Durability of environment–recruitment relationships in aquatic ecosystems: insights from long‐term monitoring in a highly modified estuary and implications for management

The environment can strongly influence the survival and population dynamics of aquatic organisms. Our understanding of these relationships, typically based on simple linear regression, underpins many contemporary resource management decisions. However, such relationships can break down over time as ecosystems change. Even when durable, relationships may not be very useful for management if they exhibit high variability, context dependency, or nonstationarity. Here, we systematically review the literature to identify trends across environment–recruitment relationships for aquatic taxa from California's San Francisco Bay and Sacramento–San Joaquin Delta Estuary. This delta is one of the most heavily modified aquatic ecosystems in North America, and home to numerous species of concern whose relationships with the environment inform regulatory actions and constraints. We retested 23 of these relationships spanning nine species using data that have accumulated in the years since they were first published (9–40 additional years) to determine their durability. Most relationships remained the same or stronger in direction and magnitude but showed declining predictive power with the addition of new data, particularly for older relationships that had not adjusted for recognized regime shifts in the system through the use of step changes or data splitting. Constantly refining these relationships may give the appearance of durability, but limit their practical value as policy tools when the present or future state of the ecosystem is unknown. We conclude by synthesizing emerging insights from the literature on best practices for the analysis, use, and refinement of environment–recruitment relationships to inform better decision making in dynamic ecosystems.