Our findings support the use of the trailing edge of the dorsal fin for long-term photo-identification studies at coastal aggregation sites. We photographically identified five white sharks repeatedly over a period of 16–22 years. Through the use of secondary characteristics and comparisons of first and last photographs of these five sharks, we found no evidence of changes in the size, shape, or arrangement of existing notches on these fins. We did, however, identify two sharks that incurred changes to their dorsal fins within the study period. These two sharks, a 4.9-m-TL female photographed each year from 2004 to 2008 (Fig. 3a) and a 4.3-m-TL male sighted November 07, 2008, and again 11 days later on November 18 (Fig. 3b), were shown to incur damage to their dorsal fins over the periods sighted. Though the trailing edge of the dorsal fin was damaged, creating a new notch, identifiable markings above and below this notch remained unchanged and sufficient for identification, illustrating how even relatively major fin trauma may results in the addition of a notch while leaving ample morphologic information for a positive identification. It is possible that the entire fin edge could be removed or altered due to major trauma preventing matching to previous sightings, but we saw no indication of this type of trauma during the 22 years of this study, which suggests this would be an extremely rare occurrence.
Dorsal fin identification also supports the long-term (≥15 years) site fidelity of white sharks at coastal aggregation sites and provides the longest empirical documentation of white shark longevity to date, >22 years. Individual identification illustrated that animals returned to these sites consistently throughout the study period and potentially during their entire adult life span. Each animal was not necessarily seen every year, but unequal effort across both spatial and temporal scales most likely contributed greatly to such gaps in identification. Additionally, the prevalence of short-term resights (<5 years) is also largely attributed to increased effort during the last 3 years of the study (Fig. 1).
This methodology, obtaining one photograph of the dorsal fin, requires far less effort than methods to document body pigmentation patterns and does not suffer from large rates of tag shedding or biofouling as traditional tagging methods do. Many of the white sharks in this study have been tagged with (on occasion multiple) pop-off archival tags, ultrasonic transmitters and/or floy tags, which have the potential to shed within a short time span (<1 year) (Jorgensen et al. 2010), precluding their use in long-term mark-recapture studies. In addition, further effort to automate the matching process will make the process more feasible with large datasets, allowing for the universal comparison of animals across different geographic regions regardless of diving effort or water clarity conditions.
Dorsal fin identification offers the foundation for mark-recapture studies to quantify the population size and trends of white sharks. If possible, we suggest all available data be recorded (e.g., fin photographs and body pigments). However, white shark studies are often very difficult and costly. Therefore, we propose that dorsal fins, because of their ease in recording, illustrated longevity and potential for universal application, be prioritized as a method to document individual white sharks. This methodology cannot only be used to monitor the status of the white shark population in the northeast Pacific, but unlike other photo-identification methods, it can be more uniformly applied to archive white sharks worldwide, standardizing documentation of individuals across vast spatial and temporal scales.