Canada Border Services Agency says there was something fishy about a shipment inspected at Toronto's airport — and it wasn't just the frozen fish.

CBSA says officers at Pearson International Airport were conducting a routine examination of cargo arriving off a flight from Lahore, Pakistan, on Tuesday when they discovered suspected heroin.

Officials say officers found two small packages deep within a large shipment of frozen fish that were similarly wrapped and loosely placed inside the contents.

The packages were cut open and the contents tested positive for heroin.

A total of four kilograms of heroin was seized and turned over to the RCMP.