According to the regulations you need a net. Also, according to several people at the DFG (some pretty "high ups") a Boga Grip or other landing device will suffice.

The regulation is so that you can safely land short fish. Requiring just a net is stupid because regular nets will thrash the fish's tail and fins giving it fin rot and making it die - which is exactly opposite of the point of bringing a net. The Boga Grip, on the other hand, is the safest way to land a fish (for the fish). There are nets made of rubber (or something) that are made for catch n' release and don't screw up the fish.

Also, if you don't have a Boga and have a cheap rip off model be careful not to break the fish's lower jaw - keep your hand loose. Those knock-offs are piles of crap.