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Unarmored Threespine Stickleback-Gasterosteus aculeatus williamsoni

This is a very small fish, about 2 inches long, This highly endangered fish can be found locally in San Francisquito Creek and the “Lagoon” at the St. Francis Dam Disaster site. This fish has no scales, is only found in fresh water, and it is highly endangered. San Francisquito Creek is one of the few places this fish can be found.  Partially plated Threespine Sticklebacks can be found in other rivers and lakes, but the species in San Francisquito have not interbred with any other sticklebacks, as there are no tributaries to San Francisquito Creek.

The males build nests with mucus threads secreted from their kidneys. They then lure their prized female into their well-built nest. After egg fertilization, the female leaves at the request of the male, while the nest, eggs and fry are fiercely protected by the male.  Despite their size of only 2 inches, the males can be very aggressive.

After the fry hatch and are old enough to leave the nest, the male will most likely make another nest to spawn again, or join a school of Sticklebacks in order to feed more efficiently.