By Peter Moyle
The California Fish Commission introduced American Shad into California in 1871 via milk crates shipped on the newly built transcontinental railroad (Dill and Cordone 1997). Shad, apparently the first non-native fish species (of 50) to become established in the state, were so well suited to California that in a few years, shad supported a commercial fishery in the San Francisco Estuary and had colonized the Columbia River to the north. Now they are found in the North Pacific Ocean and inflowing rivers from Mexico to Russia, with runs numbering in the millions in major spawning rivers.
Recent years have seen shad populations declining in native range on the East Coast (based on fishery data). Similarly, in California, the decline of shad in the Sacramento-San Joaquin river system parallels that of declining native fishes (Moyle 2002). In the Columbia River, on the other hand, shad spawning runs have increased so much that their numbers often exceed those of native salmon and steelhead. The rising shad and falling salmonid numbers on the Columbia have fueled speculation that shad are somehow responsible for the native salmonids’ decline (Quinn et al. 2024). This blog discusses American shad issues in both the Sacramento and Columbia rivers.
American Shad: the Founding Fish.
American shad (Alosa sapidissima) are moderately large (40-60 cm adult lengths) anadromous fish related to the herrings. They are native to Atlantic coastal waters and rivers from Florida to Labrador. Adults are silvery schooling fish that feed by filtering out zooplankton from the open waters they inhabit. When shad aggregate for spawning in rivers, they are sought by both anglers and commercial fishermen. Despite being rather bony, shad’s oily flesh and roe are highly regarded as a delicacy, as their scientific name suggests: sapidissima means “most delicious.” Female shad grow larger than males and produce roe, and so are particularly sought after. As an abundant fish, the shad were important to the native peoples along the Atlantic coast, as well as to the Europeans who had colonized land along rivers with large shad runs. Thus, George Washington’s wealth was partly the result of the shad fishery that operated in the Potomac River below Mount Vernon (McPhee 2002).
Introduction into California
American shad were so favored that they were one of the first fishes to be transported from Atlantic coastal rivers to California, in 1871 (Dill and Cordone 1997). Thousands of juvenile shad were captured from the Hudson River, loaded into large metal milk cans, and carried cross country in special railroad cars, to be planted in the Sacramento River near the town of Tehama. This action was repeated almost every year from 1873 to 1881. The adult shad were in high demand for flesh and roe as the non-native human population exploded during the Gold Rush. The shad population quickly expanded and peaked in the commercial fishery in 1917, when 6 million pounds were harvested. The commercial fishery was banned in 1957 because anglers thought the commercial gill-net fishery for striped bass was harming the sport fishery for both shad and bass. Despite this ban, the shad populations continued to decline (Moyle 2002).
Spread
The Sacramento shad found the river habitat ideal for spawning and larval rearing. The juveniles then moved into the ocean where the upwelling of the California Current generated abundant zooplankton, promoting rapid growth to adulthood. Shad were eventually captured in waters from Baja California in Mexico to the Kamchatka Peninsula in Russia. The most significant colonization event was in the Columbia River, where shad from the Sacramento River were first captured in 1875, just four years after their introduction in California. Additional direct plants of juvenile shad from the Hudson River to the Columbia were made in 1885 and 1886 (Quinn et al. 2024), leading to the present distribution.
Columbia River
The Columbia River shad population expanded as more habitat became available to them. Shad initially used the lower reaches of the river for spawning and rearing but eventually were able to colonize habitat above Bonneville Dam (the lowest dam) by 1938, using fish ladders built for salmon and steelhead. Such colonization continued as 13 more hydropower dams with fish ladders were built (Quinn et al. 2024). The dams created accessible spawning and rearing habitats for shad, particularly the cool, slow moving water in reservoirs favored by juvenile shad and to some extent by juvenile salmon and steelhead. Also favored by this arrangement were non-native predators.
The best data on annual shad abundance in the Columbia River is from estimates of adult fish numbers passing through fish ladders at Bonneville Dam. These estimates indicate that shad in the Columbia River have steadily, if erratically, increased in abundance since their introduction (Figure 2). The estimated number of adults reached nearly 7 million in 2020 ( Quinn et al. 2024), with an average of 2,225,797 shad from 1992-2003.
Are American Shad hurting Salmonid Populations in the Columbia River?
