The Post Office said the majority of its branches were open despite the strike.
Three hundred branches are striking, but the Post Office said around 250 of these were open.
The rest of the Post Office's network of 11,600 branches is not involved in the dispute.
Post Office group network and sales director Kevin Gilliland said: "We want to reassure customers that nearly all of our network is unaffected by the strike action."
More top news
The fish would have suffered a slow death, the RSPCA said.
The death of university student Libby Squire is being treated as a "potential homicide", police have said.
"Witness O" said he had been told by the current head of the IRA he could name those he knew were involved.