Assessing diets of California salmonines using fatty acid signatures and its impact on observed thiamine deficiency

Abstract

California Central Valley (CCV) Chinook Salmon (Oncorhynchus tshawytscha) hatchery fry were diagnosed with thiamine (vitamin B1) deficiency complex (TDC) in 2020, launching a statewide monitoring program to evaluate egg thiamine concentrations in populations of Chinook Salmon, Coho Salmon (O. kisutch), and Steelhead Trout (O. mykiss). There have been two proposed hypotheses for the drivers of thiamine deficiency: consumption of prey with increased thiaminase activity or with a high lipid content. Limited Chinook Salmon stomach content analysis showed a dominance of Northern Anchovy, a prey species with high thiaminase activity and a high lipid content, preceding the observation of thiamine deficiency. Therefore, the objective of this research was to identify the diet of Chinook Salmon, Coho Salmon, and Steelhead Trout using fatty acid signature (FAS) analysis and link their diet to the observed TDC. From 2020-2022, eggs from female Chinook Salmon, Coho Salmon, and Steelhead Trout were collected from CCV and northern California (NC) hatcheries. Main historical salmonine forage species were also collected from the Pacific Ocean. Fatty acid signatures and thiamine concentrations were quantified from eggs and prey using gas chromatography/mass spectrometry and high performance liquid chromatography, respectively. Chinook Salmon egg FASs differed significantly between CCV and NC populations suggesting NC Chinook Salmon were predominantly reliant on Pacific Herring, characterized by greater proportions of oleic acid (18:1n-9) and greater thiamine reserves. However, CCV Chinook Salmon populations appeared to consume mostly Northern Anchovy, characterized by high proportions of eicosapentaenoic acid (20:5n-3) and decreased thiamine reserves. Chinook Salmon egg FASs were also significantly different among seasonal run (fall, late fall, winter, and spring), with the winter-run and late fall-run Chinook Salmon being the most impacted by thiamine deficiency. Coho Salmon eggs from NC hatcheries had variable FASs; those indicative of Northern Anchovy consumption were thiamine deficient, while those indicative of a larger reliance on Pacific Herring had greater thiamine reserves. Steelhead Trout egg FASs presented variation among hatcheries; Trinity River Hatchery eggs were mostly rich in 18:1n-9 indicating Pacific Herring in the diet and Nimbus River Hatchery eggs were rich in 22:6n-3 which indicated consumption of Market Squid. Steelhead Trout eggs collected from Mokelumne River, Feather River, and Coleman National Fish hatcheries were generally rich in 20:5n-3 which corresponds to a diet of Northern Anchovy. Interestingly, Nimbus Steelhead Trout eggs had the greatest presence of thiamine deficiency, followed by eggs rich in 20:5n-3. Egg lipid content was not strongly correlated with decreased thiamine concentrations, but polyunsaturated fatty acid proportions were. The egg unsaturation index had a strong negative correlation with egg thiamine concentration, indicating the importance of lipid quality over quantity when considering drivers of TDC. This study highlights how FASs can be used to track the effect of changing ocean regimes on complex food webs and how diet shifts can impact salmonine health.