Taxonomic inflation in fish systematics: A critical review with illustrative evidence assessment and integrative case study of the Salmo trutta Complex.

Fish species are being described at an accelerating rate, but a substantial fraction of recent descriptions rests on evidentiary standards too weak to distinguish genuine species-level discontinuities from intraspecific variation, local adaptation and the artefacts of single-marker analyses, a pattern of taxonomic inflation with documented costs for biodiversity databases, conservation prioritization and macroevolutionary inference. This review examines the problem in fish systematics and uses the Turkish brown trout (Salmo trutta complex) as an illustrative empirical example through which the underlying evidentiary weaknesses, and the means of correcting them, can be made concrete. A structured assessment of 21 Turkish Salmo descriptions published between 2000 and 2026 shows that they rest overwhelmingly on external morphology and, in fewer than half of cases, a single mitochondrial marker, with nuclear genomic, crossbreeding and karyotypic evidence almost entirely absent. Integrative evidence runs counter to these delimitations: controlled crossbreeding within the Danubian (DA) lineage yields fully viable hybrids with shared chromosomal architecture, and multi-marker analysis across thousands of individuals and numerous drainage basins reveals widespread haplotype sharing and cross-basin admixture, indicating that much of the observed variation reflects intraspecific diversity rather than distinct species. Drawing these threads together, a four-tiered graded evidentiary framework (Standards A-D) is proposed, scaled to biological and logistical context and anchored in the concordance criterion, whereby species recognition requires convergence of multiple independent lines of evidence. The principal recommendation is sustained investment in capacity-building, open-access pipelines and reciprocal training rather than any centralized validation body, with mitochondrial-only descriptions flagged as provisional pending nuclear or reproductive corroboration.