by James Hobbs and Peter Moyle

Staff with the California Department of Fish and Wildlife trawl for Delta smelt in the Sacramento-San Joaquin Delta in 2015. (Image credit: Amy Quinton, Capitol Public Radio)

The results of 2017 surveys of Delta fishes are coming in. Already, the results are clear:  it was an unhappy year for Delta smelt.

The wet year with high outflows should have created an increase in the population, as happened in 2011.  Instead numbers stayed extremely low.  The US Fish and Wildlife Service (USFWS) estimated abundance of adults from January to February 2017 at approximately 48,000 fish. (The USFWS completed a revised adult delta smelt abundance estimation based on the CDFW’s SKT data for January and February; the point estimate was 47,786 but with confidence intervals from 22,000 to 92,000.) While this might seem like a lot of fish, for a pelagic forage species this is really low.

Abundance of Delta Smelt as reflected in the Fall Midwater Trawl index and Summer Townet index. Redrawn from data reported by the Region 3 Bay-Delta Fish and Wildlife Office.

Meanwhile the California Department of Fish and Wildlife (CDFW) Spring Kodiak Trawl Survey (SKT) index was only 3.8; this survey is aimed at adult Delta Smelt between January and May.  The CDFW SKT index has ranged from 1.8 (2016) to 130.2 (2012); the 2017 index was based on a total catch on only 39 fish. Fish surveys conducted in tidal marsh by UC Davis did not capture a single Delta Smelt in Suisun Marsh and only 1 adult in the Petaluma River-marsh in April.

Two measures of the year’s reproductive success of Delta Smelt are also taken by USFWS and CDFW. CDFW produces an index of abundance for juveniles (Summer Townet Survey-STN) and sub-adults (Fall Midwater Trawl Survey-FMWT). The STN Index ’increased’ in 2017 to 0.2, although the previous two years the index was zero. An index of zero does not mean the smelt abundance was zero, rather fish were just not captured at the index stations. We know there were still fish around because the next survey up, FMWT, caught a few delta smelt, so they were just really low in abundance. The FMWT index was lowest on record, 2, representing a total catch of only 2 individuals in October among the index stations.

The US Fish and Wildlife Service (USFWS) recently began a new monitoring program called the Enhanced Delta Smelt Monitoring Program (EDSM) which employs slightly different sampling methods to produce estimates of total Delta Smelt abundance.  Abundance estimates from USFWS varied wildly from week to week in 2017, but appeared to collapse in July and remain low through the remainder of the season.

Delta Smelt abundance estimates from the EDSM surveys, July –September 2017. Different sampling areas are represented by different colors. Vertical lines are confidence intervals around each estimate. Figures from USFWS-EDSM Sept 22 2017.

What happened in July? We can only speculate:  July is usually a month of very warm temperatures in the Delta.  Catches of Delta Smelt in the Cache Slough region and Sacramento Deepwater Ship Channel coincided with very warm water 22-23 °C  (72-73 °F).  Smelt would have been extremely stressed by these warm waters and would have likely moved downstream to the western region of the Delta where water temperatures were 2-3 °C cooler.  This may explain the increased abundance in survey 3 (Figure 2) in Suisun Bay and Marsh.  What happened after that is anyone’s guess. We certainly didn’t see these fish in our Suisun Marsh surveys.

Abundance of delta smelt remained low in all surveys after September, which interestingly, coincided with a USFWS decision to relax ESA requirements in order to maintain freshwater outflow for the fish starting in October.  Since October 1, the special EDSM program encountered only 17 Delta Smelt in a total of 366 tows.  December abundance estimates were down to less than 4,000 fish. While the abundance is extremely low, targeted efforts by UCD researchers at the Fish Conservation and Culture Laboratory were able to collect 100 delta smelt for the captive breeding program in one day, from a known area of concentration. So Delta Smelt are not yet extinct, but numbers have never been this low.

At this point it is worth reviewing some basic facts about delta smelt biology.  First, they basically have a one-year life cycle in the wild, although a few fish live a second year. Delta Smelt move upstream in the fall and aggregate in the North Delta, where presumably most spawning takes place, although beaches along the Sacramento River near Rio Vista likely are attractive to spawners (Figure 3). Fish spawn between February and June.  Most fish die after spawning, although individual smelt can spawn multiple times over several months (Le Cava et al. 2015).

Catches of delta smelt by the Spring Kodiak Trawl in January 2017, when the fish should be aggregating for spawning. This survey concentrates on delta smelt.

Most smelt spend the first few months of their short lives in western Delta (Sacramento River) or as resident fish in a few areas in the north Delta. Smelt are largely absent today from the South Delta, except when carried there by cross-Delta water movement generated by the SWP and DWR pumping plants.  Delta Smelt feed on zooplankton and spend their lives in the larger channels and bays of the upper SFE, near the surface. The life history described above actually has more flexibility than we describe; for a more nuanced view see Moyle et al. (2016) and Hobbs et al. (2017).

Some fishes with similar habitat requirements for juveniles (young of year) but with more complex life histories than Delta Smelt showed a positive response to the wet conditions.  For the FMWT, both juvenile Striped Bass and American Shad in 2017 showed some of their highest indices in recent years.  They also were abundant in Suisun Marsh and in other sampling programs.  Longfin Smelt, a distant cousin to the Delta Smelt, also showed a small uptick in abundance, while two other species monitored by FMWT, Sacramento Splittail and Threadfin Shad, showed continued low numbers. However, we note that the FMWT is a poor tool for sampling splittail and Threadfin Shad and these species are abundant in Suisun Marsh, with no strong trends (Moyle, unpublished data).

