I have answered my own question to some extent, though I would still value other's opinions. This was mainly from the Veterinary Medicines Department in the UK:
Antibiotics come with individual recommendations of withdrawal periods depending on proven depletion times. This will vary throughout the year depending on water temperature and therefore metabolic rates. The times are calculated on degree-days, so Florcol for example is 15 days at 10°C or a withdrawal figure of 150 (10 days at 15°C would be an equivalent).
If salmon or sea trout are being considered the par are usually bathed in a fungicide after grading when they may get damaged and prior to being taken to sea cages; so there is some seasonality there, spring and autumn I guess.
Likewise, these two species will be treated for sea lice at specified times of the year.
There is no withdrawal time for vaccines.
Thanks for your time.
Richard Chivers
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