I also think they do the migration,something i have always wondered about though is that they never,in my experience,turn up in cast nets. A lot of other juvenile fish seem to get attracted to the schools of inch long prawns,Queenies,threadies,grunter,moses perch. But in all the years i cast netted heavily,i never got a juvenile jack. Got quite a few legal lacks up in the headwaters where the prawns followed the brackish water,but not once a small one. I know of places we used to see an odd one,and know in mackay where there are a lot of little ones growing in freshwater. But down here it is dammed,so i find it odd i never saw one. Just another piscatorial mystery to fathom i guess..