The Haddock (Melanogrammus aeglefinus) is known in Gaelic tradition as "Peter's fish". The reason is Biblical.

In the Biblical account of Jesus and his disciples being asked to stump up the temple tax in Capernaum (Mathew 17), Jesus asks Peter to get some money in order to avoid offending the tax collectors. Go thou to the sea and cast a hook and take up the fish that first cometh up; and when thou hast opened his mouth thou shalt find a piece of money; that take and give unto them for me and thee.

We can be confident that the species of fish in question was not the haddock of our own marine waters. But as Christianity came into Gaeldom, the story of Peter and the fish became connected with the haddock because of the large dark spot the species bears on either side of its body above the pectoral fin. These were seen to be the finger marks left by Peter on the fish as he extracted the money from its mouth. The haddock became known in Gealic as isag Pheadair "Peter's fish" or isag Pheadair runaich 'the fish of loving Peter; its modern name adag however is almost certainly derived from the English "haddock".

Children in fishing communities were taught to differentiate the haddock from another whitefish, the whiting (Melangius merlangus), with the following saying: Ball dubh air an adaig, gob fad' air a'chuidhteig 'a black spot on the haddock, a long snout on the whiting'. On Christmas day in Uist, the men would traditionally row 707 oar-strokes from the land  before casting  a line, that particular number possibly  being a relic  of an earlier pre Christen practice. Any fish they caught  on that day would be called Deirc Pheadair "Peter's tribute" and would be given to the poor and needy.

This extract was taken from "The nature of Scotland" by Ruairidh Macllleathain. 2013.