"In a study in the Bahamas, lionfish abundance was found to have increased rapidly between 2004 and 2010, by which time lionfish accounted for nearly 40% of the total predator biomass in the system."
I'm quite confused with the exact year or years that 'by which time' refers to. Does 'by which time' mean 'by 2010' or 'the period of 7 years from 2004 to 2010'?
Based on the connotation that the context shows, I guess it means the former but I'm not quite sure. If the sentence was written "from 2004 to 2010, by which time ...", I could have better understood. But what about "between 2004 and 2010, by which time..."?