"An inspection found out that out of 11 boats only seven had permission to trawl in Russian waters," a spokesman said, adding that the four trawlers were later transported to a port in the Kurils. The detentions happened late on Thursday.
"One of the captains is diabetic, and the issue of providing him with medicine has been settled," the spokesman said.
On Friday, Japanese Foreign Minister Masahiko Komura called on Russia to release the Japanese sailors.
"I proposed to Russia via our ambassador to free the crew members and the vessels," Komura said, adding that he had asked the Russian diplomatic mission to release the sick sailor on "humanitarian grounds."
Confrontations over poaching between Russian authorities and Japanese fishing vessels are frequent in Russia's Pacific waters. Last August, a Japanese fisherman was shot dead near the Kuril Islands when Russian border guards opened fire at his boat after it had refused to stop.
The incident further strained relations between Russia and Japan, which have contested the ownership of the Kuril Islands for over 60 years in a dispute that has kept the two countries from signing a formal peace treaty after World War II.
Earlier this week, Russia's State Fisheries Committee announced that Russia would launch a crackdown on illegal exports of crab and other bio-resources to Pacific Rim countries from the beginning of 2008.
In 2006, a total of 80 fishing vessels made illegal catches in Russia worth over 900 million rubles ($36 million). In the last two years a total of 7 million tons of illegally-caught seafood, worth 230 million rubles ($9.2 million), has been seized in Russia's Bering and Okhotsk Seas.
