Swordfish is not considered kosher. Why? Because it does not have the kind of scales required for kosher fish. Only scales that can be easily removed from the fish without ripping the skin are recognized as scales for kosher purposes. Since swordfish scales cannot be removed in this manner (when they are even present), the fish is not considered kosher.
For more, see All About Kosher Fish.
Young sword fish have easy remove scales... are they kosher?
It takes time for them to have the scales below skin level.
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They are nevertheless not considered kosher. For more about it see Which “Scales” Make a Fish Kosher? (and subtitle "Swordfish"). Reply
how about what the sefardiy poskim that say it is? why they are being disregarded Reply
Why are mackerel kosher? It is impossible to remove the microscopic scales without tearing the skin…exactly the same as swordfish skin. Reply
Mackerel shed their skin naturally when they are removed from water. As such, the scales are considered detachable. Reply
Shalom
Tuna when adult also looses its scales and it’s kosher.
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The scales may be rubbed off, but Mackerel most definitely do not shed their skin naturally when removed from water. Reply
Not all tuna is kosher, and thus should only be purchased with reliable certification. Reply
why tuna when adult looses scales like sword fish and its kosher? Reply
I find issue with this decision since, if you've ever caught an swordfish, you'd find that it takes no ripping to remove the scales and your entire boat and self would be covered in thousands of little silvery scales whether you liked it or not.