The Delta smelt is an endangered fish species endemic to the California Bay Delta region. It serves as an important ecological indicator of environmental conditions in the Delta. However, the smelt population has declined dramatically due to water diversions that have reduced and degraded its freshwater habitat. Conservation efforts to protect the smelt under the Endangered Species Act have caused conflicts with water users. Solutions proposed to help recover the smelt population include reassessing water allocations and enacting proactive management strategies to protect dependent species.

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Mike Stoever
Strategies in Watershed Management
Johns Hopkins University
No Fish Too Small:
The Role of the Delta Smelt as an Ecological Indicator in the California Bay-Delta
The Delta smelt (Hypomesus transpacificus) is a critically endangered fish species
endemic to the California Bay Delta region (specifically the Upper Sacramento-San Joaquin
estuary). While physically small (adults range from 2-3 cm), this species serves an incredibly
important role as one of the best indicators of environmental conditions in the Delta (CBD,
2016). Due to various demands on the water comprising its habitat, the Delta smelt has recently
fallen to record-low abundance. In order to prevent the species from becoming extinct, dramatic
reductions in the freshwater allocated to central and southern California from the Delta and San
Francisco estuary have been enacted (Center for Watershed Sciences, 2016). This has placed the
smelt in the crosshairs of water users, as agricultural and consumptive users downstream are
unhappy with the reductions in allocated water. Conservationists and those tasked with ensuring
Endangered Species Act (ESA) compliance (along with the overall health of the ecosystem) have
been left to fight for protection of the species and explain its importance to all stakeholders. This
paper provides further background on the issue and proposes potential solutions.
Once the most abundant fish species in the region, the Delta smelt could be found until
the late 1950s in Suisun Bay, Napa River, and in the waterways of the south and central Bay
Delta (USFWS, 2016). The population began declining when the first of two large water export
facilities went online and began pumping water out of the Delta, with the second facility starting
up in 1967 (and correspondingly pumping out even more water) (USFWS, 2016). This decrease
in water availability for the Delta smelt coincided with an increase in the amount of pesticides
and herbicides contaminating the Delta, further degrading its habitat (USFWS, 2016). It was also
at this time that the Delta began dealing with an influx of exotic invasive species. These exotics
were likely transported via large transport ships and deposited when ballast water was dumped,
and include the exotic copepod Pseudodiaptomus forbesi, which has been replacing the main
food source of adult Delta smelt, the native copepod Eurytemora affinis (EPA, 2010; USFWS,
2016). While the Delta smelt appears to also consume this species, the full impact of its presence
is unknown (EPA, 2010). In 1986, another exotic species, the overbite clam, was discovered in
Suisun Bay and led to the elimination of much of the Delta smelt population there through its
increased consumption of phytoplankton, a vital component of the food web (EPA, 2010). From
this point forward, the ecological environment of the Bay Delta was radically changed and
damaged and Delta smelt numbers suffered as a result (USFWS, 2016). After plunging to
worrisome levels in the 1980s, the species was listed under the ESA in 1993 and has continued to
decline since (USFWS, 2016). It faces the edge of extinction today, as existing regulatory
mechanisms intended to protect the species have not proven adequate and have allowed Delta
smelt population size to decline by more than 90 percent over the past 10 years (IUCN, 2014;
USFWS, 2016). Current data shows that the remaining population of Delta smelt is so low and
scattered in such numbers that the species is at risk of falling below an effective population size
and is unable to reproduce, with only four females and two males observed in a recent count
(IUCN, 2014; Kay, 2015).

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As noted above, the Delta smelt is closely associated with the freshwater-saltwater
mixing zone found in the Upper Sacramento-San Jaoquin estuary (Moyle et al., 1992). While a
euryhaline species, it is seldom found where saltwater makes up more than one-third of the total
water (EPA, 2010). This is due to its reliance upon freshwater during the spawning season,
which often occurs during March, April, and May (Moyle et al., 1992). Due to its short life cycle
(one year) and low fecundity, the Delta smelt is particularly sensitive to changes in estuarine
conditions such as those presently occurring as a result of drought and increased river diversions
for water supply and agricultural use (Moyle et al., 1992; EPA, 2010). As more and more of the
water inflowing to the Delta is diverted, the mixing zone where the Delta smelt makes its home
is being restricted to a relatively small area of deep river channels and has increased entrainment
into the diversions themselves (Moyle et al., 1992). This poses a major issue for the Delta smelt,
as the main food source of its larvae, microzooplankton (which they require in abundance) are
limited in these compressed mixing zones, where the current found in the deeper river channels
may be faster and more turbulent and thus limiting the production of microzooplankton by the
inability of sunlight to penetrate water depths (EPA, 2010). In turn, this restricts the amount of
food available to Delta smelt larvae, if offspring make it to that stage. Females of the species
broadcast eggs during the spawning season that adhere to stable surfaces at the bottom of river
channels, the stability of which are threatened by this increase in turbulence (EPA, 2010).
Further, as movement of the species is reliant upon the outflows that are being lowered due to
water diversions, the species is effectively trapped in these new, narrower habitats with a reduced
food source on which to rely.
While it might be easy to dismiss the importance of a single, small fish species such as
the Delta smelt, doing so would be unwise. The species plays a crucial role in the ecosystem’s
food web, serving as a food source for such well-known species as striped bass, white catfish,
black crappies, longfin smelt, Chinook salmon, and sturgeon (EPA, 2010; CBD, 2016). If the
Delta smelt were to go extinct (as an extinction analysis completed in 2006 predicted could occur
within the next 10 years), these populations would undoubtedly suffer (CBD, 2016). As such, the
role of the Delta smelt is greater than simply being prey for its predators; the species’ most
important role is as an ecological indicator for the health of the Bay Delta ecosystem as a whole.
If the population of Delta smelt in the ecosystem is alive and well, then it is reasonable to believe
that other fish, birds, mammals, and plants are also thriving (Center for Watershed Sciences,
2015). Thus, its decline is a warning that other native Delta fish species that have also fallen to
alarmingly low levels (such as the aforementioned longfin smelt, Chinook salmon, and sturgeon)
may be lost as well, potentially starting a cascading effect of negative impacts and species loss.
Knowing the potentially catastrophic impacts of losing this species (and bound by federal
legislation, namely the ESA), federal and state officials have taken steps to dramatically reduce
the amount of freshwater allocated from the Delta and San Francisco Estuary to central and
southern California (Center for Watershed Sciences, 2016). These steps include the reduction of
water deliveries when pumps are found to harm the species and/or when its habitat is too saline
(Kay, 2015). In some years, this has resulted in reductions of Delta exports of 15 percent to
protect the smelt, along with winter-run Chinook salmon that as previously stated rely upon the
smelt as a food source (Center for Watershed Sciences, 2015). Increased drought conditions have
led downstream users (such as those in the agricultural and domestic water supply industries) to
lament these reductions, as 70 percent of California’s water supply originates in the north while
70 percent of its demand is in the south (LAO, 2013). This outcry has led the state to declare
emergency waivers allowing it to bypass the minimum flows required by the ESA for the species

