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The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) recognizes 16 designatable units (DUs) of Atlantic Salmon (Salmo salar) with 15 of those DUs representing extant anadromous populations. Last assessed by COSEWIC in 2010, this species is currently up for reassessment. As a primary generator and archivist of data related to Atlantic Salmon, Fisheries and Oceans Canada (DFO) is responsible for compiling and reviewing information on the species to help inform the upcoming reassessment. Here, as Part 1 of the pre-COSEWIC review of Atlantic Salmon, we focus on re-evaluating the DU structure. Over the last decade, new genetic and genomic data have become available that can be used to improve our understanding of the DU structure. COSEWIC's definition requires that a DU represents a discrete and evolutionarily significant unit of the species; therefore, we develop a framework using a weight of evidence approach to ensure that all DUs proposed here meet criteria for both discreteness and significance. Our approach incorporates genetic and genomic datasets, as well as life history and climate information. Our approach led to the subdivision of four of the previously defined COSEWIC DUs into multiple units, including the subdivision of Labrador, South Newfoundland, Gaspe-Southern Gulf of St. Lawrence, and Nova Scotia Southern Upland. In addition, based on a weight of evidence, we determined that some DUs required re-evaluations of their boundaries, which led to changes of the previously recognized DU boundaries in Quebec (between Western North Shore and Inner St. Lawrence) and in Newfoundland (between Northwest and Northeast Newfoundland). Re-evaluation of boundaries also supported that southern Gulf populations were not discrete from eastern Cape Breton populations, and thus these populations were combined into a single DU. Further, we identified two populations that belong in adjacent DUs, which would result in non-contiguous boundaries. This included de la Corneille River in Quebec (physically located in Western North Shore DU but groups with Eastern North Shore DU) and Gaspereau River in the Bay of Fundy (physically located in Inner Bay of Fundy DU but groups with Outer Bay of Fundy DU). Overall, using newly available data, we propose that there are 19 DUs of extant anadromous Atlantic Salmon that are supported by evidence of discreteness and significance, and we propose new names and numbering for these 19 putative DUs. [ABSTRACT FROM AUTHOR]