Complex sentence: Careful follow-up by Cree health authorities since the 1980s have been largely successful and they continue to promote the regular consumption of fish, with the notable exception of the predatory species living in the reservoirs, which still show high levels of mercury.
Split into simpler sentences: Careful follow-up by Cree health authorities since the 1980s has been largely successful. The authorities continue to promote the regular consumption of fish, with the notable exception of the predatory species living in the reservoirs, which still show high levels of mercury.