Han Nijssen is a Dutch

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 ichthyologist. He is curator

Curator

A curator is a manager or overseer. Traditionally, a curator or keeper of a cultural heritage institution is a content specialist responsible for an institution's collections and involved with the interpretation of heritage material...

 at Zoölogisch Museum in Amsterdam

Amsterdam

Amsterdam is the largest city and the capital of the Netherlands. The current position of Amsterdam as capital city of the Kingdom of the Netherlands is governed by the constitution of August 24, 1815 and its successors. Amsterdam has a population of 783,364 within city limits, an urban population...

.
Nijssen has worked extensively with fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 from South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

, and is the author of several species, e.g. Corydoras weitzmani and Corydoras xinguensis.
Collaborating with Isaac Isbrücker he has described e.g. the group Hypancistrus

Hypancistrus

Hypancistrus is a genus of Loricariid catfish originating from the Amazon basin in South America. Unlike many of the other Loricariids, however, some Hypancistrus species are more carnivorous and enjoy meat in their diet...

and the species Hypancistrus zebra and Corydoras panda, among others.
He also has collaborated with Sven O. Kullander

Sven O. Kullander

Sven Oscar Kullander is a Swedish biologist specialised in ichthyology.He primarily researches cichlids – notably the genus Apistogramma and the Cichlasoma-complex – and other tropical fresh water fishes....

.

The species Corydoras nijsseni and Apistogramma nijsseni

Apistogramma nijsseni

Apistogramma nijsseni is a species of cichlid fish, endemic to the black water habitats in the Carahuayte river drainage, Peru. The species is popular aquarium fish amongst dwarf cichlid hobbyists....

are named after him.

Publications (incomplete)

• The Cichlids of Surinam: Teleostei: Labroidei - Sven O. Kullander and Han Nijssen (1989)