Delta smelt have nearly ceased to appear in “pelagic” fish surveys carried out in their narrow geographic range in the upper San Francisco Estuary. As trawl-generated index values for delta smelt have declined over the past quarter century – understand there is no reliable estimate of the size of the delta smelt population — the chorus of voices advocating for captive rearing and releases of the species has grown louder. When in 2019 the Bureau of Reclamation proposed to include development of a conservation hatchery as a component of its ongoing operation of the federal Central Valley Project, it was building on work to culture delta smelt that was initiated in the early 1990s, just after the fish was listed as a threatened species by the U.S. Fish and Wildlife Service. 

While the conservation community has a long history of developing and implementing captive breeding programs, the effort to develop a successful captive breeding program for delta smelt is ambitious and faces significant challenges. Nonetheless there are compelling reasons to pursue the captive breeding program, including the seemingly precarious status of delta smelt in the wild, combined with the systematic failure of previous costly conservation measures to yield tangible benefits for the species.


Captive breeding is a tool of last resort in conservation biology. It is intended to bring back species from the very brink of extinction. Among the obstacles to its success are poor reproduction in captivity, resulting from but not limited to domestication, disease, and genetic introgression. Several decades of captive breeding targeting diverse, imperiled species have yielded important lessons for conservation planners charged with protecting and ultimately recovering listed species in Bay and Delta. Albeit with uneven progress, the Fish and Wildlife Service has employed captive breeding as a key component of broader conservation strategies for the California condor, whooping crane, and Mexican wolf, among other species. Every known individual California condor was brought into captivity in 1987; today the condor has re-established populations in three western states and Baja California. Captive breeding and release of individuals in the wild played a central role in increasing the whooping crane population from a low of just 22 individuals in the 1940s to more than 700 today. Similar efforts brought Mexican wolves back from a low of just six individuals in the 1980s to over 160 in the wild in the southwestern United States in 2020.

 

Captive breeding also is one of the core conservation measures identified by the Service in its recovery plan for the pallid sturgeon. At the same time, the agency emphasizes the need to restore the species’ habitat including ensuring the availability of sufficient food resources in order to support the hatchery-reared and wild populations.  Captive breeding and release of at-risk species cannot contribute to conservation if the threats to the species in the wild that caused the decline to go unaddressed. It is for this reason that to complement the Service’s captive breeding program for pallid sturgeon, researchers recently completed a study of the productivity of its habitat on the Missouri River in order to determine its present carrying capacity for the species.

 

Condors, cranes, wolves, and pallid sturgeon have much in common. They are large bodied, long-lived species with low reproductive rates. Those characteristics are frequently attributed to species referred to as K-selected species. In contrast the diminutive delta smelt are short-lived with high reproductive rates. These characteristics are frequently attributable to species referred to as r-selected species. The concept of r/K selection was first set forth by McDonald and Wilson in 1967 based on the notion that species may pursue a life history strategy that emphasizes explosive breeding or invest more in fewer offspring so that its population remains at or near carrying capacity. The two categories can be conceived of as the ends of a continuum. Notably, most captive breeding efforts have focused on K-selected species.

 

Key characteristics of r/K selection

 

r

K

Number of offspring

many

few

Size of offspring

small

large

Reproductive maturity

early

late

Lifespan

short

long

Early mortality

high

low


It is unclear whether and to what extent captive rearing and release of delta smelt could augment the diminished numbers of the r-selected fish, sustaining the species until other conservation efforts succeed in restoring its habitat and recovering delta smelt numbers in the wild. However, Lessard et al. (2016) considered practical constraints and the distinctive ecology of delta smelt in critically assessing the prospects for using captive rearing to support species recovery efforts. They observe that developing policies that allow for experiments releasing cultured delta smelt in their natural habitat are a precursor for advancing policies that would allow for population supplementation actions. They have described a framework for evaluating the potential ecological, demographic, and genetic risks that come with supplementing wild populations, and propose how those risks can be addressed and the uncertainties that always accompany those efforts can be accommodated. Importantly, the study group observed that releases of cultured fish, either experimentally or as a management action, need to be conducted within an adaptive management program that is integrated with other conservation strategies, particularly habitat enhancement and restoration. They conclude there is sufficient baseline information for the existing delta smelt culture program to proceed with targeted field research that utilizes captive-reared fish to explore population supplementation. 

