Can Southern Blue Whiting replace Cod? Can you use Southern Blue Whiting (which is a small codfish) in place of Atlantic Cod in most recipes? Other than the size difference between the two, are there any important differences to bear in mind when substituting?

(Southern Blue Whiting from a few miles up the Queensland coast was the only cod-like fish the local fishmonger had in stock)
Cod is cod, a smaller variety's characteristics won't be so different as to make much difference in a recipe. 

Really any mild-flavored white fish with a medium strength flesh is a reasonable substitute for cod, in fact due to cod stock depletion you'd be doing nature a favor by picking something locally caught.