I think its possible that overfishing in those areas has had negative effect. Its true they are huge areas but I wonder how much of it is actually fished for discus...if certain areas are more heavily fished because of ease of access its likely the gene pool is affected in some way locally
Unfortunately whats needed is more natural history studies of discus in the wild. We really know little about how far discus travel in an average lifetime or season to season. If they tend not to move much the effect of collecting on a particular area would have the potential of having a greater influence on the genetic diversity and health of local populations. If Discus are largely transient and cover lots of area the effect will be minimized greatly due to the influx of new individuals to the collection areas.
al