OSLO Goldman Sachs Asset Management (GSAM) has offered 6.6 billion Norwegian crowns ($598.8 million) in cash to buy Norwegian aquaculture service group Froy, the two companies said in a statement on Monday.

GSAM said it had reached a deal to buy a 72% stake in Froy from a unit of salmon farmer Salmar for 76.50 crowns per share, and will make a bid for the rest of the shares at the same price.

Salmar in January this year said it was seeking potential buyers for the stake, and Froy's share price has since risen by almost 50% to close at 72 crowns on Friday.

Froy's fleet of specialised wellboats and other vessels provides maritime transportation services to fish farmers and other aquaculture companies.

"Wellboats and service vessels are vital to enabling best-in-class farming practices and Froy is at the forefront of driving sustainability," GSAM Global Co-Head of Infrastructure Tavis Cannell said in a statement announcing the offer.

($1 = 11.0213 Norwegian crowns)

(Reporting by Terje Solsvik; Editing by Kim Coghill)