Traditionally, fish that spend at least part of their lives in brackish or fresh water were considered unsuitable for sashimi because of the possibility of parasites. For example, salmon, an anadromous fish, is not traditionally eaten straight out of the river.[citation needed] A study in Seattle, Washington, showed that all wild salmon had roundworm larvae capable of infecting people, while farm-raised salmon did not have any roundworm larvae.[9]
Freezing is often used to kill parasites. According to European Union regulations,[10] freezing fish at −20 °C (−4 °F) for 24 hours kills parasites. The U.S. Food and Drug Administration (FDA) recommends freezing at −35 °C (−31 °F) for 15 hours, or at −20 °C (−4 °F) for 7 days.[11]