Gasterosteus aculeatus williamsoni

Fricke et al. (2024) treat Gasterosteus aculeatus williamsoni as a full species that includes G. aculeatus microcephalus as a synonym. A plateless form occurring in Shay Creek, San Bernardino County, California, has been identified as G. a. santaeannae (or santa-annae) but is recognized as synonymous with G. a. williamsoni (Ross 1973; Moyle et al. 1989).

This taxon's range and abundance have been significantly reduced relative to historical levels. Many of the threats that precipitated steep declines continue today.

This taxon occurs in isolated locations within three separate watersheds in California: the upper Santa Clara River and its tributaries in Los Angeles County; San Antonio Creek on Vandenberg Air Force Base in Santa Barbara County; the Shay Creek vicinity, in San Bernardino County; and San Felipe and Pine Valley Creeks in San Diego County (USFWS 2009, 2021). Based on analysis of element occurrences by HUC 8 watershed and considering the discontinuous distribution of this taxon within these watersheds, the range extent of this taxon is between about 10,000 and 20,000 km2.

The California Natural Diversity Database (CNDDB) identifies 13 element occurrences of this taxon and, when combined with two occurrences in San Diego County not identified by CNDDB, there are 15 occurrences of this taxon.

USFWS (2009, 2021) identify threats as stream and river channelization, loss and fragmentation to urban development, indirect impacts of agricultural run-off, excessive groundwater extraction, invasive, non-native plant species, introduction of non-native predators, and toxic spills and discharges into waters supporting this taxon. Channelization of rivers and stream has occurred extensively throughout the current and historical range of this taxon. Channelization generally renders waterways unsuitable by reducing or eliminating nesting substrate. Extensive urban development in south coastal watersheds occupied by this taxon have reduced populations by introduction of pollutants and stormwater run-off containing pesticides and herbicides. Similarly, siltation and potentially toxic run-off from agricultural adjacent to some occupied streams may reduce available prey and adversely affect water quality. Many of the basins in southern California have experienced lowered water tables from long-term extraction of groundwater without sufficient recharge thereby reducing, or in some cases eliminating surface flows. Non-native plants, especially giant reed (Arundo donax), dominate the water profile in many areas reducing availability for this taxon and other fish and wildlife. The potential for accidental toxic chemical spills from railways and roadways exists throughout this taxon’s range. Many of the rivers and streams occupied by this taxon now harbor non-native predators including African clawed frogs, bullfrogs (Rana catesbeiana), red swamp crayfish (Procambarus clarkii), signal crayfish (Pacifastacus leniusculus), and various species of fishes, especially bass (Micropterus spp.), catfish (Ictalurus spp.), sunfish (Lepomis spp.), and mosquito fish that feed on various life stages of this taxon. Finally, impacts from a changing climate change include changes in water quality (especially water temperature), changes in timing and duration of flows, and extended drought. Increased temperatures and variable water availability from uncertain precipitation and snow run-off patterns may be primary factors effecting this species (USFWS 2019b). In a comprehensive review of climate vulnerability of at-risk freshwater fish, Moyle et al (2013) assessed 164 taxa against exposure and sensitivity criteria within four generally accepted and relevant climate scenarios. This taxon was classified as “moderately vulnerable”.