Is "management science" a subsection of economics?
The journal Management Science is a decent one. Many economists do publish on that journal. In fact, a quick look at its editorial board reveals many notable names, with the recent Nobel laureate in Economics Richard Thaler among the associate editors.

The field of management science is more interdisciplinary in nature. See Wikipedia's entry for it. To me, it's like operations research for business or applied microeconomics in the context of business decision-making. To call it a "separate science" is a bit too much, though.