A humpback whale, likely lured by a trawling net capturing masses of Antarctic krill, became entangled last month and died in the Southern Ocean. Three dead juveniles were caught in the same company’s krill nets last year.

Scientists say the humpbacks may have been malnourished while forced to compete for food with a burgeoning industry harvesting the tiny crustaceans — the linchpin in the Antarctic food web — for use in pharmaceuticals and fish feed...