Results of previous multilocus sequence and whole-genome-based analyses have suggested that a homogeneous group of isolates belonging to the genus Tenacibaculum , represented by strain TNO020^T and associated with skin ulcer development in sea-farmed fish, represents an as-yet-undescribed species. Comparative whole-genome analysis performed in the present study clustered five isolates, including TNO020^T, in a distinct lineage within the genus Tenacibaculum . Phenotypic differences, high intra-cluster average nucleotide identity (ANI) values and low ANI values with other Tenacibaculum species support the proposal of a novel species, for which we propose the name Tenacibaculum piscium sp. nov. with strain TNO020^T (=CCUG 73833^T=NCIMB 15240^T) as the type strain. Further, large-scale genome analyses confirmed the existence of two different phylogenetic lineages within ‘ T. finnmarkense ’, a species effectively but not validly published previously. ANI values just above the species delineation threshold of 95–96 % confirmed that both lineages belong to the same species. This result was also supported by DNA–DNA hybridization values. Phenotypically, the two conspecific lineages are distinguishable by differences in growth temperature range and ability to degrade l-proline. For the group of isolates already commonly known as ‘ T. finnmarkense ’, we propose the name Tenacibaculum finnmarkense sp. nov., with strain TNO006^T (=CCUG 73831^T=NCIMB 15238^T) as the type strain. We further propose the subdivision of T. finnmarkense sp. nov. into two genomovars, T. finnmarkense genomovar finnmarkense with strain TNO006^T (=CCUG 73831^T=NCIMB 15238^T) as the type strain and T. finnmarkense genomovar ulcerans with strain TNO010^T (=CCUG 73832^T=NCIMB 15239^T) as the type strain.

This comprehensive population study substantially improves our understanding of the epizootiological history and nature of an internationally important fish-pathogenic bacterium. The MLVA assay developed and presented represents a high-resolution typing tool particularly well suited for Yersinia ruckeri infection tracing, selection of strains for vaccine inclusion, and risk assessment. The ability of the assay to separate isolates into geographically linked and/or possibly host-specific clusters reflects its potential utility for maintenance of national biosecurity. The MLVA is internationally applicable and robust, and it provides clear, unambiguous, and easily interpreted results. Typing is reasonably inexpensive, with a moderate technological requirement, and may be completed from a harvested colony within a single working day. As the resulting MLVA profiles are readily portable, any Y. ruckeri strain may rapidly be placed in a global epizootiological context.