Les in Chorlton,
I have sung a song by that title for many years now and it may be the one you seek info for. However, I am not sure who wrote it or when. The info I have is that it may have been written by a Folkrock band by the name of either Green Anthem or Reynard. The words which may help someone are as follows:
Title: Oh You Great Leviathan.
Ch:
Oh you great leviathan,
Swimming in your sea blue green,
You're the greatest of all creatures,
And you're like will never again be seen.

In the vast and trackless oceans,
Live many creatures great and small.
But the Blue whale and his family,
Are the greatest of them all.
Ch:
Man has troubled you for centuries,
Hunted you both far and wide.
Countless thousands of your number,
For the sake of commerce died.
Ch:
Listen all you whaling nations,
Must you kill so needlessly.
For all future generations,
Let the whale swim in the sea.
Ch:
Hope this helps you Les and triggers some info out there as to writer/s etc.