the online version of this article ( doi:10.1007/s00227 - 011 - 1643 - 5 ) contains supplementary material , which is available to authorized users . white sharks , carcharodon carcharias , observed off the coast of california belong to a genetically distinct population unique to the eastern pacific ( jorgensen et al . 2010 ) . subadult and adults travel long distances offshore from california and northern mexico during yearly migrations ( boustany et al . 2002 ; weng et al . 2007 ; domeier and nasby - lucas 2008 ; jorgensen et al . these offshore movements traverse international waters where individuals are susceptible to fishing mortality , yet few data are available from potential offshore fisheries , confounding efforts to determine white shark population status . therefore , fishery - independent methods must be employed to begin to estimate their population size . mark - recapture methods have been used to estimate white shark population sizes in south africa ( cliff et al . 1996 ) , australia ( strong et al . 1996 ) and the northeast pacific ( chapple et al . 2011 ) . these studies are dependent on the assumption that white sharks return to the same location over multiple sampling periods . recent work has illustrated the short - term [ e.g. , < 8  years or 25% of estimated lifespan ( cailliet et al . 1985 ) ] seasonal site fidelity of sharks to coastal aggregation sites . white sharks tagged off coastal california and guadalupe island , mexico returned to their tagging location following offshore movements ( domeier and nasby - lucas 2008 ; jorgensen et al . similarly , a combination of a satellite tag and dorsal fin identification described the movement of a shark in south africa to australia and back in 9  months ( bonfil et al . 2005 ) . however , no studies have tested the long - term [ e.g. , > 14  years or 50% estimated lifespan ( cailliet et al . these mark - recapture frameworks also assume external identification tags or unique markings are conserved and identifiable throughout the study . shedding and therefore , current research on white sharks has focused on the use of distinctive natural markings as individual identifiers ( klimley and anderson 1996 ; strong et al . 1996 ; bonfil et al . 2005 ; domeier and nasby - lucas 2007 ; gubili et al . one method proposed for individual identification in white sharks at guadalupe island utilizes pigmentation patterns ( domeier and nasby - lucas 2007 ) . though resighting requires only one photograph of the gill flaps , pelvic fin or caudal fin on either side , initial identification requires photographs of all three of these locations on both sides of the animal . in addition , the conditions necessary to support this type of identification , superb water visibility and high recreational diving effort , are unique to only guadalupe island . collecting these data is likely to be too labor intensive and impractical in areas outside of guadalupe island with low water visibility ( e.g. , south africa ) and/or limited diving effort ( e.g. , california ) . additionally , pigmentation patterns in several sharks were shown to naturally change in shape and/or size from year to year ( domeier and nasby - lucas 2007 ) . alternately , dorsal fins can be photographed from either side and are often observed out of the water , precluding issues with water clarity or the need for divers . dorsal fin identification , requiring limited effort , is possible at all white shark aggregation sites globally . a comparison of matched individuals using genetic markers and dorsal fin photographs showed a high degree of concordance ( 85% ) between the two techniques over short ( 5  year ) time periods ( gubili et al . white sharks have been shown to exhibit short - term site fidelity and repeatable individual identification through dorsal fin photographs ( klimley and anderson 1996 ; strong et al . 2009 ) and body pigmentation ( klimley and anderson 1996 ; domeier and nasby - lucas 2007 ) ; however , in order to understand the long - term status of white sharks , mark - recapture studies must be expanded out to longer time series . therefore , it is imperative to determine whether assumptions regarding site fidelity and identification are valid across these longer time periods ( e.g. , > 14  years ) . here , we aimed to determine ( 1 ) the stability of posterior edge dorsal fin morphology as individual identifiers over long time periods and ( 2 ) use this information to illustrate the long - term site fidelity of individual mature and subadult white sharks at coastal aggregation sites . this study began at the south farallon islands in 1987 ( see anderson et al . , the study was expanded to include coastal areas near tomales bay , california and beginning in 2005 , a small amount of marine mammal blubber was used as bait to increase encounter rates and duration . high - resolution photographs and video were taken of each shark when they came near the surface to investigate a seal - shaped decoy . we extracted dorsal fin images from these photographs and , if above water photographs were not available and water clarity permitted , from underwater video . estimates of total length ( tl ) were also made when possible as sharks swam alongside a research skiff of known length . images were assessed for quality based on four criteria : angle , size , focus , and contrast . images of insufficient quality were removed ( see online resources 1 for discussion of photograph processing methods ) . experts ( individuals with demonstrated expertise matching dorsal fin photographs ) then matched the remaining accepted