Differences between haddock and cod
Haddock and Cod are two quite different fish. However, they can be used interchangeably in many fish recipes. So if you want to substitute one for the other you can easily do so. When cooked, many people can't easily tell them apart. Even large food producers and supermarkets get them mixed up.


  ...Three of the 59 samples purchased from Asda failed to contain the right fish. 
  Its cod fishcakes actually contained haddock whilst a haddock pie and a haddock fillet were made using Atlantic cod.
  
  At Waitrose two out of 28 samples tested were mislabelled.
  One of them, a mini smoked haddock pie offering ‘flaked North Atlantic haddock’, actually contained cheap Pacific cod.


Read more: http://www.dailymail.co.uk/news/article-1380164/Mislabelled-fish-products-Asda-Tesco-Sainsburys-Morrisons-Waitrose-Lidl.html#ixzz1UCukDoa2

The main points where they may be treated differently are in long term storage. Haddock is not usually salted where as cod is. Haddock can easily be dried or smoked.