Can a country choose to only partially ratify a treaty?

Question

Let's say that Country A wants to enter into UNCLOS. However, it doesn't like certain provisions of UNCLOS. Is it legally possible de jure (not de facto, since countries clearly can just establish the stuff they like in UNCLOS as their own laws) to only ratify parts of the treaty (not abiding by the opt-out clause)? Or is this against international norms/is not regarded as legal? (I also do not mean countries who are in the process of adopting all parts of a treaty since their intent is still to adopt the entire treaty.) Now, let's say that the country was abiding by the opt-out clause and did not exclude any other parts of the treaty. Did they technically/semantically speaking still ratify the entire treaty?

Answer 1

Broadly speaking: "yes, but..."

Such declarations are called 'reservations'.

A reservation is a declaration by a state made upon signing or ratifying a treaty that the state reserves the right not to abide by certain provisions of the treaty.

Reservations are formally defined in Article 2.1(d) of the 1969 Vienna Convention on the Law of Treaties. Articles 19-23 govern the process of making and withdrawing reservations. Generally, it outlines, among other things, that:

• Reservations cannot be incompatible with the object and purpose of the treaty.
• A treaty may also prohibit reservations for some or all of the treaty's provisions. Other states may object to a reservation.
• A reservation may also be withdrawn at a later date.
• Reservations and objections must be in writing.

In comments here, Henry linked to the UNCLOS page where you can see its signatories and their declarations and reservations, and pointed to UNCLOS articles 309 and 310.

Article 309

Reservations and exceptions

No reservations or exceptions may be made to this Convention unless expressly permitted by other articles of this Convention.

Article 310

Declarations and statements

Article 309 does not preclude a State, when signing, ratifying or acceding to this Convention, from making declarations or statements, however phrased or named, with a view, inter alia, to the harmonization of its laws and regulations with the provisions of this Convention, provided that such declarations or statements do not purport to exclude or to modify the legal effect of the provisions of this Convention in their application to that State.