Individual identification of bony fishes using unique body markings: Implications and applications.

The natural variation in animal body markings, such as spots, stripes and blotches, offers a powerful tool for researchers, conservationists, citizen scientists and resource managers to distinguish specific individuals within species. By building libraries of photo-identified individuals, we can track and differentiate individuals over time and space, thereby non-invasively and often cost-effectively studying species' biology and behaviour. This, in turn, enhances our understanding of species' ecological roles, and informs and supports effective conservation strategies. Although photo-identification has been successfully applied to various aquatic species, including cetaceans, sharks and rays, it remains surprisingly underutilised for bony fishes. Indeed, despite their striking diversity of colours and patterns, relatively few studies have used natural markings to non-invasively investigate bony fish biology and conservation. In this review, we highlight the potential of photo-identification as a valuable research and management tool for these fishes in both field and laboratory environments. We outline the scientific, practical and ethical benefits of this approach, illustrating how individual identification can advance our understanding of fish biology and support their conservation efforts. We also discuss previous applications of photo-identification in bony fishes, examine barriers to its broader adoption and address common misconceptions that may limit its use. We propose strategies to overcome these challenges driven by advancements in camera technology and artificial intelligence, and discuss scenarios where photo-identification may prove particularly effective. Through this review, we therefore aim to encourage the broader use of natural body markings as a non-invasive method in bony fish research, management and conservation.