Complex sentence: During 1997 and 1998, a precipitous decline occurred in the catch (see the graph on the right), which can be attributed to changes in the sea surface temperature that accompanied the 1997--98 El Niño.
Split into simpler sentences: During 1997 and 1998, a precipitous decline occurred in the catch (see the graph on the right). This decline can be attributed to changes in the sea surface temperature that accompanied the 1997--98 El Niño.