Your text is mostly clear, but there are a few minor grammatical issues and opportunities for improved clarity. Below is a corrected version:
"When scientists tagged a big 9-foot great white shark, they were surprised when the tracker washed up on shore. Scientists believed it must have been something larger than that great white shark that ate it. However, when people heard of this, they suspected that the megalodon must still be alive. However, the chances of this being true are low. It is more likely that a killer orca or a larger cannibalistic shark ate the 9-foot shark."
Changes made:
- Changed "bigger" to "larger" for better word choice.
- Added "that" after "suspected" for clarity.
- Changed "bigger" to "larger" in the last sentence for consistency.
- Added "that" before "a killer orca" to improve reading flow.