While there have been relatively few studies on O‘opu in general, Stenogobius hawaiiensis remains one of the two least studied species. More work needs to be done to understand the basic biology and ecology of this endemic fish. S. hawaiiensis is one of the smaller species, reaching a maximum length of 4-5 inches. This species can be found on all Hawaiian Islands, but is surprisingly abundant on Oahu, where it can be found in both altered and unaltered streams. Although it possesses a sucking disk, it is a poor climber, and is thus restricted to the lower reaches of streams as well as estuaries. Gut content analyses indicate that algae composes a major portion of the diet of S. hawaiiensis. This species has the ability to scrape algae from rocks, and is also capable of picking-biting and filtering food particles from the water column and sediments. Thus, S. hawaiiensis likely feeds substantially on algae, but occasionally feeds on invertebrates as well. There have been no formal studies done to examine the reproductive biology of S. hawaiiensis. Continuous research has been taking place, however, on this species at the Anuenue Fisheries research facility in Honolulu. This O‘opu is thought to breed year round due to the constant presence of juveniles in streams. During courtship, both males and females display 9-11 black bars on their flanks as well as a red border on the edge of their dorsal fins. Males develop a red flush on their throats and lower jaw. Eggs are laid on the upper surfaces of flat rocks or in crevices, but it is unclear whether the eggs are defended by either of the parents.
Photo by Yamamoto and Tagawa