Some of them starting to spawn a bit early?
True.
Also, if an Iceland Gull is 55cm long then these fish seem to be 1-1.5m long, too big for the average atlantic salmon.
Wikipedia says this about cod:
"Atlantic cod are a shoaling species and move in large, size-structured aggregations. Larger fish act as scouts and lead the shoal's direction, particularly during postspawning migrations inshore for feeding. Cod actively feed during migration and changes in shoal structure occur when food is encountered. " and that cod can grow up to 2m in length.
So that seems more likely. They had a tendency for none of them to show for a while and then lots to appear at the surface together.
I think that the gulls must have been attracted because these fish would have been feeding on schools of small fish which they pushed up to the surface where the gulls could then get them.