This topic surfaces every now and again and imo don't think a vertical or horizontal hold has any fatal impact on any fish that are being caught. The handling part / time out of the water will determine it's faith. The more time it takes to return it to it's environment the less chance it has at surviving.
Any fish, big or small will loose a lot of energy fighting to free itself from the hooks that are lodge into it's mouth, so the faster you can get them to the boat the better.... A net will play a crucial role for the handling part.... if you have a livewell, you can use it too.... put it in there and leave the pump running to create oxygen so that it regains it's strength until you release it.
So that being said, for me and this is just my opinion the holding part is no concern. The quicker you land and release the fish, the better it's survival.
Leechman