Identifying the environmental factors that are responsible for the decline in numbers of delta smelt is prerequisite to identifying management actions that can contribute to reversing the trend and recovering the species. A number of empirical models that consider multiple environmental factors and employ innovative multivariate-statistical approaches have attempted to identify the causes of the decline, but they have produced inconsistent and even contradictory results. That explanatory ambiguity has been unhelpful to resource managers responsible for protecting the delta smelt and its habitat in the degraded upper San Francisco Estuary, and has resulted in the implementation of ineffective and often costly management actions. In this paper we approach the problem by employing a basic ecological tenet – the law of limiting factors – which asserts that specific individual environmental factors limit the abundance of a species in certain seasons and years, but not in all seasons or years. The inconsistent influence of important environmental determinants of delta smelt population responses presents a dilemma for quantitative analyses of species-habitat relationships using conventional analytical methods. By utilizing a unique approach to identify limiting factors, the authors found that a small number of factors explained 88% of the variation in the abundance of delta smelt. Management actions that address food availability in summer, reduce the abundance of silversides (an introduced fish species 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.
Hamilton SA, Murphy DD. 2018. Analysis of limiting actors across the life cycle of delta smelt (Hypomesus transpacificus). Environmental Management 62:365-382.