by Jay Lund
California’s water policy community continues to be embroiled on how best to manage what remains of California’s native aquatic ecosystems, particularly for the Sacramento-San Joaquin Delta and its tributaries. One aspect of this controversy is the dedication and use of habitat and flow resources to support native fishes.
There is general agreement that California’s native fishes need both water and aquatic habitat. After this, water management for native ecosystems becomes more complex, uncertain, and controversial.
Various authors have produced or explored relationships between native fish abundance, flow, and habitat for California’s Sacramento-San Joaquin Delta (see some under further readings below). For policy-making, there is a tendency and probably a need to simplify the world by trying to believe such relationships. Scientifically, the policy insights from such relationships might be quite limited, but such curves might have some utility nonetheless for helping us stagger towards better management.
Consider the alternative general fish abundance-outflow-habitat curves in the figure:
Axes. Even the axes for this plot will be a gross simplification. For net outflow, it is quite important to know: a) when outflows occur seasonally, together with Delta inflow conditions, b) internal Delta flow conditions, c) the recent time history of Delta inflow and internal flow, and d) upstream habitat conditions that supply nutrients, prey, and young for species of interest. Similarly, the habitat area is a gross amalgam of different types of habitat being managed differently in different parts in the Delta and upstream to provide different habitat, food, and nutrients at different times of year suitable for different native species. Such grotesque simplification is a cost of the simplicity needed to start organizing a problem.
Thresholds for extinction. On the diagram, there should be general agreement that some minimum thresholds of Delta outflow and habitat are needed for native species to subsist (dotted lines). We should despair on knowing such thresholds exactly. Thresholds for extinction are unlikely to be fixed and could easily vary with species, hydrologic conditions, antecedent conditions, and how these annual conditions are managed seasonally and locally within the Delta. These thresholds should be seen something like vibrating limits, where even getting near them increases risks for extinction and costs for future species recovery. Quantifying such limits as policies will be unavoidably controversial and scientifically perilous.
More fish and habitat help fish abundance. There is general agreement that more flows and more habitat, if properly managed seasonally and geographically, should lead to more fish. Consider the dashed diagonal line on the diagram, along which fish abundance always increases with northeast movement.
Substitution of resources for fixed fish abundance. Now pick some arbitrary point on the diagonal, beyond the zone of certain extinction, a point that supports a fish abundance of n with equal dedications of aggregated habitat and flow. Is this the best way to have an abundance of n fish? Maybe a little more flow with a little less habitat (or vice versa) might result in more fish or less economic cost for providing flow and habitat for n fish?
Four possible shapes for a habitat-flow-fish abundance curve are suggested for achieving n fish. These are curves A, B, C, and D on the diagram. These curves would shift outward for greater fish populations, and inward towards the extinction thresholds for smaller fish populations. (For explication, the quantification of fish here is just as grotesque, averaged, and un-nuanced as the quantification of flow and habitat. These curves also vibrate stochastically, implying some probability of stable recovery increasing with higher populations.)
No substitution of flow and habitat. Curve A shows no possibility for substituting flow for habitat in supporting fish. Here, each fish needs fixed amounts of both habitat and flow, with fish abundance limited only by the most limiting resource. Any extra flow or habitat above the limiting amount is wasted, except that, given uncertainties, an additional resource reduces the likelihood that resource is limiting, but makes it more likely that other resources are limiting.
Fixed rate of substitution. If a constant fixed substitution of flow for habitat exists, then line C describes how flow and habitat trade-off to produce a fixed aggregated fish abundance. From a management perspective, if flow is more easily gotten politically or economically than habitat, then fixed substitutability would lead investing all in flow (as near the threshold as one dared), to get the most fish.
Substitutions that encourage balance. Where the total habitat and flow consists of heterogeneous sub-areas of habitat and flow with different abilities to support fish abundance, then some substitutability of flow and habitat will occur from investments in different mixes of sub-areas. So spatial heterogeneity could create a degree of substitutability, perhaps like curve B. For this curve, fish abundance would likely increase quickly as one exceeds a flow or habitat extinction threshold, and be maximized with a balancing of fish and flow investments. For curve B, maintaining a given fish abundance n with less and less flow or habitat requires more rapid increases in the other resource to compensate (perhaps across different sub-areas), until the extinction threshold is approached.
Some substitutions encourage extremes. If additional flow or habitat above an extinction threshold only slowly improves fish abundance, then curve D seems the likely flow-habitat-abundance curve shape. This seems a perilous ecosystem to manage because, like the linear case of curve C, one is tempted to maximize abundance by investing all in flow or habitat (not both) as close to the extinction threshold of the other resource as one dares.
Keep out of the zone of extinction. Of course, this discourse is only useful if enough flow and habitat resources exist to escape the zone of extinction, defined by the thresholds. Indeed, one needs to invest in both resources to be far from them, as their exact locations vary with time and have uncertainty.
Other important things. Alas, this diagram tempts suggests that flow and habitat (even as grossly considered here) are the only important factors. The management of invasive species, ocean conditions, climate change, and other things may shift these curves with time, gently or abruptly.
Conclusions. So, what can be learned from this?
- Managing flow and habitat for native Delta fishes is an unavoidably grotesque, complex, and uncertain problem. Whatever policies and management are adopted will probably not work as hoped for. So a considerable effort is needed in developing institutions, resources, science, and synthesis that can adaptively manage.
- If we think we know the general shape of the substitutability of flow and habitat in supporting fish abundance, it leads us to either investing predominantly one resource or the other as near the extinction threshold as we dare if substitutability is constant (curve C) or concave (curve D), with more balanced investments in flows and habitat if there is no substitution (curve A) or weak substitution (curve B). The shape of substitution trade-offs determines the best strategic approach.
- If there is no substitutability of flow and habitat for fish abundance (curve A), yet both are vital, then it is important to seek the proper balance of resources. A balanced strategy also is best, but less strongly, if increasing flow or habitat would be needed to make up for a scarcity of the other (curve B) and both resources are costly.
- In all cases, managers should stay away from the extinction zone. Some flow-habitat-abundance shapes tempt one to find and manage at the edge of extinction edge, despite its instability. For these substitution shapes (curves C and D), how close should we dare get to the extinction threshold?
- To find the proper balance and avoid extinction thresholds requires vigorous science-based adaptive management, based on ecological or mechanistic theory and models supported by field data and experiments.
- Although these general policy lessons have some value, settling on an initial resource policy and allocation can obscure the more difficult problems of how to manage these resources locally within the Delta seasonally and inter-annually. This more challenging detailed level of management and science must be workable for the ideal curve shapes and policies discussed above to hold true.
Hopefully this adds more insight than turbidity.
Jay Lund is the Director for the Center for Watershed Sciences and Professor of Civil and Environmental Engineering at the University of California – Davis. This blog benefited (probably not enough) from conversations with John Durand, Peter Moyle, Wim Kimmerer, and Cathryn Lawrence.
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