The owner of a Kensington seafood market where police seized 40 illegally large rockfish last month was placed on 12 months probation yesterday and ordered to pay $5 in court costs by a Montgomery County District Court judge.

Steven Himelfarb, 32, who pleaded guilty to one count of illegal purchase and possession of rockfish, told Judge Thomas A. Lohm he was unaware of the Maryland state law that prohibits the possession of rockfish longer than 32 inches.

"It is your duty to inform yourself of the law," the judge replied.

Nine armed agents of the Maryland Natural Resources police raided Himelfarb's fish market, U. S. Fish Inc., on June 15, seized 40 large rockfish, and arrested Himelfarb.

Natural Resources officer Robert Mullen said Maryland law protects the larger rockfish because they are "cow fish" that travel up freshwater tributaries in the spring and lay as many as a million eggs each.

Himelfarb's attorney, Sam Williamowsky, told the judge his client has been in the fish business in New York and in Washington D.C. and that most of the rockfish in question were shipped from New York and New England.

"There are no size limitations in New York and the District of Columbia," Williamowsky said. "This is not a case where he was trying to hide fish. When people are trying to get around the law they fillet the fish to try and disguise the size. My client has gotten an education out of this experience. You will not see him before you again on this charge."

Williamowsky said that the state also has received a benefit from the incident--40 rockfish valued at $1,600. Thirty-eight of them have already been consumed by inmates at the Maryland House of Correction. The other two, which were kept on ice as evidence, will now also go to a state institution, officials said.