Student or Learner
What's the exact meaning of the following sentence in blue? What time does the word 'then' refer to?
According to their latest paper published in Nature, the biomass of large predators (animals that kill and eat other animals) in a new fishery is reduced on average by 80% within 15 years of the start of exploitation. In some long-fished areas, it has halved again since then.Thank you in advance.
Of the 20% remaining at the end, half (a further 10% of the original) have gone in some areas.