The astonishing increase in Columbia River shad in recent decades coincides with a decline in numbers of adult salmon and steelhead (5 species) in the river (figure 2), and some have suggested the two trends could be linked. Quinn et al. (2024) reviewed the evidence for a link and found that “…despite the great abundance of American shad, harmful effects on salmon are neither clear from empirical studies nor from ecological principles. Rather, life histories and habitat use patterns largely separate spawning adult salmon and their offspring from American shad in space and time. Available evidence indicates that this separation … results in weak or no effects on salmonids.” Quinn et al. (2024) recommend additional study of shad populations in the Columbia to better understand “the species’ life history, population dynamics, and role in the ecosystems they share with salmon,” particularly in light of climate impacts on the ecosystem.
American shad in the Sacramento River
In the Sacramento River system, shad populations were likely in the millions around the turn of the 20th Century and supported both commercial and recreational fisheries. Over the next 100 years or so there were presumably fluctuations in shad numbers that accompanied a long-term decline. Dill and Cordone (1997) speculated that in the 1970s shad runs in California were likely in the range of 2-3 million adults each year, although studies were lacking on the catch of shad in the sport fishery. This lack of data on the spawning population continues to the present day, although there is still a sport fishery. The lack of studies apparently reflects the lack of interest in their management by CDFW, which in turn reflects the current low status of the fish, as an oily non-native species with lots of sharp little bones imbedded in the flesh.
Fortunately, the abundance of juvenile shad in fresh water and the San Francisco Estuary has been monitored fairly closely because multiple agencies have vital monitoring programs for pelagic fishes, focused on endangered smelt and salmonids. This data indicates that from 1980 onward, pelagic species in the Delta have declined, including American shad (Stompe et al. 2022). The simultaneous decline of five pelagic species indicates that their continued decline in recent years is mostly likely due to multiple unfavorable habitat conditions (especially outflow) for rearing in the river, the Delta, and other parts of the SFE.
I find it is amazing that American Shad are still present in the highly altered rivers of California and that they still support a fishery. They also have some value as a species for independent monitoring of the ‘health’ of the Bay-Delta system because they are not listed as a threatened species, but do use the same pelagic habitats as listed native species.
Conclusion
Shad populations in both the Columbia River and the Sacramento River have persisted in the face many possible threats, in their estuaries, rivers, and ocean. Developing an understanding of factors affecting the populations in both river basins should help fish agencies make decisions on how to manage them (or not manage them). In both rivers, limited evidence suggests that American Shad are well integrated into the ecosystems in which they live, so understanding how they survive in one ecosystem could help fisheries managers on both coasts.
Peter B. Moyle is a Distinguished Professor Emeritus at the University of California, Davis and is Associate Director of the Center for Watershed Sciences.
Further Reading
Cavallo, B.J. 2022. American shad — the Delta’s most-abundant and least-considered anadromous fish. Center for California Water Resources Policy and Management. https://calwatercenter.org/american-shad-the-deltas-most-abundant-and-least-considered-anadromous-fish/
Dill, W.A. and A.J. Cordone.1997. History and Status of Introduced Fishes in California, 1871-1996. California Department of Fish and Game. Fish Bulletin 178.
McPhee, J. 2002 The Founding Fish. New York: Farrar Straus, and Giroux.
Moyle, P.B. 2002. Inland Fishes of California. Berkeley: University of California Press.
Quinn, T.P., J. Epifano, P. B. Moyle, S. Gregory, D.J. Hassel, E, Merrill, K. Rose T. F. Turner, T.C. Wainwright. 2024 Ecological Interactions Between Non-Native American Shad and Pacific Salmon: The Columbia River Case Study. Reviews in Fisheries Science and Aquaculture. https://doi.org/10.1080/23308249.2024.2337434
Stompe, D.K., P.B. Moyle, K. Oken, and J. R. Durand. 2022. A spatiotemporal history of key pelagic fish species in the San Francisco Estuary, California. Estuaries and Coasts https://doi.org/10.1007/s12237-023-01189-8
Stompe, D.K., P. B. Moyle, A. Kruger, and J. R. Durand 2020, Comparing and integrating fish surveys in the San Francisco Estuary: Why diverse long-term surveys are important. San Francisco Estuary and Watershed Science 18 (issue 2). https://doi.org/10.15447/sfews.2020v18iss2art4
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