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Pinnipeds are K-strategist species with relatively low intrinsic population growth rates (Härkönen et al. 2002). They have evolved life history traits, such as low fecundity and high adult survival, which make their populations resilient to periods of unfavorable environmental conditions (Stubbs 1977; Schaffer 1983; Stearns 1989; Kirkwood et al. 2000). The long potential lifespans of many K-strategist species, such as harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus), enable individual high-quality animals to make a disproportionately large lifetime contribution to population growth (Kirkwood et al. 2000; Jagers and Harding 2009; Badger et al. 2023). We have recently observed wild harbor seals which are more than 20 years of age, with both a male and a female aged 34. These individuals were marked as part of studies carried out between the 1980s and early 2000s, meaning detailed information on their early-life morphology (e.g., mass, length, and axial girth) is available. They are the oldest observed harbor seals in the wild, close to the maximum observed age for the species (36 years) as estimated from growth layers in teeth during necropsies (Härkönen and Heide-Jørgensen 1990; Dietz et al. 1991). Given the long potential lifespan of harbor seals and the short time span of most research granting schedules, there have been few studies of individuals across their entire lifetime (Härkönen et al. 1999; Lindenmayer et al. 2012; Cordes and Thompson 2015). Our observations present a rare opportunity to study the contribution of these older animals, with well documented early-life conditions, to population growth. Harbor seals are a widely distributed species of true seal which are often used as a model for population growth in marine mammals (Blanchet et al. 2021; Liu et al. 2022). In the Kattegat–Skagerrak region of the North Sea, harbor seals have been the subject of monitoring both for use as an indicator of ecosystem health and for research into seal behavior, diet, and population dynamics since the late 1970s (Heide-Jørgensen and Härkönen 1988; Harding et al. 2005; Carroll, Infantes, et al. 2024). As the result of hunting in the early twentieth century, the Kattegat–Skagerrak population fell from a minimum estimate of 14,000 individuals (based on an assumed annual growth rate of 5%) in 1890 to approximately 2500 in 1979 (Heide-Jørgensen and Härkönen 1988). Following protection in the 1960s and 1970s, the population grew exponentially (Heide-Jørgensen and Härkönen 1988; Olsen et al. 2010). Mass mortality events in 1988 and 2002, caused by outbreaks of Phocine Distemper Virus (PDV), led to the death of up to 66% of the population on each occasion (Dietz et al. 1989; Härkönen et al. 2006). Over the past decade, growth of the Kattegat-Skagerrak harbor seal population has slowed with a peak in 2017 and counts of more than 14,000 individuals (Harding et al. 2024; Carroll et al. 2025). There are now indications that the population may be in decline, including reduced pup production, decreases in somatic growth rates, and reduced counts during annual molt surveys (Harding et al. 2018; Infantes et al. 2022; Carroll et al. 2025). Similar declines have been observed in harbor seal populations in the UK and North-Western Atlantic, although causes may vary (Bowen et al. 2003; Thompson et al. 2019). The Kattegat-Skagerrak has been subject to widescale ecosystem changes, likely caused by the collapse of several key fish stocks as a result of industrial fishing (Bartolino et al. 2012; Boström et al. 2014; Capuzzo et al. 2018). It is likely that these changes are placing stress on the harbor seal population as more energy must be expended foraging, potentially compounded by increased hunting pressure (Silva et al. 2021; Kappa et al. 2025). As population growth slows, the long potential lifespan of harbor seals may become increasingly important. Adult survival in harbor seals is high and stable (often > 95%) relative to subadults or pups (Härkönen and Heide-Jørgensen 1990; Härkönen et al. 2002; Bowen et al. 2003; Harding et al. 2005). Survival is generally higher for females than males (Hastings et al. 2012). Mature females can give birth to a maximum of one pup per year, with the age of first reproduction occurring between four and six years of age, and fertility rates generally being above 90% (Härkönen et al. 2002). Older females (> 25 years of age) have lower fertility rates; however, they have the potential to remain reproductively active throughout their lifetime (Härkönen and Heide-Jørgensen 1990; Härkönen et al. 2002). In Northern Europe, birthing occurs in June (Härkönen and Heide-Jørgensen 1990; Härkönen et al. 1999). Mature female seals can survive periods of resource scarcity or environmental stress by skipping breeding (Kjellqwist 1995). This means that, under stress from limited prey availability, adult survival is generally maintained, while pup survival and birth rates are the first demographic rates to fall (Kjellqwist 1995; Harding et al. 2005). The age structure of a population plays an important role in determining its growth rate and generation time. In the short term, a higher ratio of mature to immature individuals can mean higher birth rates; however, as the age structure shifts towards older animals, generation time will increase, eventually lowering growth capacity (Härkönen et al. 2002; Hoy et al. 2020; Jackson et al. 2020; Jonasson et al. 2022). Between 1984 and 2002, 211 harbor seals along the West Coast of Sweden were captured and given permanent unique markings through freeze branding. Marked seals were mostly pups of the year or subadults (Härkönen et al. 1999). For each marked individual, details on sex, mass, length, axial girth, and age class were taken. In subsequent years, systematic surveys of the region during a series of mark-recapture studies provided insights into harbor seal survival as well as age- and sex-specific behavior (Härkönen et al. 1999; Harding et al. 2005). Following the most recent outbreak of PDV in 2002, these surveys were halted due to the large numbers of marked individuals that had died and the high cost of survey methods available at the time (Härkönen et al. 2006). Since 2021, the Koster seal colony (Figure 1A) has been surveyed annually in June, late August, and early September in order to determine pup counts and collect remote estimations of seal body