 

That assessment understood, it would be natural to look to existing captive rearing and release programs in Sacramento-San Joaquin Delta. An r-selected species that is native to the Delta and the river systems that support it, which has been the subject of captive breeding is Chinook salmon. In a recent article critical of the Service for pursuing captive breeding of delta smelt (Bork et al. 2020), several researchers affiliated with UC Davis point to Chinook salmon and other West Coast salmonids as a cautionary tale. It is true, as they argue, that captive breeding in efforts to maximize salmonid production for commercial and recreational fisheries, and without sufficient regarding for the adverse effects of such efforts on wild populations, can undermine efforts to conserve wild populations rather than facilitate them. But it also is the case that salmon hatcheries operated for the purpose of conservation can be effective as, Neil Thompson’s recent work with Sacramento River winter-run Chinook salmon demonstrates. The contention of Bork and his colleagues at Davis – that the federal government’s effort to develop a captive breeding program ignores historical problems with salmon hatcheries – is a red herring.

 

The notion that we should be developing a conservation hatchery for delta smelt was not novel when it appeared in the Reclamation’s biological assessment that informed the Service’s 2019 biological opinion. In 2016 “A Backup Plan for Delta Smelt” appeared in Bay-Nature Magazine describing existing and planned delta smelt captive breeding efforts. That same year an article in San Francisco Watershed and Estuary Science suggested that culture techniques used in Japan to introduce captive-bred wakasagi (a close cousin of delta smelt) offered promise that the program could perhaps be replicated for delta smelt. 

 

A terrestrial r-selected species that is native to the adjacent Central Valley along the San Joaquin River and that has been the subject of ongoing captive breeding efforts is the riparian brush rabbit, a close relative of the ubiquitous cottontail rabbit, that is listed as endangered under both the federal and California Endangered Species Acts. While efforts to rear the brush rabbits in captivity and relocate them in the wild had early halting success, impediments to the program’s performance are a function of the long-term reductions of available brush rabbit habitat and stochastic environmental events, including flooding of its remaining habitat, rather than shortcomings in the design of the breeding program. A recent status assessment for the species showed that between 2002 and 2013 nearly 1,500 captive-bred riparian brush rabbits were released on the San Joaquin River National Wildlife Refuge and refuge-managed properties, and that the captive breeding effort established the largest riparian brush rabbit population in the wild, dramatically increasing the species’ distribution and abundance.

 

The extent to which we can rely on past experience, including precedent involving r-selected species, to inform a program for delta smelt is limited. That said, over a half century of captive breeding efforts has produced a body of knowledge that informs efforts to conserve at-risk species using a combination of tools, including captive breeding and a focus on environmental factors that limit survival and recruitment in the wild. At this juncture and in light of what we know about the status of delta smelt, advancing a captive breeding effort in combination with habitat restoration in a rigorous adaptive management framework is sound policy based on a substantial body of reliable knowledge. If there is anything regrettable about these developments, it is that resource managers did not act sooner.

 

Until the most recent apparent decline of the wild delta smelt population, the captive breeding program produced more than 50,000 delta smelt adults annually. Yet, from its beginning in the early 1990s until 2019, all delta smelt produced by the captive breeding program were culled (or sacrificed in laboratory studies) because regulations prohibited experimental release of them into the waters of the upper San Francisco Estuary.  Had the full research value and conservation potential of the captive breeding program been recognized earlier, wild delta smelt would likely still be in trouble, but conservation planners would have gained years of valuable insight regarding how best to help them.

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