American shad are the Chinook salmon of herring. Like Chinook salmon, adult American shad spend several years in the ocean before returning to rivers for spawning — they’re anadromous. Also, like salmon, anglers love shad because they’re fun to catch and good to eat. Well, okay, shad are not as good to eat as ocean caught salmon, but they are delicious when prepared well.
American shad are native to the east coast of North American. They were introduced to the Sacramento River after repeated releases of shad fry — transported via the then newly completed transcontinental railroad — between 1871 and 1880. Shad quickly became abundant in the Sacramento River and proceeded to colonize many other rivers along the West Coast. In the Columbia River system, shad are now the most abundant anadromous fish species migrating upstream through impoundments on the lower Columbia River. At Bonneville Dam — 140 river miles upstream of the Columbia River mouth — an average of 2.2 million adult shad migrated upstream each year between 1992 and 2003. In contrast, an average of 940,000 adult salmonids migrated annually past Bonneville Dam during the same period.
Adult shad return to rivers for spawning in April through June. They are serial broadcast spawners, meaning they release gametes into open waters in small pulses over a period of days or weeks. Shad have a very high fecundity, producing upwards of 300,000 eggs per female. Embryos drift downstream just above the river bottom. Once hatched, juvenile shad rear in slower moving waters — river edges and backwaters — feeding on zooplankton, insects, and other invertebrates. Most shad move out to sea in the autumn of their first year of life, and then rear in the ocean for two to four years before returning to spawn. Some fraction of adult shad will return to spawn more than once — not all die after spawning.
The Sacramento River and its tributaries host what appears to be a very consistent and relatively large population of adult American shad, while fish species like striped bass and delta smelt have declined over the last 40 years. It is reasonable to ask, have adult American shad returns declined similarly? Unfortunately, no data is available to provide a definitive answer. A well-known journalist and long-time editor of Fish Sniffer magazine reported shad fishing on the American River is as popular now as it was in his youth. Having regularly fished for shad since 1985, I agree with his assessment. Adult shad have returned like clockwork in seemingly great abundance every year that I have gone looking for them. The quality of shad fishing in a given river changes from year to year, but that seems to be a consequence of how returning shad respond to available river flows that they experience, not a consequence of flows that occurred during prior-spawning and years that produced early rearing. For example, more shad tend to return to the American River and to stay in the river longer when spring-season releases from Nimbus dam are greater. In years with low spring flows on the American River, shad fishing is likely to be better on the Sacramento or Yuba rivers.
Despite their abundance, populations of American shad on the West Coast have not been the subject of targeted monitoring or targeted management. Fish passage facilities built for anadromous salmonids on the Columbia River fortuitously allowed adult shad to pass upstream and have provided an opportunity to enumerate how many fish move upstream each year. The abundance of Columbia River shad expanded rapidly beginning in the early 1960s, reaching an all-time peak abundance of more than 7 million fish in 2019 (ISAB 2021). Explosive growth in the Columbia River shad population seems to have been enabled by construction of the Dalles Dam in 1957 — the dam inundated Celilo Falls a natural barrier to shad migration and provided access to run-of-the-river reservoirs behind Bonneville, John Day, and McNary dams (Hasselman et al. 2012). Slow moving waters, warmer temperatures, and ample food supply make these reservoirs favorable for rapid growth of juvenile shad. According to an Independent Science Advisory Board report, juvenile shad avoid predation by occupying lake and stream bottoms during daylight hours and then move higher into the water column at night to feed on abundant zooplankton. Although Columbia River shad have become increasingly abundant since 1957, year-to-year fluctuations do occur. Analysis suggests interannual variability in adult shad returns is most closely associated with coastal water temperatures, rather than with freshwater conditions (ISAB 2021).
Several long-term fish monitoring programs in the Delta capture larval or juvenile American shad. However, it is unclear how, or if, catch in these surveys relates to subsequent abundance of adults. Considering the large numbers of adult American shad spawning in Sacramento tributaries each year, juveniles have never been particularly abundant in Delta fish survey returns. Catch of juvenile American shad in the San Francisco Estuary’s Fall Midwater Trawl (FMWT) has been highly variable but was consistently low from 2007 through 2016 (Figure 1). Was there a subsequent, multi-year decline in adult shad that followed these years with low FMWT catch? If there was, it went unnoticed by anglers. Juvenile shad are also salvaged at the south Delta export facilities — an average of nearly 1 million fish each year. This may seem like a very large number, but nearly 75 million juvenile offspring would be produced by just 250,000 adult female spawners, and that is likely a conservative estimate of adult abundance.
