Understanding Salmon Migration Dynamics in a Data‐Limited Environment

Abstract

Effective management of natural resources and migratory fish populations depends on understanding the relationship between life cycle parameters, management decisions, and environmental conditions. However, the difficulty of data collection can result in small or nonrepresentative data sets, thus limiting the accuracy and utility of modeling results. This is especially true for studies of migratory species in changing environments. In such situations, reevaluating previous results in light of new data is an essential exercise. We used three additional years of acoustic telemetry data to assess and update previous statistical models of survival of juvenile fall‐run Chinook Salmon Oncorhynchus tshawytscha smolts emigrating through the southern portion of the Sacramento–San Joaquin River Delta (hereafter, “Delta”) in California. Some previously identified relationships were maintained with the additional data, including the significance of water flow measures in areas outside the main‐stem migration route and the role of water temperature and river discharge into the system for upstream reaches. The net downstream‐directed discharge in the interior Delta was associated with fish movement to a large water pumping facility, but its perceived importance in the original analysis depended on a single dominant study year. Survival in dry years was predicted to be higher in the upstream region of the main‐stem San Joaquin River when a temporary rock barrier limited entry of fish and river flow to the first distributary encountered in the Delta. In normal and wet years, through‐Delta survival was predicted to be higher for fish that departed the main‐stem river at that distributary. However, considerable uncertainty remains in the modeling results. Additional monitoring will be necessary to further reduce uncertainty and ensure that the modeling results reflect the system as it responds to climate change.

Route use of emigrating steelhead in a heavily modified river delta

“…Fish and Wildlife Service [USFWS] 2023). Furthermore, there has been no evidence that a survival benefit is accorded to juvenile fall-run Chinook Salmon by remaining in the San Joaquin River at the head of Old River (Buchanan and Whitlock 2022), and the survival differences between these two routes have been equivocal for steelhead as well (Buchanan et al 2021). Thus, the barrier has not been installed in recent years.…”

“…Thus, the barrier has not been installed in recent years. However, for both steelhead and salmon, there is evidence that the presence of the rock barrier has sufficient effect on hydraulics to improve survival in the upper reaches of the San Joaquin River route, presumably by preventing San Joaquin River water from entering Old River and instead causing it to move downstream in the San Joaquin River (e.g., survival difference of ~0.1 due to the barrier between the head of Old River to Turner Cut for steelhead in 2016 [Buchanan et al 2021] and up to ~0.4 for Chinook Salmon in dry years [Buchanan and Whitlock 2022]). Thus, the barrier and its effects remain of interest to managers due the potential survival benefit gained from the barrier's effect on flow (i.e., increased San Joaquin River flow).…”

“…Predation risk near the facilities and risk of entrainment in the pumps and water conveyance canals have prompted efforts to segregate migrating salmonids away from the facilities, including the installation of a temporary rock barrier during the spring at the head (i.e., source) of Old River, the first diffluence on the San Joaquin River that leads to the facilities. However, there are other entries to the interior Delta further downstream, and survival to the Delta exit is particularly low for both steelhead and Chinook Salmon O. tshawytscha that leave the main-stem river in the more tidal region downstream at Turner Cut (Figure 1; Buchanan et al 2021;Buchanan and Whitlock 2022). Thus, an ability to predict route use at several junctions (i.e., diffluences) is needed, both to limit mortality at the facilities and to predict overall migratory survival through the Delta in response to management decisions (e.g., barrier installation; Salmonid Scoping Team 2017).…”

“…However, there are other entries to the interior Delta further downstream, and survival to the Delta exit is particularly low for both steelhead and Chinook Salmon O. tshawytscha that leave the main‐stem river in the more tidal region downstream at Turner Cut (Figure 1; Buchanan et al. 2021; Buchanan and Whitlock 2022). Thus, an ability to predict route use at several junctions (i.e., diffluences) is needed, both to limit mortality at the facilities and to predict overall migratory survival through the Delta in response to management decisions (e.g., barrier installation; Salmonid Scoping Team 2017).…”

Hydropower peaking and stalled salmon migration are linked by altered reservoir hydraulics: A multidisciplinary synthesis and hypothesis

“…Perhaps most important is whether the pattern of migration delay and overwintering continues today, considering that most of the fish-migration data were collected more than a decade ago. Revisiting data-limited, early models of salmon migration dynamics as new data become available may reduce uncertainty and ensure that the models reflect the system as it responds to long-term environmental and management conditions (Buchanan & Whitlock, 2022).…”