Background/Objectives

Understanding adaptive reservoir management strategies that balance ecological outcomes with other objectives necessitates properly articulated environmental objectives. Aside from flood pulse extent related metrics, residual-based descriptors provide robust descriptions of fish assemblage structure and harvest. Previous work suggested maximizing variation and capturing essential low and high flow characteristics for total fish catch could be an effective method for maximizing fish harvest in the presence of hydropower constraints. But seasonal-scale timing and phase shifts of flow regime remained unexamined.

Approach/Activities

We proposed a model framework based on spectral analysis of hydrologic variation and the Multivariate AutoRegressive State Space (MARSS) model to statistically quantify the effect sizes of hydrologic variation impacts on total fish catch and functional group (migration types) fish harvest and applied it to 17 years of fish harvest data from the Lower Mekong River Basin (LMB). Then we compared future fishery yields by MARSS forecasts using Fourier series flow simulations and hydrological anomaly effect size from our result.

Results/Lessons Learned

First, we discovered that hydrologic anomaly magnitudes and duration matter as much as their relative timing when estimating total catch of all species combined. Second, when differentiating between distinct migratory traits of fish, impacts of hydrological anomalies were generally comparable to those of the total catch model for all hydrologic metrics but longitudinal migrators were more impacted. Third, fishery catch forecast showed our good designs were generally beneficial, but the continuous delayed flood hydrograph led to a deterioration in fishery catches.

Our findings suggest that duration and timing of hydrologic anomalies matter as much as their magnitude. Anomaly droughts coupled with strong pulse can benefit species if timed correctly. Longitudinal migrators were more sensitive to anomalous floods and droughts than other migratory species. Fish catch projections using effect sizes derived from historical data revealed that a well-timed and protracted drought followed by a powerful flood pulse would be advantageous to the fishery, but a flood delay could negate such benefits.