The question then becomes, why didn’t Delta Smelt respond to improved conditions as did other fishes?   One possible explanation is that there was so much water last winter that smelt were more dispersed than usual and had a hard time finding mates. This dispersion is reflected in the distribution of reproductive fish in January 2017 from the Spring Kodiak Trawl Survey. Reproductive smelt were scattered from the Sacramento Deepwater Ship Channel in the North Delta to the Napa River, with the majority of catches being single individuals. As further evidence for the distribution problem, we caught one adult in the Petaluma River in April, where they have never been encountered before. 

The abrupt decline in abundance detected by the EDSM survey in late July (Figure 2) may be an indicator that Delta Smelt recruitment (survival from larvae to adulthood) is impacted by summer temperatures. Research on temperature tolerance suggests Delta Smelt are very sensitive to warm waters (Komoroske et al. 2014; 2015, Jeffries et al. 2016). Since the beginning of drought in 2012 water temperatures have been creeping up, warming earlier and cooling off later in the year. Is this a signature of climate change? It may be too early to say, but if this year stays dry with low outflows in late summer, smelt that survive will have to have find cool-water refuges somewhere, perhaps the lower Sacramento River.

When numbers are so low, as they clearly are for smelt, random factors in sampling, in distribution of spawners, in spawning success, and other factors can make a big difference to the total population or to the indices.  If smelt are concentrated in just a few places for spawning, then physical changes in the spawning habitat or coincidence of spawning with concentrations of egg and larval predators (e.g., Mississippi Silverside) can be lethal.   Competition from the introduced, similar Wakasagi Smelt could be problem, which seems to be increasing in numbers.  Wakasagi also hybridize with Delta Smelt, but the offspring are apparently sterile, so this could interfere with spawning in the wild.  In other words, when Delta Smelt numbers are low, many things can keep them from rebounding, pushing them closer to extinction.

So the coming year does not look like a happy one for Delta Smelt. So far, California appears to be back in a drought pattern. Furthermore, protections from freshwater exports appears to be in question. In late December the U.S. Bureau of Reclamation announced their intentions to maximize water deliveries, as well as review and consider modifications to the 2008 RPAs for protecting smelt. One of these RPAs, the Fall X2 Action (RPA Component 3), calls for maintenance of higher than normal flows in the fall to maintain low-salinity habitat in Suisun Bay following years of above normal or high flows. In October, these regulations were relaxed by USFWS under petition from the US Bureau of Reclamation³. This will likely be something the Bureau will pursue further in their request for modifications. In addition, the Interior Department announced they will be working on changes to the Endangered Species Act under the guise of the Trump administrations Unified Agenda of Regulatory and Deregulatory Actions. Its unclear what changes to protective measures for Delta Smelt will occur in 2018, but it appears the changes will not favor the fish.

If our wishes were fishes, Delta Smelt might survive.

James Hobbs is a research scientist with the UC Davis Department of Wildlife, Fish and Conservation Biology. Peter B. Moyle is a UC Davis Professor Emeritus of fish biology and an associate director of the Center for Watershed Sciences.

Further reading:

Hobbs, J.A, P.B. Moyle, N. Fangue and R. E. Connon. 2017. Is extinction inevitable for Delta Smelt and Longfin Smelt? An opinion and recommendations for recovery.  San Francisco Estuary and Watershed Science 15 (2):  San Francisco Estuary and Watershed Science 15(2). jmie_sfews_35759. Retrieved from: http://escholarship.org/uc/item/2k06n13x

Jeffries, K.M., R.E. Connon, B.E. Davis, L.M. Komoroske, M.Britton, T.Sommer, A,E. Todgham, and N.A. Fangue. “Effects of high temperatures on threatened estuarine fishes during periods of extreme drought.” Journal of Experimental Biology 219, no. 11 (2016): 1705-1716.

Komoroske, L. M., R. E. Connon, J. Lindberg, B. S. Cheng, G. Castillo, M. Hasenbein, and N. A. Fangue. “Ontogeny influences sensitivity to climate change stressors in an endangered fish.” Conservation physiology 2, no. 1 (2014).

Komoroske, L.M., R.E. Connon, K.M. Jeffries, and N.A. Fangue. “Linking transcriptional responses to organismal tolerance reveals mechanisms of thermal sensitivity in a mesothermal endangered fish.” Molecular ecology 24, no. 19 (2015): 4960-4981.

LaCava, M., K. Fisch, M. Nagel, J. C. Lindberg, B. May, and A. J. Finger. 2015. Spawning behavior of cultured delta smelt in a conservation hatchery. North American Journal of Aquaculture 77: 255-266.

Moyle, P. B., L. R. Brown, J.R. Durand, and J.A. Hobbs. 2016. Delta Smelt: life history and decline of a once-abundant species in the San Francisco Estuary. San Francisco Estuary and Watershed Science14(2) http://escholarship.org/uc/item/09k9f76s

Moyle, P.B., J. A. Hobbs, and J. R. Durand.  2018.  Delta smelt and the politics of water in California.  Fisheries. In press (February).