The current study establishes that photo-ID can be used effectively to identify T. lymma individuals using dorsal spot patterns. Our results reveal a high degree of individual variation in T. lymma spot patterns (Supplementary Fig. 4). I3S photo-matching software correctly identified 161 different T. lymma individuals and proved to be an effective method to rapidly recognise individual spot patterns among T. lymma individuals. The physical recapture of individuals demonstrated that the overall pattern of larger spots persists and allows for accurate identification for at least 496 days. Given the longevity of spot patterns and the high probability of tag loss, we suggest photo-ID as a preferable, non-invasive alternative to traditional tagging methods for individual identifications of T. lymma.
The implementation of computer-aided identification with I3S confirmed that the spot patterns of T. lymma were unique and distinguishable, and validated its use for photo-ID of individuals. With a true-positive detection rate exceeding 90%, the results of our study affirm the reliability of I3S as a photo-matching tool. These findings are consistent with previous studies which also used I3S to identify individual teleost fish such as Paracirrhites forsteri23 and Stereolepis gigas10, and more commonly elasmobranchs such as A. narinari, Carcharodon carcharias, and Carcharias taurus12,24,25. Among these studies focusing on elasmobranchs, only C. taurus achieved a higher true-positive detection rate of 92% exceeding that established here, highlighting the suitability of T. lymma for photo-ID based identification. Our study also establishes a threshold score of 8.39 for the identification of T. lymma individuals, where match scores below this value could be considered true-positive matches (Fig. 2). This threshold score is also comparable to those previously established for other species of the family Dasyatidae, such as M. microps (10.0613).
Assessing the long-term applicability of photo-ID requires knowledge of the degree of change in T. lymma dorsal markings over time. Of the 29 individuals that were recaptured, only one individual had an observable change in the spot pattern (Fig. 3; adult female). However, this change was only observed in smaller spots (< 5 mm diameter) located on the outer edge of the disc and on the cranium, both of which had not been selected for the I3S fingerprint file. Despite this, the larger spot patterns in the centre of the disc remained unchanged, allowing for successful re-identification of the individual for up to 16 months using I3S. Specifically, the image of the re-captured individual had the lowest match score to the image from the initial capture event, and retained its position as the most likely match within the database. This emphasises the importance of considering the stability of individual spots, as well as the role of larger and more central spots in maintaining the overall stability of the pattern for long-term identification. While the positive correlation between spot size and disc width suggests that photo-ID may be more reliable for adult individuals of T. lymma, it is unclear whether spot patterns change with age or developmental stage. For example, some species, such as the reticulate whipray, Himantura uarnak, appears to gain new spots with age instead of having its existing spots grow in size14. To fully understand population dynamics and ontogenetic distribution shifts, it will be essential to confirm the stability of spot patterns over longer durations (i.e., > 500 days). While this study was conducted on wild populations of T. lymma, it would be valuable to independently validate these findings in a captive setting, such as those in aquaria. Based on these observations, it is advisable to use only spots larger than 5mm in diameter and those located in the centre of the disc to ensure matching accuracy and minimise errors due to potential spot pattern changes over time. Due to the limited tag retention times observed in the current study, alternative marking methods (such as, microchips or Passive Integrated Transponder (PIT) tags26,27) should also be considered in future studies to assess spot-stability in T. lymma over greater time scales.
While substantial scars or deformities can serve as secondary features for identification (e.g., flapper skate, Dipturus intermedius26), the high healing capacity of T. lymma means that scar tissue cannot be relied upon as a defining feature for photo-ID. Anecdotally, in the current study recaptured individuals did not have any visible scarring at the tag attachment site three months after losing their tag. It is also important to note that injuries to the dorsal surface may not result in long-term scarring but may still have the capacity to alter boundaries of individual spots (see Supplementary Fig. 5). Though injuries may temporarily alter spot boundaries, they do not affect the relative position of spots or their pattern, and thus will not impact the placement of points in the fingerprint files and subsequent I3S matches, as the software relies solely on the central position of the spots rather than their shape.
Photo-ID offers several advantages over conventional tagging methods for T. lymma. Firstly, the physical risks associated with collecting stingrays for tagging, including the potential harm from their barbed tails, pose a significant challenge to both researchers and the animals. Additionally, the physical attachment of the tags can restrict movement and cause capture/handling stress, potentially impacting the survival or behaviour of the individual28,29. In contrast, photo-ID will not always require stingrays to be captured, thus avoiding the risks and impacts associated with traditional tagging methods when identifying individuals. It is still important to note, however, that the successful application of photo-ID in the wild is dependent on several factors, such as the clarity of the water, the behaviour of the animals towards observers, and the accessibility of the population in terms of depth and habitat30. Nevertheless, photo-ID is likely to be more cost-effective than conventional tagging methods, particularly when the cost of tagging equipment and the labour required to attach and monitor the tags are considered31. Additionally, since tags can be shed, photo-ID is more effective at identifying individuals over longer durations as the dorsal spot pattern of T. lymma appears to be stable over the course of the study. However, future research is needed to evaluate the factors that may affect dorsal spot pattern stability in T. lymma and to determine which parts of the body future photo-ID efforts should focus on to ensure maximum pattern stability. Overall, photo-ID offers a safer and more cost-effective alternative to traditional tagging methods, making it a more advantageous method for individual identification of T. lymma, especially in areas where the collection of organisms is prohibited. Lastly, photo-ID can provide longer monitoring time scales while avoiding the negative impacts associated with invasive methods, making it a suitable option for long-term research on this species.
The current study highlights the potential for computer-aided photo-ID to improve the efficiency of long-term monitoring of T. lymma populations. Future applications of photo-ID may not require physical capture as photographs of T. lymma can be easily taken alongside common research methods, such as dive surveys10,28,29 or aerial surveys over shallow-water habitats20 (Supplementary Fig. 1). With increasing capabilities in machine learning, the process of identifying individual animals from photographs could likely be fully automated, making large-scale photo-ID projects even more feasible. Collaborative efforts between researchers and local stakeholders can also expand the spatial coverage of data collection and complement non-invasive monitoring programmes to increase knowledge on the biology and conservation of this species31. Recreational scuba divers or fishermen can also help collect photo identification data on stingrays by participating in community science programs or by contributing their own photographs to ongoing research projects, similar to MantaMatcher31 and the Common skate photo-ID database for Scotland32. Overall, by expanding photo-identification databases in both size and duration, the performance of photo-matching software will improve, as there will be increased availability of matches to draw upon24,33.