If you gut them, but leave them whole, they can last longer than you think.
Fish markets have fresh fish that could be up to a week old, but are still fresh as they were kept near freezing, approx 33- 35 deg.
After a few days they eyes will start to get cloudy, but the smell is an important factor ...... I have found keep them coverd in ice, not a slush or any water.
The nasty water is what will smell .... just keep them draied but covered in ICE.