Complex sentence: Overfishing for grayling reduced its numbers in the Chena to "dangerous levels" by the mid- 1980s; in the 21st century, sport fishing for grayling, which grow to long in the upper river, is limited to catch and release.
Split into simpler sentences: Overfishing for grayling reduced its numbers in the Chena to "dangerous levels" by the mid- 1980s. In the 21st century, sport fishing for grayling, which grow to in the upper river, is limited to catch and release.