mass index (Infantes et al. 2022; Amorosi et al. 2024; Carroll, Infantes, et al. 2024). During surveys, videos of known haul outs are taken using a DJI Matrice-200 drone mounted with a DJI Zenmuse-Z30 camera, while still images are taken using Mavic II Zoom and Mavic II Pro drones. In May 2023, a marked seal (Seal 15, Figure 1B) was opportunistically sighted and photographed (using a Cannon EOS 6D Mark II DSLR camera with a Tameron SP 150–600 mm V2 lens). Following the sighting of Seal 15, videos and images from drone surveys carried out in June and September 2021 and 2022, along with images from an aerial survey carried out using light aircraft in June 2022, were reviewed in retrospect. Two more marked individuals (Seal 216, Figure 1C, and Seal 13) were noted. In July 2022, a member of the public uploaded a sighting of Seal 216 approximately 70 km away outside the town of Lysekil to the citizen science website Artportalen (Figure 1A) (Melin 2022). During surveys in 2023, 2024, and 2025, an intentional effort was made to note and photograph any additional marked individuals. This resulted in 11 additional sightings of six marked seals captured by drone (Figure 1C) or DSLR camera (Figure 1B, Table 1). Two additional marked individuals were sighted, but the number could not be determined. Sighted seals were originally marked between 1990 and 2002 as pups of the year (n = 7) or subadults (n = 1). This means they ranged in age at the time of sighting between 20 and 34 years. Seal 216 was observed with a pup in both 2022 and 2023, while seal 214 was observed with a pup in 2025. It was concluded that these were likely to be their offspring based on proximity and interactions, such as nose touching and nursing, although allosuckling has been observed in harbor seals (Arso Civil et al. 2021). Only two marked males were sighted (Seals 13 and 183). It should be noted that the most intensive surveys occurred in June during pupping when the majority of hauled-out seals are female (Härkönen et al. 1999). All individuals were marked within Koster National Park (Figure 1A). With the exception of the sighting of seal 216 in a different seal colony 70 km away in July 2022, all resightings occurred within 4 km of their original catching sites. Seal 216 was also sighted within Koster National Park in subsequent years during the breeding season. This provides independent evidence for the high degree of breeding site fidelity noted for harbor seals during individual ID studies elsewhere and suggested by genetic analysis (Olsen et al. 2014; Cordes and Thompson 2015; Liu et al. 2022). The observation of these marked individuals was unexpected given the length of time since the original studies took place and the occurrence of multiple mass mortality events since the time of their capture (Härkönen et al. 1999, 2006; Zohari et al. 2014; Mollerup et al. 2024). Survival is one key parameter derived from mark-recapture studies (Harding et al. 2005; Hastings et al. 2012; Badger et al. 2023). Using mark-recapture data from the Kattegat-Skagerrak harbor seal population, Harding et al. (2005) found a strong correlation between first year over-winter survival probability and autumn mass. Through restarting systematic surveys for marked individuals from these original field studies, it may be possible to assess the influence of early life environmental conditions or morphological traits on long-term survival probability. Drone-based image collection methods could now streamline survey efforts. They can also provide additional information such as body mass and length estimates (Corcoran et al. 2021; Infantes et al. 2022; Carroll, Infantes, et al. 2024). Individual identification of harbor seals based on natural markings (Photo-ID) is an alternative to branding studies and has contributed to a number of long-term monitoring projects (Cordes and Thompson 2015; Graham et al. 2017). Automated or semi-automated methods for individual identification of harbor seals are also increasingly popular (Langley et al. 2021; Birenbaum et al. 2022). As birth rates decline in the Skagerrak (Infantes et al. 2022), it is possible that older age classes are not being replaced by younger seals (Bowen et al. 2003). Adaptations such as long potential lifespan and skipping reproduction in harbor seals allow older individuals to act as a buffer for population growth potential against poor environmental conditions (Härkönen et al. 2002; Hoy et al. 2020). In this study, four of the six females were sighted during the breeding season in June. Although only two of these females were confirmed to be nursing a pup, it is not implausible that the others remain reproductively active. The present study highlights the potential importance of high-quality individuals with high survival and reproductive output to population development supporting similar findings in gray seals (Badger et al. 2023). These observations have implications for seal management as the loss of these individuals to hunting could disproportionately impact population development (Silva et al. 2021; Carroll, Ahola, et al. 2024). Daire Carroll: conceptualization, data curation, formal analysis, investigation, visualization, writing – original draft, writing – review and editing. Jessica Harvey-Carroll: data curation, writing – review and editing. Tero Härkönen: data curation, writing – review and editing. Karin C. Harding: conceptualization, data curation, funding acquisition, writing – review and editing. The authors would like to thank the staff of Koster National Park for facilitating this research. They would like to thank J. Melin for his submission to Artpotalen, E. Infantes, E.V. Pagan, F. Kappa, and J. White for their assistance in collecting data, and L. Rydberg for his time reviewing images. They would also like to thank A.M. Carlsson and M. Ahola at the Swedish Museum of Natural History for contributing images taken during aerial surveys funded by the Swedish Agency for Marine and Water Management. Images were collected opportunistically during non-disruptive surveys as described in Infantes et al. (2022) and Carroll, Infantes, et al. (2024) with permission to conduct research in Koster National Park from the County Council of Västra Götaland (521-14883-2021 and 1720-2023). Original branding studies were conducted according to permits given by the Ethical Board of the County Court in Gothenburg, Sweden. The authors declare no conflics of interest.