Figure 1 — Annual abundance indices for juvenile American shad in the Fall Midwater Trawl
Interestingly, the number of juvenile American shad salvaged each year is reasonably well correlated with the index of juvenile shad provided by the Fall Midwater Trawl. The correlation between FMWT and salvaged juvenile shad might be interpreted as evidence for the validity of the FMWT (or salvage) as an index of how many juvenile shad were produced each year. But that interpretation assumes juvenile American shad are dependent on the Delta for rearing. Is that assumption reasonable?
In the Columbia River, juvenile shad certainly occur in its estuary, but the bulk of juvenile rearing is understood to occur upstream (ISAB 2021). Unlike the Columbia system, there are no run-of-the-river reservoirs accessible to juvenile shad upstream of the San Francisco Estuary. However, there are hundreds of miles of slow-moving river channels, sloughs, and backwaters that would appear to provide ideal summer rearing habitat. If Sacramento River basin juvenile shad are primarily rearing upstream of the Delta, then the correlation between the FMWT and salvage may demonstrate that existing monitoring efforts effectively index juvenile shad in the Delta, but not the abundance of the population as a whole.
As a fish species that lacks reliable, long-term adult abundance data, American shad are in the company of other well-known Central Valley anadromous fish species – including longfin smelt, striped bass, and steelhead. Among those species, indices of juvenile abundance are available only for longfin smelt and, arguably, for striped bass. In the absence of robust adult abundance estimates that can be linked to indices of juvenile recruitment, it’s very difficult to assess population viability or to determine what factors or habitats contribute to success — or species decline. Given a choice between juvenile abundance indices and adult abundance estimates, the latter provides much greater value to conservation and management planners. Yet, for species like American shad and striped bass we continue to rely on Delta juvenile abundance indices that do not appear to be representative of overall population trends. Catch of juvenile striped bass in the FMWT, for example, is fifty-six times lower now than it was from 1967 to 1976. The population of adult striped bass has undoubtedly declined since 1976, but the decline in adults has not been proportional to the decline of juveniles observed in the FMWT. Just like American shad, juvenile striped bass are rearing somewhere—just not in the Delta waters sampled by the FMWT.
While American shad aren’t among state or federally protected species that reside in or pass through the Sacramento-San Joaquin Delta, obtaining a better understanding their population dynamics would be a prudent monitoring endeavor. That American shad has apparently defied the “pelagic organisms decline” that has ensnared a number of Delta fishes making it a particularly valuable species to study.
An effective monitoring program would allow us to assess how American shad are responding to today’s environmental conditions. Resource managers could document how the shad population responds to new water-project facilities like north Delta diversions or to modified operations of existing facilities like those that will accompany new Biological Opinions or new State Water Resources Control Board flow standards.
Such needs could be addressed by a well-planned and thoughtfully executed effort to estimate annually adult abundance for both American shad and striped bass. It probably makes sense to consider monitoring these non-native species together because both return to Central Valley rivers for spawning in the spring. New technologies and analytical techniques would make such a monitoring effort more effective and less expensive than ever before.
Securing funding for a new monitoring scheme to estimate the abundance of adult American shad and striped bass will likely be challenging. The standard, default approach, less appealing to funders and less likely to yield reliable population estimates, would revive past data-collection efforts without carefully considering what information is needed or how best to obtain it. A better approach — one we rarely see in the Delta — would involve scientists making a compelling case for how and why a proposed monitoring method and its attendant sampling design will yield population estimates that fully satisfy management and conservation needs.
References
Hasselman, DJ, RA Hinrichsen, BA Shields, and CC Ebbesmeyer. 2012. The rapid establishment, dispersal, and increase in abundance of invasive American shad in the Pacific Northwest. Fisheries 37(3): 103-114.
Independent Scientific Advisory Board (ISAB). 2021. American shad in the Columbia River: past, present, future. ISAB 2021-4, October 22, 2021.
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