Examining the viability of dorsal fin pigmentation for individual identification of poorly-marked delphinids

Scientific Reports, Aug 2018

Dolphin photo-identification has traditionally relied only on distinctive markings on the dorsal fin—this is problematic for delphinids whose populations exhibit a low mark ratio. We used common dolphins (genus Delphinus) as a model species to assess the viability of using pigmentation for photo-identification. Using a photo-identification catalogue of 169 adult individuals collected between 2002 and 2013, we extracted features that quantified pigmentation in a manner that was robust to lighting artefacts and dorsal fin orientation. We determined the proportion of individuals which exhibited pigmentation and examined temporal stability by (i) visually examining individuals and (ii) testing for seriation. We found 88–91% of images could be manually matched to the correct individual in the catalogue based on pigmentation patterns alone. A linear discriminant analysis classifier correctly identified the correct individual 77% of the time. We found 95% common dolphins exhibited distinctive pigmentation—all of which were temporarily stable. Our work challenges the current thinking that pigmentation is an unreliable feature for delphinid photo-identification and suggests that this feature could be applied to common dolphins and other poorly-marked delphinids.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://www.nature.com/articles/s41598-018-30842-7.pdf

Examining the viability of dorsal fin pigmentation for individual identification of poorly-marked delphinids

Abstract Dolphin photo-identification has traditionally relied only on distinctive markings on the dorsal fin—this is problematic for delphinids whose populations exhibit a low mark ratio. We used common dolphins (genus Delphinus) as a model species to assess the viability of using pigmentation for photo-identification. Using a photo-identification catalogue of 169 adult individuals collected between 2002 and 2013, we extracted features that quantified pigmentation in a manner that was robust to lighting artefacts and dorsal fin orientation. We determined the proportion of individuals which exhibited pigmentation and examined temporal stability by (i) visually examining individuals and (ii) testing for seriation. We found 88–91% of images could be manually matched to the correct individual in the catalogue based on pigmentation patterns alone. A linear discriminant analysis classifier correctly identified the correct individual 77% of the time. We found 95% common dolphins exhibited distinctive pigmentation—all of which were temporarily stable. Our work challenges the current thinking that pigmentation is an unreliable feature for delphinid photo-identification and suggests that this feature could be applied to common dolphins and other poorly-marked delphinids. Introduction The need to monitor cetacean populations for management purposes typically requires assessments of abundance1,2, site fidelity3, movement patterns4,5 and social structure6. A range of methods have been used to examine these parameters including: distance-sampling where line-transect surveys are used to count animals to assess their density7,8, and mark-recapture (MRC) analysis using sightings (and re-sightings) of naturally or artificially marked individuals as a sample of the population9. Distance-sampling techniques assess species at the population level but do not give information about specific individuals. In contrast, MRC methods, including photo-identification (photo-id), samples individuals—which in turn, may give an understanding of individual-level site fidelity, movement patterns and social structure. Photo-id is a cost-effective approach that utilises unique naturally-occurring marks, eliminating the need to physically capture or tag the organism. This method has been applied to a variety of species through the identification of unique natural features. Historically, the use of photo-id has been highly applied to well-marked coastal delphinids (such as bottlenose dolphins, Tursiops truncatus), but less successfully to the poorly-marked, gregarious common dolphins (Delphinus spp.) due to the difficulties in identifying individuals. For most delphinid photo-id studies, the most frequently used identifying feature is nicks and notches present on the leading and trailing edges of the dorsal fin10,11. However, the use of nicks and notches for individual identification is problematic for populations which have a low mark ratio (ratio of marked to unmarked dorsal fins). For example, common dolphins are poorly marked and therefore difficult to identify—they have the second lowest mark ratio of any delphinid (ca 10–46% of animals have identifiable dorsal fin nicks/notches12,13). This difficulty (along with the general logistical problems associated with studying any large aggregated population) is reflected by the very small number of studies that have been published on Delphinus worldwide—only six in total12,14,15,16,17,18. Finding other features unique to individuals could assist in the recognition of poorly-marked delphinids. Increasing the proportion of animals that can be catalogued and providing a secondary feature to confirm matches would likely have a great impact on those studies that rely on individual identification. In this context, we consider pigmentation to be a feature worthy of exploration. Pigmentation has been successfully applied as a feature for individual recognition of delphinids previously (see Table 1), but its use has traditionally been limited, mainly due to three concerns: (1) pigmentation may not be prevalant in populations; (2) pigmentation may be not stable over time, and (3) pigmentation may not have sufficient discriminatory power to identify individuals in the presence of imaging artifacts. Table 1 Studies of delphinids which have used pigmentation patterns for individual identification, specifically outlining if pigmentation prevalence, stability or discriminatory power has been examined via visual or statistical analysis. Full size table Some studies have examined pigmentation prevalance within populations and visually assessed pigmentation stability over time (see Table 1 for summary). However, there are no published dolphin studies which have devised methods to quantitatively verify pigmentation stability or measure its discriminatory power when identifying individuals. Moreover, while dorsal fin pigmentation has been used as a secondary feature to confirm individual common dolphins (...truncated)


This is a preview of a remote PDF: https://www.nature.com/articles/s41598-018-30842-7.pdf

M. D. M. Pawley, K. E. Hupman, K. A. Stockin, A. Gilman. Examining the viability of dorsal fin pigmentation for individual identification of poorly-marked delphinids, Scientific Reports, 2018, Issue: 8, DOI: 10.1038/s41598-018-30842-7