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Coldwater fish in a warm water world: implications for predation of salmon smolts during estuary transit

Citation

Nobriga, Matthew; Michel, Cyril; Johnson, Rachel; Wikert, J.D. (2022), Coldwater fish in a warm water world: implications for predation of salmon smolts during estuary transit, Dryad, Dataset, https://doi.org/10.5061/dryad.h70rxwdjn

Abstract

Predator-prey systems face intensifying pressure from human exploitation and a warming climate with implications for where and how natural resource management can successfully intervene. We hypothesized young salmon migrating to the Pacific Ocean face a seasonally intensifying predator gauntlet when warming water temperature intensifies a multiple predator effect (MPE) from Striped Bass Morone saxatilis, and Largemouth Bass Micropterus salmoides. We evaluated this hypothesis using data synthesis and simulation modeling. 2. Contemporary studies based on acoustically-tagged fish reaffirmed older observations that Chinook Salmon smolts must transit the Delta before water temperature reaches 20°C or mortality will be nearly 100%. Striped Bass attack rates on tethered smolts were insensitive to distance from shore and water temperature whereas Largemouth Bass attack rates were highest near shorelines in warm water, supporting the temporal aspect of the hypothesis. Whether the combined effects of the two predators produces an MPE remains unconfirmed due to limitations on quantifying salmon behavior. 4. We used multiple simulation models to try to reconstruct the empirical relationship between smolt survival and water temperature. Simulations reinforced attack rate results, but could not recreate the temperature dependence in smolt survival except at higher than observed temperatures. We propose three hypotheses for why and recommend discerning among them should be a focus of research. 5. We found significant linear relationships between monthly mean inflow to the Delta from each of its two largest tributaries and monthly mean water temperatures along associated salmon migration routes, but these relationships can be nonlinear, with most of the correlation occurring at low inflows when water temperature is largely controlled by air temperature and day length. As the global climate warms, changed circumstances in predator-prey relationships may present important challenges when managing species vulnerable to extinction in addition to presently more abundant species. --

Methods

This data set is a mixture of Microsoft Excel files where data were compiled and then coverted into .csv files that were analyzed in the R environment and simulation model code in R that generates its own data and does not require an external data source. The field data come from various websites and previously published studies of Chinook salmon smolt survival in California's Sacramento-San Joaquin Delta.

Each Excel file has an associated ReadMe included as the first tab in the workbook. The ReadMe files for the .csv's derived from the Excel files are in an Excel file called CSV_ReadMe.xlsx. The ReadMe's for the R scripts are included as #comments interspersed with the active code.

Raw flow data for the Sacramento-San Joaquin Delta can be downloaded here: https://data.cnra.ca.gov/dataset/dayflow

Raw temperature used in our study can be extracted from the 20-mm Survey and Spring Kodiak Trawl Survey databases housed here: https://iep.ca.gov/Data/IEP-Survey-Data

We apologize but the second link above does not match what is in the paper. These fish survey data were migrated to the link above while our manuscript was in press and we did not notice the update until after we had finalized the page proofs.

Usage Notes

The excel files should guide the user through them sufficiently well for others to reproduce the analyses that come from them. The R code creates some of its own data. In other places, a user is pointed to particular associated .csv files.