"trade declines" is correct and usable in written English.
You can use it to express that trade is decreasing or going down. For example: "Research shows that trade declines when there is an economic downturn."
Countries focus inwards, trade declines as a proportion of GDP and growth shrinks.
On the other hand, purpose-built aerotropolei have problems of their own: cities built to facilitate global trade will be prone to suffering when trade declines during economic downturns.
The best estimates I have seen for trade declines in the Depression were of a drop of between 50% and 66% in world trade, but my guess is that the big drops were concentrated among a smaller number of large players back then.
Global trade expands by 11percent while intra-EU trade declines by 2percent.
Global trade declined by 8% between December 2014 and May 2015.
From 1929 to 1934, overall world trade declined by some 66percentt.
Industrial production grew last year and, thanks to ties with China, trade declined only slightly.
The coal trade declined drastically after 1918 and was diverted from Port Talbot completely after 1953.
As a result, the terms of trade declined precipitously for producers of primary commodities.
Atlantic City's resort trade declined in the decades following World War II.
Cuba's overall trade declined sharply after the Soviet Union dissolved in 1991.
