Unarmored threespine stickleback, Gasterosteus aculeatus williamsoni

Unarmoured threespine stickleback (Photo ©  Sam Stewart)

Federal status: Endangered
State status: Endangered

(=) Stable


Gasterosteidae, related to the seahorse family

Creeks and rivers from Santa Barbara to San Bernardino counties


A small (5 cm), scaleless, freshwater fish that lives for only one year. As the name implies, it has three spines on its back. The two spines closest to the head are smaller and sharp.

Habitat & Behavior

Unarmored threespine stickleback occurs in slow-moving, freshwater micro-habitats in streams and rivers shaded by dense vegetation. They require clear, flowing, well-oxygenated water with associated pools and eddies.  Reproduction occurs from October to January in areas with adequate aquatic vegetation and slow-moving water where males can establish and vigorously defend territories. They feed primarily on benthic insects, small crustaceans, and snails. 


The primary threats to the unarmored threespine stickleback are urbanization, eutrophication (excess nutrients), stream channelization, groundwater removal, and reduced water quality. Recent prolonged droughts from 2012 to 2016 have significantly impacted the year-round freshwater levels of pools and streams and creeks.


A number of federal and state agencies have identified critical habitat for the unarmored threespine stickleback in Los Angeles County. Introductions were undertaken by the California Department of Fish and Wildlife in the Santa Francisquito Creek in 2015 to increase the range of the species.

Where can you spot it?

The unarmored threespine stickleback is almost impossible to see in its native habitat. However, an appreciation of their habitat can be gain from designated trails along Santa Francisquito Creek.


  1. Unarmored Threespine Stickleback (Gasterosteus aculeatus williamsoni) 5-Year Review: Summary and Evaluation. 2009. U.S. Fish and Wildlife Service, Ventura Fish and Wildlife Office, Ventura, California. [link]