Fill net up at sea when fish are thus? (9)

I believe the answer is:

plentiful

I'm a little stuck... Click here to teach me more about this clue!

'fish are thus?' is the definition.
I can't tell whether this defines the answer.

'fill net up at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'fill'+'net'+'up'='fillnetup'
'fillnetup' anagrammed gives 'PLENTIFUL'.

'when' is the link.

Can you help me to learn more?

(Other definitions for plentiful that I've seen before include "Productive" , "Abundant - profuse" , "Abundant, copious" , "common" , "Existing in great quantities" .)

I've seen this clue in The Telegraph.

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