Swordfish (Xiphias gladius) does not seem to have scales when one looks at a sample. Some say that it has scales that are embedded to such an extent that it is impossible to remove them without making a hole. Others say that it has kosher scales on parts of its body which fall off during its development. Still others claim that it may have some kosher scales even at the time of harvest. The Orthodox Union traditionally treats swordfish as non-kosher.
See Sh’ailos and Teshuvos Tzitz Eliezer 9:40 who discusses a statement made by the Knesses HaGedolah about “cherev hadag” and explains why we cannot use the statement to permit swordfish.
14. Why is swordfish not kosher?
The adult swordfish has no scales. Even the scales of a young swordfish are not true scales. A fish is only kosher if it has scales that can be removed from the skin. The scales of swordfish cannot be removed without destroying the skin. Therefore swordfish is not kosher.