fin photographs within and across all years simultaneously . if individuals were identified multiple times over the study period , we compared the earliest and latest photographs to determine long - term stability of dorsal fin morphology . when possible , secondary characteristics ( e.g. , tags , permanent scars , mutilations , etc . ) were used to validate these matches . . experts matched 20 randomly chosen photographs from 12 sharks , which had obvious secondary characteristics ( not evident in the fin photographs ) . matching results from each expert were compared to the true matches based on these known secondary characteristics . confirmed males were sighted nearly twice as often as confirmed females ( 1.8:1 ) , though this ratio may be biased because it is easier to confirm the presence of claspers than the absence . if the sex could not be confirmed , the animal was labeled  unknown . four experts accurately identified individuals and matches 98% of the time with no false positives and only one false negative . similarly , gubili et al . ( 2009 ) used genetic data to show nearly 85% accuracy in experts matching dorsal fin photographs . annually , limited effort and photograph quality resulted in the identification of few sharks before 2006 when effort increased and digital slrs were used . prior to 2006 , over 60% of sharks were resighted ( 29 of 48 ) . in 2006 and 2007 , significantly more new sharks were recorded ( 76 ) , but by 2008 , 35% ( 27 ) of these new sharks had been resighted at least once . twenty - one individuals were repeatedly identified over periods 5  year , ten individuals 10  years , and five individuals 15  years ( fig .  1 ) . to illustrate the stability of markers and the empirical evidence of longevity , we describe five individuals resighted over periods 15  years at south farallon islands ( fig . 1cumulative number of individuals identified repeatedly over time periods ranging from one to twenty - two yearsfig . 2individuals a tj , b ct , c ft , d bh , and e rf repeatedly identified by dorsal fin morphology over periods > 15  years . the year indicates the date the photograph was takentable  1yearly identification of five white sharks resighted between 1987 and 20081987198819891990199119921993199419951996199719981999200020012002200320042005200620072008bhxxxxx xxxxct  x  x  xxx  x  xxxxxxxx  ftx  xxxxxxrfxxx  xtjxx  xxxxxxx depicts when an animal was sighted . all individuals were identified in 1993 when effort was increased cumulative number of individuals identified repeatedly over time periods ranging from one to twenty - two years individuals a tj , b ct , c ft , d bh , and e rf repeatedly identified by dorsal fin morphology over periods > 15  years . the year indicates the date the photograph was taken yearly identification of five white sharks resighted between 1987 and 2008 x depicts when an animal was sighted . all individuals were identified in 1993 when effort was increased the longest record was shark tj , a 4.5-m - tl male . tj was sighted 8 times over 22  years , initially in 1987 and most recently in 2008 ( fig  2a ) . shark ct , a 3.5-m - tl male was sighted on 14 occasions over 20  years ( fig . ft , a 4-m - tl male , was sighted 7 times in 19  years ( fig . bh and rf , a 4-m - tl male sighted 9 times ( fig . all tl estimates given refer to the size at the most recent sighting of each shark . 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 1622  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.fig . 3identification of fins after observations of fin damage . a a female shark photographed in 2004 with a complete fin , in 2005 with large lacerations on the fin , and in 2008 with scars from the damage , but all other distinguishing markings intact . b a second shark showing new damage over a two - month period in 2008 identification of fins after observations of fin damage . a a female shark photographed in 2004 with a complete fin , in 2005 with large lacerations on the fin , and in 2008 with scars from the damage , but all other distinguishing markings intact . b a second shark showing new damage over a two - month period in 2008 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 ) 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 can not 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 . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
mark - recapture techniques can be used to estimate white shark ( carcharodon carcharias ) population abundance . 
 these frameworks are based on assumptions that marks are conserved and animals are present at the sampling location over the entire duration of the study . though these assumptions have been validated across short - time scales for white sharks , long - term studies of population trends are dependent on these assumptions being valid across longer periods . 
 we use 22  years of photographic data from aggregation sites in central california to support the use of dorsal fin morphology as long - term individual identifiers . 
 we identified five individuals over 1622  years , which support the use of dorsal fins as long - time individual identifiers , illustrate strong yearly site fidelity to coastal aggregation sites across extended time periods ( decades ) , and provide the first empirical validation of white shark longevity > 22  years . 
 these findings support the use of fin morphology in mark - recapture frameworks for white sharks.electronic supplementary materialthe online version of this article ( doi:10.1007/s00227 - 011 - 1643 - 5 ) contains supplementary material , which is available to authorized users .