The fish is the plural for sure. As is the wood. Can I just confirm that the meaning is the first three words not the first six words as previously stated. The subject of a great Alan Sillitoe short story (spoiler).
Not as easy for me as others found. Bottom half went in much more easily than the top. And many words I learned for the first time.
FOI 18d. LOI 17, 16.
COD 20d. Not sure why. Thanks Pasquale for a fun solve and for broadening my vocabulary.