Cooperative Fish and Wildlife Research Units Program: Oregon
Education, Research and Technical Assistance for Managing Our Natural Resources

Occupancy models are often used to analyze long-term monitoring data to better understand how and why species redistribute across dynamic landscapes while accounting for incomplete capture. However, this approach requires replicate detection/non-detection data and many long-term monitoring programs lack temporal replicate surveys at a sample unit. In such cases, it has been suggested that surveying subunits within a larger sample unit may be an efficient substitution (i.e., space-for-time substitution). Still, the efficacy of fitting occupancy models using a space-for-time substitution has not been fully explored and is likely context dependent. Herein, we fit multistate occupancy models to Delta Smelt (Hypomesus transpacificus) and Longfin Smelt (Spirinchus thaleichthys) catch data collected by two different monitoring programs in the San Francisco Bay-Delta, USA. We demonstrate how our inferences concerning the distribution and relative abundance of these species changes when using a space-for-time substitution. Specifically, we found the probability that a sample unit was occupied and contained a large number of fish was much greater when using a space-for-time substitution, presumably due to the change in the spatial scale of our inferences. Furthermore, our results indicate that as the spatial scale of our inferences increased our ability to detect environmental effects on system dynamics decreased, which we suspect is related to the tradeoffs associated with spatial grain and extent. Overall, our findings highlight the importance of considering how the unique characteristics of monitoring programs influences inferences, which has broad implications for how to appropriately leverage existing long-term monitoring data to understand the distribution and relative abundance of species.