Translate the following English sentence to French:
"But this is no more than the price which must be paid in order to guarantee the cleanliness of the marine environment, especially since the directive envisages that it should be possible to identify the vessels which cause less pollution and to offer them reduced rates?"
Mais n' est-ce pas là le prix à payer pour garantir la propreté de l' environnement marin, d' autant que la directive prévoit que les navires moins polluants devraient pouvoir être identifiés et bénéficier de redevances réduites ?