Eucyclogobius newberryi was split into two and the southern portion of its range is now attributed to Eucyclogobius kristinae (Swift et al. 2016).

This species inhabits mouths of creeks in a relatively small range in California. Due to pervasive coastal development, its range has been reduced and continues to decline. Its estimated area of occupancy (AOO) is 600 km². Additional major threats include competition from invasive species and climate change. Although the species occurs in more than ten locations, its population is considered severely fragmented. It is continuing to decline and is assessed as Vulnerable (VU B2ab(i,ii,iii,iv,v)). Some conservation measures are in place, but additional work is needed.

This species is endemic to California, USA, and is patchily distributed from Tillas Slough in the mouth of the Smith River south to Santa Monica Artesian Springs (Swift et al. 2016, Love et al. 2021). The species is naturally absent due to lack of suitable habitat between the Eel River (Humboldt County) and Ten Mile River (Mendocino County), between Lagoon Creek (Mendocino County) and Salmon Creek (Sonoma County), and between the Salinas River (Monterey County) and Arroyo del Oso (San Luis Obispo County) (Swift et al. 1989, USFWS 2007). Records of this species south of Santa Monica are now attributed to Eucycylogobius kristinae (Swift et al. 2016). The depth range for this fish is 1–15 m (Love et al. 2021).

Based on a minimum convex polygon drawn around the extent of its extant range, its estimated extent of occurrence (EOO) is 115,971 km². Based on a 2x2 km grid overlaid on the localities, the estimated area of occupancy (AOO) is 600 km². It sis estimated to occur in more than 10 locations.

Populations of this species are naturally highly dynamic (Lafferty et al. 1999). When lagoons are breached due to flood events during the rainy season, populations decrease and then recover during the following summer (Lafferty et al. 1999). There is no explicit marine life history phase; hence, frequency of population interactions and genetic exchange among different coastal lagoon subpopulations is restricted but sometimes does occur from nearby subpopulations (Lafferty et al. 1999, McCraney and Kinziger 2009). It occurs in isolated populations and sustains a limited ability to recolonize beyond 10–15 km, making it easier for populations to become extirpated (Swift et al. 2016).

According to USFWS (2007), no range-wide, long-term monitoring program was currently being conducted and about 50% of the remaining populations were considered vulnerable to extinction due to severe habitat degradation. Surveys indicate that this species no longer occurs in 16% of the 135 historically documented localities, and with its southernmost localities now attributed to Eucycylogobius kristinae, this number will increase (USFWS 2014, Swift et al. 2016). Note that the loss of the species from these southernmost sites is due to the taxonomic change rather than extirpation of the species from those sites. The future persistence of populations in 50 to 75 of these localities is uncertain (USFWS 2014). Populations from the San Francisco Bay no longer exist (USFWS 2014). Populations in Humboldt Bay are believed to be among the largest for this species, although studies suggest that isolation and low genetic diversity in these populations may increase the risk of extirpation (USFWS 2014). The Lake Earl population may also be vulnerable to extirpation due to severe population bottlenecking (USFWS 2014).

The largest and most stable populations of this species occur in estuaries and lagoons ranging from 2 to 50 hectares (0.02 to 0.5 km²), and are localities that remain mostly untouched by humans (USFWS 2014). There are only 10 of these stable source populations, while 55 to 70 localities are so small that extended persistence is uncertain (USFWS 2014). Even populations inhabiting larger areas and with close proximity to other populations sustain the risk of extirpation (Lafferty et al. 1999).

This benthic fish occurs in small coastal lagoons, lower reaches of streams, and uppermost portions of large bays (NatureServe 2014). It is most abundant in the upper ends of lagoons created by small coastal streams (Swift, in Moyle 1976). In lower sections of coastal streams, it occurs in fresh to brackish water (preferably less than 10 ppt) (NatureServe 2014). It occurs in vegetated pools of slow (but not stagnant) areas of streams and has been documented in ponded freshwater habitats as far as 8 km upstream from San Antonio lagoon in Santa Barbara County (NatureServe 2014). Generally, it occurs in water 25–100 cm deep and can complete its life cycle in fresh or brackish water (NatureServe 2014). It tolerates temperatures of 8–23°C and salinities of 0–40 ppt and prefers mud substrates and areas of high dissolved oxygen (NatureServe 2014).

Spawning occurs on substrates of coarse sand, in burrows dug by males usually in water 25–50 cm deep (Swift et al. 1989). Larvae are found midwater around vegetation until they become benthic (Swift et al. 1989). The juvenile stage is short, lasting only a few weeks or until individuals reach sexual maturity between 2.5 and 2.7 cm standard length (Chase et al. 2016). This species generally only lives for one year in the wild (Chase et al. 2016). The maximum standard length is 5.7 cm (Love et al. 2021).

This species is threatened by habitat loss resulting from anthropogenic modification and climate change, habitat fragmentation and decreased genetic exchange, and competition and predation from native and non-native fish species (USFWS 2014, Swift et al. 2016, Chase et al. 2016).

Though conservation efforts exist to limit habitat degradation, the United States Fish and Wildlife Service (2014) states that degradation and habitat alteration is ongoing across this species' current distribution, and is expected to continue. This includes development and conservation projects such as incompatible habitat restoration initiatives, annual dredging, and bridge widening (USFWS 2014). Additionally, ongoing environment factors such as sea level rise and droughts threaten to further isolate and diminish populations (Swift et al. 2016). Sea level rise is anticipated to pose a significant threat to this species by engulfing coastal lagoons and bays, raising salinity levels throughout its preferred habitat, and consequently disrupting its typical reproduction cycles (USFWS 2014). Severe droughts that occurred from 2013 to 2015 in central and southern California have caused several known population extirpations, with drought frequency and intensity in California projected to increase in coming years (Swift et al. 2016, Stewart et al. 2020).

Competition between juveniles of this species and the native Threespine Stickleback (Gasterosteus aculeatus) may limit successful reintroduction after extirpation (Chase et al. 2016). In most cases, competition with G. aculeatus lead to reduced growth and increased stress and mortality, making reintroduction in many localities across its range more challenging (Chase et al. 2016).

This species is not utilized.

This species has been listed under the U.S. Endangered Species Act of 1973 since February 1994 (USFWS 2014). This limits the scale of coastal development throughout its range, and requires strict procedures to be followed to ensure minimal damage to this species and its habitat (USFWS 2014). It also requires state governments and military leadership to comply with conservation regulations (USFWS 2014). Several attempts have been made to reintroduce this species to previously populated localities (Swift et al. 2016).

Research is needed to understand the effectiveness of reintroduction efforts, as this species' variable population dynamics seem to make locality based persistence following reintroduction unclear (USFWS 2014).