I am going to guess the otter did not catch the arapaima for a few reasons:
The fish has fins and the otter feet, the water in which that size fish would be found would not allow footing for the otter and the arapaima would have a considerable advantage.
The otter lacks claws like a big cat to grip and hold making it near impossible for it to latch onto the arapaima long enough to deliver a death bite to the spine.
There is very little possibility of an otter that size dragging a fish of that weight out of the river (where they normally feed I believe).
Otters are wary and shy. In the picture there is a boat in the background so I am guessing he is snacking on a fisherman's catch and is a tame otter. Or at least one that has lost its fear of humans.
Just my 2c.