Foreign Minister Dimitrij Rupel said the decree, which goes into force in a week, was adopted in reaction to regulations on the borders of the Croatian fishing area which the Croatian government adopted in mid-December.
"The Croatian regulations encroached even on our territorial and internal waters, which was why we protested on that occasion," he told the press, adding the Slovene decree was temporary and would remain in force until a border agreement or fishing rules in the border area were adopted.
Agriculture Minister Marija Lukacic said the decree divided the Slovene fishing area into three zones.
Zone A covers the entire Piran Bay, which the decree says is part of Slovenia's internal waters. Zone B covers Slovenia's territorial waters, which Lukacic said extended from the T5 point in the north, agreed by the former Yugoslavia and Italy in a sea border deal, to Cape Savudrija in the south. She added these points were mentioned also in a border deal initialled in 2001 by the then Croatian and Slovene prime ministers, giving Slovenia access to the high seas.
Zone C covers the Slovene protected ecological zone and the high seas in accordance with a law passed by the Slovene parliament last autumn.
"With the decree, we wish to step up Croatia's negotiations with the European Union on fishing regulations," said Rupel, adding the area served for commercial fisheries on the basis of a border trade and cooperation agreement existing between Croatia and Slovenia for which the EU has to adopt implementation rules.
Rupel said the agreement was a valid document ratified by both countries.
Asked who would protect Slovene fishermen fishing in the area of Piran Bay proclaimed part of the Slovene fishing area which crosses the middle of the disputed bay, Rupel said the two governments last year reached an agreement on the avoidance of incidents.
"No one needs any special protection. This is within the jurisdiction of others and not the foreign ministry."
Rupel said the decree was an attempt to settle a problem for which Croatia was to blame because its fishing area regulations encroached on Slovene territory.