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and continue to export water, which it has done as recently as 2014 (Kay, 2015). Citing this
management strategy as being “arbitrary and capricious”, water contractors, agricultural
irrigation districts, and the Metropolitan Water District sued the federal government in 2009 over
pumping restrictions made on behalf of the Delta smelt (Kay, 2015). However, the Court of
Appeals for the Ninth Circuit ruled in 2014 that any diversion resulting from bypassing federally
mandated minimum flows jeopardizes the fish and its habitat, a view supported by the National
Research Council (NRC) (NRC, 2010; Kay, 2015).
As the current drought facing California shows no sign of abating, it is imperative that the
importance of the Delta smelt is highlighted to all parties and further steps be taken to protect the
species and those that rely upon it as a food source. Water for the environment (and thus the
Delta smelt) is under-allocated in the state, while water for humans is over-allocated and based
upon a time when rain and snowfall was abundant; therefore, a reassessment of these allocations
using current data and projections that include continual drought conditions is necessary (Center
for Watershed Sciences, 2015; Kay, 2015). Further, proactive management strategies need to be
enacted in order to prevent other fish and animal species from facing fates similar to the Delta
smelt, strategies that require learning more about their requirements and managing parts of the
ecosystem specifically for their benefit. Yet another potential solution includes conducting
further research focusing on reducing exports versus increasing inflows on behalf of the Bay
Delta ecosystem, such as what was suggested by NRC (2010). This leads to the final potential
solution suggested here, that as both human and wildlife populations rely on the same limited
water supplies, a water use balance must be struck and its consequences truly understood and
accepted by all parties, echoing Simonetti (2015).
References:
CBD (Center for Biological Diversity). 2016. Delta Smelt. Available online at:
http://www.biologicaldiversity.org/species/fish/Delta_smelt/. Accessed August 5, 2016.
Center for Watershed Sciences. 2015. Q & A on survival of California’s delta smelt. Available
online at: https://californiawaterblog.com/2015/05/17/q-a-on-survival-of-delta-smelt-on-
air-and-off-air/. Accessed August 9, 2016.
Center for Watershed Sciences. 2016. Ecology of Delta Smelt. Available online at:
https://watershed.ucdavis.edu/project/ecology-delta-smelt. Accessed August 5, 2016.
EPA (U.S. Environmental Protection Agency). 2010. Endangered Species Facts: Delta Smelt.
Available online at: https://www.epa.gov/sites/production/files/2013-08/documents/delta-
smelt_0.pdf. Accessed August 9, 2016.
IUCN (International Union for the Conservation of Nature). 2014. IUCN Red List of Threatened
Species: Hypomesus transpacificus. Available online at:
http://www.iucnredlist.org/details/10722/0. Accessed August 10, 2016.
Kay, J. 2015. Delta Smelt, Icon of California Water Wars, Is Almost Extinct. Available online at:
http://news.nationalgeographic.com/2015/04/150403-smelt-california-bay-delta-
extinction-endangered-species-drought-fish/. Accessed August 9, 2016.
LAO (Legislative Analyst’s Office). 2013. Imported Water as a Percent of Use by Water Region.
Available online at: http://www.lao.ca.gov/reports/2013/calfacts/Cal-Facts-2012_for-
HTML-web-resources/image/10922.jpg. Accessed August 9, 2016.

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Moyle, P. B., B., Herbold, D. E. Stevens, and L. W. Miller. 1992. Life History and Status of
Delta Smelt in the Sacramento-San Joaquin Estuary, California. Transactions of the
American Fisheries Society 121(1):67-77.
NRC (National Research Council). 2010. A Scientific Assessment of Alternatives for Reducing
Water Management Effects on Threatened and Endangered Fishes in California's Bay-
Delta. Washington, DC: National Academies Press.
Simonetti, J. 2015. The Dwindling Delta Smelt Population: What does it mean for the species
and humans that rely on the Delta? Available online at:
http://hydrowonk.com/blog/2015/04/06/the-dwindling-delta-smelt-population-what-does-
it-mean-for-the-species-and-humans-that-rely-on-the-delta/. Accessed August 9, 2016.
USFWS (U.S. Fish and Wildlife Service). 2016. Delta Smelt (Hypomesus transpacificus).
Available online at: https://www.fws.gov/sfbaydelta/species/delta_smelt.cfm. Accessed
August 9, 2016.