Despite being dwarfed by the giant, the shark dragged the animal down, keeping its blowhole underwater.
Mr Johnson said the proceeded to suffer a lingering and agonising death lasting almost one hour.
The whale was under observation when the shark attacked, as it had mottled skin and had become separated from its pod – both indicating its poor state of health.
Mr Johnson added: “Helen looked very informed about what she was doing, which made me curious about whether she was an experienced whale killer, was acting on instinct or on plain intelligence by detecting her prey was weak.
source: express.co.uk