Every year fish are getting lighter; ... (6)
I believe the answer is:
paling
'getting lighter' is the definition.
'every year fish' is the wordplay.
'every year' becomes 'pa' (per annum).
'fish' becomes 'ling' (ling is a kind of fish).
'pa'+'ling'='PALING'
'are' is the link.
(Other definitions for paling that I've seen before include "Fence post - turning white" , "is becoming whiter" , "Fence made of stakes" , "Barrier of stakes" .)