Designing Optimal Flow Patterns for Fall Chinook Salmon in a Central Valley, California, River

Abstract

Widespread declines in stocks of Pacific salmon in the genusOncorhynchushighlight the need for research to find new and effective management strategies for recovery. Two recovery objectives are (1) to ensure that recruitment is adequate to rebuild self-sustaining populations and (2) to maintain phenotypic diversity. This study seeks to understand how seasonal flow patterns in a flow-regulated California river might be managed to attain each of these recovery objectives, specifically for the fall and late-fall runs of chinook salmon O. tshawytscha. We ask two questions: (1) Does the optimal pattern of seasonal flows change as the amount of water available is constrained by droughts or diversions of flows? and (2) How do optimal flow regimes designed for the two conservation objectives differ? We coupled simulated annealing with a recruitment model to find flow regimes that maximize either the number of smolt out-migrant ''recruits'' (MR) or the variation in spawning times among recruits (MV). Optimal flow regimes identified for both the MR and MV objectives changed as we increased the annual quantity of water available, allocating higher flows during the spring and fall seasons. Flow regimes that optimized the MR and MV objectives were different. For example, the MV flow regime with unlimited annual flow provided a pulse of high flow 2 weeks before the peak spawning date of the minority late-fall run. Simulated recruits produced by MV flow regimes were fewer in number—and had parents that spawned later and over a wider range of dates—than recruits produced by MR flow regimes. Although these results have not been verified by empirical studies, they demonstrate the potential for managing species with special conservation status by combining state-of-the-art numerical optimization methods with mechanistic ecological models. Anadromous salmonids play a significant role as keystone species in the river ecosystems drain- ing to the Pacific coast of North America (Willson and Halupka 1995; Cederholm et al. 1999). His- torically, Pacific salmon in the genus Oncorhyn- chus permeated coastal rivers and streams from Alaska to southern California. A tendency to re- turn to spawn in their natal river allowed salmon populations to adapt to local environmental con- ditions (Waples 1995) and led to an adaptive ra- diation in life history traits for chinook salmon O. tshawytscha (Healey 1994). Collectively, salmon populations diversified into an array of populations (also known as ''runs,'' ''races,'' or ''stocks'') with life histories distinguished by spawning time and place. The temporal diversity and geographic distribution of salmon populations resulted in the presence of one life history type or another during