Much of it is due to the amount of offshore processing done in China after fish are caught in US waters - which means the US companies catching them are paying high duty rates on various types of seafood, including salmon, tilapia, and tuna, crab and shrimp, according to the Epoch Times, which notes that American companies have been sending their ocean harvests to China for processing for more than a decade in order to take advantage of historically cheaper labor and operating costs.
In fact, some 65% of the estimated 6.3 billion pounds of seafood consumed by Americans every year is reimported - a move which saves US companies around 80 cents per pound of fish in labor costs. Given the roughly 30 million pounds of salmon alone being sent to Chinese processing plants, the savings add up.