Complex sentence: English sole is an important commercial fish, primarily caught off Washington, Oregon and California and, though biomass is increasing, catches have been declining since the 1960s and are currently almost at an all-time low.
Split into simpler sentences: English sole is an important commercial fish, primarily caught off Washington, Oregon and California. Though biomass is increasing, catches have been declining since the 1960s and are currently almost at an all-time low.