Who are selling it?
Check also with Long John Silver and Burger Kings both are serving fish burger derive from Hoki fish, as for Mcdonald may use Hoki fish and Pollack (Kosher) please check with your country to confirm and some Japanese food stores outlets may serve. Most odd or strange looking design fish are usually non-kosher. Rattail shows like eel like structure. More information about this fish.
The blue grenadier, hoki, blue hake, New Zealand whiptail, whiptail or whiptail hake, Macruronus novaezelandiae, is a merluccid hake of the genus Macruronus, found around southern Australia, and New Zealand, at depths of between 10 and 1,000 m. Its length is between 60 and 120 cm.
The blue grenadier is similar in appearance to the rattails, with large eyes and a very elongate tapering body ending in a pointed tail consisting of the fusing of the second dorsal fin and the anal fin. There is a short spinous first dorsal fin. This fish-eating carnivore has a large mouth containing sharp pointed teeth used to catch and eat lantern fish, its main prey, but crustaceans and squid are also taken.
Body colour is pale blue-green above, silver on the sides and belly, and the fins are darker.