Complex sentence: The Japanese eel population, along with anguillid eel populations worldwide, have declined drastically in recent years, presumably due to a combination of overfishing and habitat loss or changing water conditions in the ocean interfering with spawning and the transport of their leptocephali.
Split into simpler sentences: The Japanese eel population, along with anguillid eel populations worldwide, have declined drastically in recent years. This is presumably due to a combination of overfishing and habitat loss or changing water conditions in the ocean interfering with spawning and the transport of their leptocephali.