Complex sentence: Petropavlovsk was a great source of fish (particularly salmon) and crab meat for the Soviet Union in the 20th century; however, since the end of the Soviet era fishing rights have been granted to foreign interests.
Split into simpler sentences: Petropavlovsk was a great source of fish, particularly salmon, and crab meat for the Soviet Union in the 20th century. Since the end of the Soviet era fishing rights have also been granted to foreign interests.