Complex sentence: Chinese paddlefish are now believed to be extinct as there have been no sightings of specimens in the wild since 2003, and past attempts of artificial propagation for restoration purposes have failed because of difficulties encountered in keeping captive fish alive.
Split into simpler sentences: Chinese paddlefish are now believed to be extinct as there have been no sightings of specimens in the wild since 2003. Past attempts to artificially propagate Chinese paddlefish in captivity have failed because of the difficulties encountered in keeping captive fish alive.