de-parasiting mackerel for sushi
The FDA recommends below, which is how every restuarant I have ever heard of does it as well.

A recent survey of U.S. gastroenterologists has confirmed that seafood-borne parasitic infections occur in the U.S. with sufficient frequency to make preventive controls necessary during the processing of parasite-containing species of fish that are intended for raw consumption...[]..Freezing and storing at -4°F (-20°C) or below for 7 days (total time), or freezing at -31°F (-35°C) or below until solid and storing at -31°F (-35°C) or below for 15 hours, or freezing at -31°F (-35°C) or below until solid and storing at -4°F (-20°C) or below for 24 hours is sufficient to kill parasites. FDA's Food Code recommends these freezing conditions to retailers who provide fish intended for raw consumpution