Conservation science has offered one widely appreciated truism above all others. Habitat loss and fragmentation is the primary cause of imperilment and extinction for very many species. This is certainly the case for the delta smelt, which has seen its habitat diminished and transformed by human activities dating back to California’s gold rush era. Congress recognized the importance of habitat when it enacted the federal Endangered Species Act nearly five decades ago, declaring as its purpose — “to provide a means whereby the ecosystems upon which endangered species and threatened species depend may be conserved” — in other words, to protect and restore the habitats that support imperiled species. The directive for the federal wildlife agencies to designate Critical Habitat for listed species is one of a suite of provisions in the Act that are intended to contribute to the conservation of endangered and threatened species by protecting their habitats.


On its very first day the Biden Administration released “a non-exclusive list of agency actions that heads of the relevant agencies will review in accordance with the Executive Order — Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis.” Near the top of the list of “agency actions for review” at the Department of the Interior are revisions made to regulations designating Critical Habitat made by the outgoing administration, including a regulatory definition of habitat for listed species that was proffered by the Fish and Wildlife Service and National Marine Fisheries Service. 


Those wildlife agencies in late 2020 defined habitat as the “…physical places that individuals of a species use to carry out one or more life processes. Habitat includes areas where individuals of the species do not presently exist but have the capacity to support such individuals, only where the necessary attributes to support the species presently exist.” Notable due to its omission from that definition is extending the concept of habitat to landscape areas that are not presently habitable, but that may in the future be habitable, either due to restoration efforts or to shifts in landscape occupancy that can be expected by the species as a consequence of climate change. In the coming months, we will see whether the Biden Administration embraces the now-existing definition of habitat or seeks to expand it to encompass landscape areas with the potential to become habitat.


While the debate over the definition of habitat within the narrow confines of a regulatory regime for designation of Critical Habitat likely will have limited real-world impacts, a robust definition of the habitat of a threatened or endangered species is a near absolute prerequisite for the conservation of listed species. That being the case, one might reasonably presuppose that federal wildlife officials would have a highly refined working definition of habitat for the delta smelt, a fish that has been on the list of threatened and endangered species for more than a quarter century and that has been targeted by conservation efforts costing to the tune of billions of dollars. Unfortunately, such a definition is wanting. 


Ample evidence shows that delta smelt are not widely distributed across the waterways and bays of the upper San Francisco Estuary, rather the species is selective in its haunts and hideouts; it has unique habitat requirements. And 25 years of studies relate delta smelt occurrences to environmental factor conditions, both physical and biotic – water temperature, salinity, turbidity, flow velocity, food availability, landscape features, and more. Delta smelt have an affinity for specific locations in different seasons and at different times of day. Nonetheless, the Fish and Wildlife Service has demurred from integrating available data on the fish’s resource requirements and patterns of landscape occupancy into a synthetic definition of the delta smelt’s habitat — a definition that can guide a much-needed, updated conservation strategy for the fish. 


Absent an adequate definition of habitat to help guide the design of a delta smelt monitoring scheme, the Service has defaulted to “monitoring” the status of delta smelt and trends in its numbers by relying on long-standing general fish surveys that are mostly limited to the bays and wide channels of the upper estuary. Predicably those surveys taken in open water circumstances from large trawlers miss the delta smelt’s habitat in the shallows and on the near-shore bathymetric gradient, and just as predictably they miss the fish where it is known to occur in greater numbers. So ill-designed to sample delta smelt are those surveys that the frequency of delta smelt encounters at historical open-water survey stations has plummeted to mostly zeros, despite the known presence of the fish from well up the lower Sacramento River and Sacramento Ship Channel west across the delta and Suisun Marsh and Suisun Bay all the way to and beyond the Carquinez Strait. The effect of using survey returns from a sampling footprint that by design misses known delta smelt habitat is to leave those charged with conserving and recovering the fish with no idea how large, or importantly how small, the population of delta smelt is and where individuals are located at any given time. Despite statements from the Service and fish biologists from their university perches — warning that the fish’s extinction is imminent — there never has been a reliable census of delta smelt. Absent focused sampling in delta smelt habitat, there is no way to predict the potential for the delta smelt’s extinction or its recovery, or whether the expensive management agenda intended to benefit the fish is effective or not.


Absent an adequate definition of delta smelt habitat, both the Service and the California Department of Fish and Wildlife in regulatory documents and other determinations prescribing management actions intended to conserve the fish have simply asserted that a portion of the low-salinity zone in the upper estuary – that area where salty bay water meets fresh river-water outflow – should serve as a proxy for the fish’s habitat. For purposes of conservation planning for delta smelt, the wildlife agencies contend that the availability of its habitat varies wildly in years of drought and deluge, moves seasonally with through-Delta flows, and adjusts in extent daily with tidal cycles. But that’s not true. The low-salinity zone fails as a surrogate measure of delta smelt habitat on first principles. Much of the low-salinity zone is unoccupied by delta smelt much of the time. And, importantly, portions of the delta smelt population in the upper estuary occur outside of the low-salinity zone as it’s defined by the wildlife agencies. Much of the current conservation agenda for delta smelt is predicated on the notion that the extent of its habitat fluctuates dramatically and is determined by flows through the Delta, when in fact the area of delta smelt habitat is largely static; it’s anchored on the landscape. It is delta smelt habitat conditions that vary with flows, particularly the distribution of the fish’s zooplankton prey. That’s a wholly different ecological phenomenon that requires conservation planners to identify where and when outflow through the Delta might be directed and otherwise managed to enhance the quality of habitat where it occurs across the upper estuary.


Absent an adequate definition of delta smelt habitat the Service designated Critical Habitat for the delta smelt for purposes of regulatory decision-making in 1994. At the time, Critical Habitat was determined to be “areas of all water and all submerged lands below the ordinary high-water mark and the entire water column” across the legal Sacramento-San Joaquin Rivers Delta. Not differentiating between areas occupied by delta smelt from areas seldom or never occupied by delta smelt, the Critical Habitat designation included vast areas of waterways and embayments in the east and south Delta that are now known to provide no actual delta smelt habitat. Ninety percent of survey records of delta smelt from the past 40 years occur in an area that constitutes well less than 20 percent of the designated Critical Habitat. Ninety-nine percent of delta smelt survey records occur in less than 40 percent of the designated area. As problematic when considered in the context of contemporary scientific information, the Critical Habitat designation excluded the Napa River and its estuary where delta smelt frequently occur, as well as the shoals of north San Pablo Bay. The discordance between the outdated Critical Habitat designation and our present understanding of habitat actually used by delta smelt provides compelling grounds for a revised designation of delta smelt Critical Habitat. And that revised designation should integrate contemporary knowledge and be informed by a robust definition of delta smelt habitat.


The absence of a refined working definition of delta smelt habitat has had profound implications for conservation planning targeting the species. Seasonal fish surveys don’t sample delta smelt habitat, the proxy used to assess the status of delta smelt habitat is demonstrably not valid, and even Critical Habitat designated for delta smelt misses the geographic mark. As a result, the conservation agenda for delta smelt includes poorly conceptualized management actions, the performance of which can’t be assessed for lack of a monitoring scheme. An important first step toward a more defensible conservation agenda for delta smelt is the development of a robust definition of its habitat. If the Biden Administration is interested in contributing to the conservation of delta smelt, it would do well to begin by developing a definition of delta smelt habitat that is informed by the best available scientific information, elicited from experts under the structured decision-making effort targeting delta smelt that is now in progress. 

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