It’s not lost on those concerned with the environmental health of the Sacramento-San Joaquin Delta and adjacent upper San Francisco Estuary that efforts to reverse declines of its imperiled fishes are failing. All the region’s fishes that are protected under the federal Endangered Species Act appear to have suffered reductions, some dramatic reductions, in abundance over the past two decades. The failure to respond to a native fishery in crisis falls in substantive part on the collective shoulders of the well-funded scientific community. From long-term surveys incapable of accurately monitoring the status and trends in numbers of those imperiled fishes to laboratory studies that can’t possibly mimic in-situ estuary conditions, much of the Delta’s “science” agenda has no clear application in conservation planning. For that matter, the many directed investigations that should have unambiguous relevance to the Delta’s conservation decision-makers seldom draw from ecological theory, few of them gather data in an experimental design, and fewer yet confront management-relevant hypotheses, leaving resource managers guessing at the conservation responses needed to save the Delta’s at-risk fishes. Academic researchers and agency scientists seem unable to provide expert guidance the resource managers and policy makers need to restore the native species that ply the Delta’s troubled waters.

One consequential oversight by scientists trying to explain the cause of the decline of the once-abundant delta smelt has led to a conservation agenda that is at best misdirected and actually may be contributing to that fish’s imperilment. Researchers have neglected to consider the law or theory of “limiting factors” in collecting and analyzing data and interpreting their findings for resource managers who are designing the conservation agenda for delta smelt. Limiting environmental factors have been commonplace considerations in agriculture for a century and a half. Liebig’s “law of the minimum” acknowledges that growth of individual organisms and of populations is not determined by the total of resources available, rather by the scarcest resource – the limiting factor – and that increasing the amounts of plentiful nutrients does not increase yield, rather yield is increased by increasing the amount or availability of the limiting factor. Applying more phosphorus-rich fertilizer to a field crop that is not constrained by phosphorus availability, but is deficient in nitrogen, will not increase productivity. To consider limiting factors in resource management is to operationalize the old adage — “a chain is only as strong as its weakest link.” 

The concept of limiting environmental factors has long been incorporated into ecosystem and population models and applied in natural resource management. And, limiting factors have obvious application in conservation planning for imperiled species. Insightful conservation planners recognize that species and their habitats in disrupted ecosystems are virtually always impacted by multiple hostile environmental conditions. However, at any given time – or stage in a species’ life cycle – one environmental stressor limits the capacity of an imperiled species to increase its numbers. If that specific environmental factor is addressed through directed management actions, and other stressors are impacting the species, another stressor will emerge as the limiting factor, and it then must be targeted by appropriate management actions. It is understood that when management actions target environmental factors that are not the limiting factor, it is unlikely that the imperiled species will benefit. Instead, its numbers will continue to decline, reducing its chances for recovery, and perhaps even reducing the time to its extinction. Conservation efforts cannot succeed if they do not specifically address the limiting environmental factor or factors.  

The embattled delta smelt is a poster fish for application of the limiting factors approach. It’s a textbook case of a species suffering a protracted decline in numbers as the extent and quality of its habitat in the upper San Francisco Estuary has diminished precipitously over decades. The physical and biotic conditions in the habitat areas that remain have degraded in multiple dimensions. Investigations have shown that multiple environmental stressors are impacting the delta smelt. Summer water temperatures in the Delta have increased over the past several decades, at times spiking in the south Delta to levels that are close to lethal to delta smelt. Within its narrow range the delta smelt is most frequently encountered in turbid waters. But the Delta’s waters are clearing as the sediment load in tributaries from gold mining more than a century and half ago was exhausted and new sediments that should be mobilized downstream into the Delta are entrained behind dams throughout the watershed. The Delta food web is awash with invasive species, predatory fishes that eat delta smelt, other fishes that compete with delta smelt for food resources, and aquatic plants that have expanded into delta smelt habitat altering its structure and composition. Contaminant loading into the system surely effects delta smelt habitat quality throughout the Delta, if not causing direct mortality of the fish. The list of environmental stressors goes on. 

Identifying the environmental factor that actually limits the size of the delta smelt population from those factors and a welter of others that could plausibly effect delta smelt and impact its habitat is complicated by the fish’s annual life cycle. To grow to maturity and reproduce in a single year, delta smelt move rapidly through life stages that have different resource needs and tolerances to habitat conditions. So, the species is subjected to different environmental regimes and a sequence of different limiting factors during its short life span. Ignoring the delta smelt’s shifting resource needs and shifting susceptibility to limiting factors virtually assures that management actions targeting the fish and its estuarine habitat will miss their intended mark.   

Previous univariate studies – analyses that consider just one environmental factor at a time – are inadequate to identify limiting factors and do not address environmental-factor effects on delta smelt in appropriate spatial and temporal context. Scott Hamilton and I raised these points in two recent journal articles. In the first, Analysis of limiting actors across the life cycle of delta smelt (Hypomesus transpacificus), we identified areas of the Delta that most reliably provide the habitat conditions that favor delta smelt survival and the factors that appear to limit delta smelt abundance there. Low turbidity and elevated water temperatures render a large portion of the estuary seasonally unsuitable for delta smelt. Within areas (subregions) in which water quality is actually suitable, delta smelt habitat occurs where channels of intermediate depth adjoin shallow water. By utilizing a unique approach to identifying limiting factors, we found that a small number of factors explained 88% of the variation in the abundance of delta smelt. Management actions that address the availability of the delta smelt’s zooplankton prey in summer, reduce the abundance of silversides (an introduced fish that preys on and competes with delta smelt), and directly enhance adult abundance, perhaps through propagation, should be at the top of the list of candidate actions under consideration by resource management agencies.

In the second article, Use of affinity analysis to guide habitat restoration and enhancement for the imperiled delta smelt, we found even in the subregions of the Delta that currently provide the very best habitat for delta smelt, conditions are frequently inadequate for at least one of four environmental attributes – temperature, turbidity, salinity, and prey availability — for periods up to several months. Those factors can limit local habitat quality for different delta smelt life stages as the fish matures through its annual life cycle in different areas of the delta. Moreover, the areas of the Delta that offer delta smelt adequate habitat conditions differ depending on outflow through the Delta — an artifact of water-year, whether it’s a drought year or a year with big precipitation. In other words, limiting environmental factors vary seasonally and between years and act on a local scale. 

Through the lens of limiting factors, it is now apparent that the optimal conservation strategy for delta smelt is one that restores adequate habitat conditions at a number of locations spread across the full remaining geographic range of the species. Each of the fish’s brief life stages then can thrive somewhere in the upper estuary through expected variation in seasonal and annual through-Delta flow levels. That understanding is absolutely essential to identifying where and for what ecological purpose habitat restoration and other management actions should target delta smelt.

It is high time to revisit and re-prioritize the multi-pronged management agenda that is intended to benefit delta smelt. Two currently proscribed management actions — reducing water exports from the south Delta to reduce entrainment of young delta smelt and enhancing outflow through the Delta in the autumns of particularly wet years – were drawn from simple correlations between delta smelt numbers and environmental phenomena. Those actions do not address environmental factors that limit delta smelt performance. And, that seems to explain the failure of those actions over the past decade to contribute to recovering delta smelt numbers. Addressing limiting environmental factors has to become an obligatory consideration in conservation planning for delta smelt, informing the resource management agenda that targets the fish and the monitoring scheme that needs to accompany it. In an era of constrained budgets for meeting the conservation needs of the Delta’s increasingly scarce native fishes, we should recognize that ignoring limiting factors sucks precious conservation dollars away from management actions that have real promise of benefiting those fishes and their habitats.

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