Development and Genetic Monitoring of a Putatively Thiamine Deficiency Complex Tolerant Atlantic Salmon Broodstock

The concept of assisted evolution suggests that hurdles to species restoration in modified environments might be overcome by selective breeding of individuals better-suited to the contemporary evolutionary landscape. Here, we introduce an experimental program aimed at developing an Atlantic salmon broodstock resilient to dietary thiamine deficiency complex (TDC). Dietary TDC is caused by overconsumption of lipid-rich prey items containing thiaminase, leading to insufficient maternal allocation of thiamine to eggs and high offspring mortality. Within Lake Champlain, USA, where TDC is widespread due to consumption of invasive Alewife, we conducted offspring survival trials of 114 wild-reared salmon pairs. We documented among-family patterns in offspring survival to yolk-sac absorption. Some females allocated sufficient free thiamine to eggs to avoid severe offspring mortality from TDC, while most did not and suffered complete or severe reproductive failure. Offspring from "resilient" families with high survival were used as founders of a putatively "TDC-Tolerant" broodstock but at the expense of low effective population size (N _e = 48). We also developed a control broodstock that included additional families that only survived after supplemental thiamine treatment (MAX-Gen, no selection, N _e = 152) to represent a status quo scenario. From 2016 to 2025 we (1) developed a Genotyping-in-Thousands by Sequencing (GT-seq) marker panel (2) implemented parentage-based tagging (3) curated two experimental broodstocks and released ~1.6 million of their offspring into Lake Champlain and (4) implemented a sampling program to monitor demographic and evolutionary outcomes. In Lake Champlain, TDC continues to be a major restoration bottleneck afflicting 42%-76% of salmon evaluated annually since 2007 and is an increasing conservation issue globally. Continuation of this work will determine whether assisted evolution improved TDC-resistance in the wild and also evaluate the genetic and demographic consequences of contrasting broodstock strategies.