Marlin (in the family Istiophoridae) is considered a billfish, due to its spear-like bill (snout). It is a well-fished species and is usually consumed in smoked form. However, some tend to stay away from marlin consumption due to its relatively high mercury content.
However, if you follow a kosher diet, it doesn’t matter that marlin has a high mercury content, because its not fit for consumption anyway: According to Chabad, who received their information from the Orthodox Union, marlin is NOT a kosher fish.
According to the Torah, the markers for a kosher fish are scales and fins. If they have scales you can assume they have fins as well. Striped marlin has scales and fins, therefore is kosher