itself or themselves?

hello everyone , I met with a question when reading different texts .They're listed as follows

The Ocean Cleanup will use a 100 km-long floating barrier to let the ocean currents concentrate the plastic themselves
(Pacific cleanup sets sail

23 Apr 2015 Maritime Journal)

The Ocean Cleanup uses long floating barriers to let the ocean currents concentrate the plastic itself.(linkedin)

I wonder whether I should use themselves or itself . I prefer themselves but during a recent exam the answer gave me itself(I am from China)

thanks for any help