Is Sea Shepherd in violation of the

United Nations Convention of Law of the Sea (UNCLOS)?

Consider the predatory actions against the Japanese whaling fleet in the Southern Ocean, please read the below sections from UNCLOS, practically Article 101.  

United Nations Convention of Law of the Sea (UNCLOS) related articles: 

Article 101

Definition of piracy
Piracy consists of any of the following acts:

 (a) any illegal acts of violence or detention, or any act of Depredation (*1 = see below for definition), committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:

     (i) on the high seas, against another ship or aircraft, or against persons or property on board      such ship or aircraft;
     (ii) against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;

 (b) any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;

(c) any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b).

Article 102

Piracy by a warship, government ship or government aircraft whose crew has mutinied
The acts of piracy, as defined in article 101, committed by a warship, government ship or government aircraft whose crew has mutinied and taken control of the ship or aircraft are assimilated to acts committed by a private ship or aircraft.

Article 103

Definition of a pirate ship or aircraft
A ship or aircraft is considered a pirate ship or aircraft if it is intended by the persons in dominant control to be used for the purpose of committing one of the acts referred to in article 101. The same applies if the ship or aircraft has been used to commit any such act, so long as it remains under the control of the persons guilty of that act.

Article 104
Retention or loss of the nationality of a pirate ship or aircraft
A ship or aircraft may retain its nationality although it has become a pirate ship or aircraft. The retention or loss of nationality is determined by the law of the State from which such nationality was derived.

Article 105
Seizure of a pirate ship or aircraft
On the high seas, or in any other place outside the jurisdiction of any State, every State may seize a pirate ship or aircraft, or a ship or aircraft taken by piracy and under the control of pirates, and arrest the persons and seize the property on board. The courts of the State which carried out the seizure may decide upon the penalties to be imposed, and may also determine the action to be taken with regard to the ships, aircraft or property, subject to the rights of third parties acting in good faith.

Link to the full text of United Nations Convention of Law of the Sea (UNCLOS):

http://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf

*1 Depredation =

     1. A predatory attack; a raid.
     2. Damage or loss; ravage