The Recapy context you provided only gives a brief summary and doesn't explain the specific way Marcellus uncovers Erik’s fate.
Short, non‑spoiler answer: Marcellus investigates by watching, listening, and collecting clues at the aquarium and in town; his observations and a few clever actions let him piece together what happened and help Tova learn the truth.
Do you want a full, spoiler‑filled explanation of exactly what Marcellus discovers and how he does it?
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