predator."

The Decline of Great White Sharks at Seal Island

The combined pressures of orca predation and human-induced mortality through lethal netting programs have led to a significant decrease in great white shark populations at Seal Island.

[16:02] Rachel Cusick: "Nets in the water to protect swimmers. And something like 30 sharks get caught in these nets and killed every year."

These factors collectively undermine the sharks' dominance, allowing orcas to ascend as the new apex predators in the region.

The Spread of Fear Among Sharks

Dr. Neil Hammerschlag and Colin Brown explore how fear disseminates through shark populations. They describe a chemical contagion—schreskof—released when sharks are injured or killed, inducing a widespread fear response that alters shark behavior.

[18:14] Rachel Cusick: "Chemical, which in the German word is called schreskof, which literally translates to scary."

This behavioral contagion results in great white sharks abandoning their traditional haunts, such as Seal Island, exacerbating their decline.

[18:31] Colin Brown: "Shreskov can set off a contagion, effectively a behavioral contagion, and an entire population could potentially develop a fear response."

Conclusion

"Mystery Bay" paints a complex picture of marine ecosystems, where apex predators like great white sharks and orcas influence each other's existence and the lives of their prey in unforeseen ways. Through meticulous research and compelling storytelling, Radiolab uncovers the delicate balance of fear, survival, and adaptation beneath the waves.

[19:37] Rachel Cusick: "So next time you're afraid of a shark, just remember they have feelings too."

This episode underscores the interconnectedness of marine life and the far-reaching consequences of human interventions on natural predator-prey relationships.

Notable Quotes:

• Alison Cook [05:35]: "There was no fear. I don't know if it was ignorance. I felt awe. I just saw majesty. I just saw this incredible beauty."

• Dr. Neil Hammerschlag [16:02]: "The nets in the water to protect swimmers. And something like 30 sharks get caught in these nets and killed every year."

• Colin Brown [18:05]: "Yeah, there's always a bigger fish in the sea. Right. And that is virtually true of every animal."

Production Credits:

Reported by Rachel Cusick and produced by Rachel Cusick, Simon Adler, and Maria Pazguchi Tierrez, with production assistance from Becky Lacks. Edited by Pat Walters, fact-checked by Diane Kelly, and mixed with sound design by Jeremy Bloom. Special thanks to Katie Ayers.

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This summary captures the essence of the "Mystery Bay" episode, providing an engaging overview of the discussions and discoveries surrounding the intricate dynamics of marine predators at Seal Island.