Global trends, biases, and gaps in biogeochemical tagging studies of migratory fish

Migratory fishes connect ecosystems and support fisheries and water security, yet their movements and population linkages remain poorly resolved at global scales. Biogeochemical tags (trace elements or stable isotopes) recorded in fish bioarchives provide retrospective evidence of migration, natal origin, and connectivity, offering a temporal perspective that extends beyond the inherently short-term scope of direct tracking methods. Here, we synthesize 1305 studies published between 1973 and 2023 to quantify global patterns, biases, and gaps in the biogeochemical tagging of migratory fish. Organizing the literature into ten research themes and distinguishing research effort (publication-level attention) from sampling effort (empirical spatial coverage), we find that publication growth has not translated into proportional gains in taxonomic, geographic, or life-history representativeness. Approximately one-sixth of known migratory fish species have been investigated using biogeochemical tagging, with research disproportionately concentrated on commercially important taxa, later life stages, and otolith-based archives. Coverage patterns are consistent with interacting constraints in bioarchive availability, tracer sensitivity, environmental baseline coverage, and analytical tractability, concentrating applications in contexts that generate strong chemical contrasts. Where environmental baselines are sparse or mismatched, biogeochemical inference is less comparable across studies and difficult to scale across regions and systems. Reconciling monitoring demand with research effort and baseline supply across global fishing regions highlights extensive areas where high migratory fish richness coincides with limited inferential capacity. We outline priorities for advancing biogeochemical tagging from case studies to decision-ready monitoring systems through improved baselines, interoperability, uncertainty treatment, and management-oriented implementation strategies.