Interesting. Your idea of seasonal availability definately corresponds with how the piraiba are imported to Thailand only once a year and noticeably, within a 3 months range apart in terms of time of the year. Maybe there are other factors are running, but a possibility stands it could be accredited to how they are caught indeed. The fact that piraiba (like the dourada) are migratory and would form schools in such manner that would also allow them to be captured in decent numbers, like you've explained. I have seen a similar migratory school of juveniles in Hemibagrus species, and they are relatively easy to catch.I've not seen Brachyplatystoma species being "farmed" in the same manner or volume that Pseudoplatystoma have been in South America. There are tons of places farming TSN species and hybrid versions of them with Leiarius or RTC's for food production though and pay lake fishing with some aquaculture mixed in as well.
I know we get Juruense and Tigrinus at small sizes and often times in near perfect condition with barbels/fins in tact but they still appear to be mostly seasonal which leads me to believe they are being wild caught generally and not captive bred. That said however the way a lot of the fish farming works in South America is with ponds tied directly into the river system for "filtration" so it could simply be the seasonality related to water levels on when the captive fish are breeding or harvestable from the ponds. To my knowledge acquiring small "Piraiba" whether they be Fila or Capa is not like catching small Dourada (Rousseauxii) where they are easily caught at certain points during their migration across Suriname. I cannot make the same assumptions for Brazilian or Peruvian specimens. I would also make the assumption that collecting small Brachyplatystoma of any type is easier and more widespread in Peru versus Suriname simply due to infrastructure and quantity of collectors operating in the region.
I know the table fare of the Brachyplatystoma species is considered lower than that of the Pseudoplatystoma species so it may not be so much that we don't have the means to produce them in aquaculture as much as it is lack of demand and slower growth rates providing smaller yields for their time investment.
The volume of them coming out of Peru I think could be related to what I mentioned above, more collectors available and more infrastructure. The way a lot of their aquaculture works they could be farm raising them in ponds tied to the river which would still create seasonal availability and the ability to provide small ones easily in relatively decent shape if they were seining them out and giving them some time to heal up prior to export. I just don't think there are a lot of breeders focusing on this if so simply based on the volume of them you see imported compared to that of say TSN or RTC. I would think it would be niche breeders if this was the case and not "commercial" because the $$$ just aren't there for them. Also with the better infrastructure there may be dependable collection points for juveniles that have become established over the years so the collectors can find small ones in schools at the right time of year. A complete assumption on my part again but seems reasonable based on the fact that I can find schools of baby channel cats or bullheads in the hundreds at the right times of year here in the states.
I previously though that piraiba meat was valued however, so its new knowledge to me that they fetch low table fares locally.