Is it cheating to do it in two stages something like this?:
create view RegionTotalFishStock as select sum(Quantity) as subcount from LOCA where stock='fish' union all select sum(Quantity) as subcount from LOCB where stock='fish'
then
select sum(subcount)as 'Number of Fish' from RegionTotalFishStock
(I'm relatively new to this, so use with caution...)