When choosing fish, we are told to always check the gills. They must be red or the fish is not fresh.
The gills of a freshly caught fish are bright red, but will darken over time. However, they should not be dark red or brownish red.
They should be a bright colour and not slimy. They should be clean and cold. Mucus builds up with age.
And the eyes should be bright, plump and shiny, not sunken and cloudy. Cloudy eyes means that deterioration has set in.
The fins and tail must not be torn or ragged, showing poor handling after the fish was caught.
The flesh of a fresh fish is firm. When pressed, it should spring back to its natural shape. Soft flesh means the fish is not fresh.
The scales should also be shiny and firm, and should not come off when you run your hand over them.