A buckling is a form of hot-smoked herring

Herring

Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...

 similar to the kipper

Kipper

A kipper is a whole herring, a small, oily fish, that has been split from tail to head, gutted, salted or pickled, and cold smoked.In the United Kingdom, in Japan, and in some North American regions they are often eaten for breakfast...

 and the bloater

Bloater (herring)

Bloaters are a type of smoked herring. They are different from the kippers as they are smoked whole, with its innards intact which gives it its unique gamey flavour. The bloater is associated with Great Yarmouth, England; the kipper with Scotland and the Isle of Man...

. The head and guts are removed but the roe

Roe

Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...

 remain. They may be eaten hot or cold.

Origin

The word may come from the German Bückling or the Swedish böckling both words denoting a hot smoked variety of the kipper

Kipper

A kipper is a whole herring, a small, oily fish, that has been split from tail to head, gutted, salted or pickled, and cold smoked.In the United Kingdom, in Japan, and in some North American regions they are often eaten for breakfast...

.

Difference from kipper

Buckling is hot-smoked whole, as opposed to the kipper, which is split and gutted, and then cold smoked.