Increasing numbers of harbour seals and grey seals in the Solent

Abstract Harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) both occur within the UK, but display regional contrasting population trends. While grey seals are typically increasing in number, harbour seals have shown varying trends in recent decades following repeated pandemics. There is a need for monitoring of regional and local populations to understand overall trends. This study utilized a 20‐year dataset of seal counts from two neighboring harbours in the Solent region of south England. Generalized additive models showed a significant increase in the numbers of harbour (mean 5.3–30.5) and grey (mean 0–12.0) seals utilizing Chichester Harbour. Conversely, in Langstone Harbour there has been a slight decrease in the number of harbour seals (mean 5.3–4.0). Accompanying photographic data from 2016 to 18 supports the increase in seal numbers within Chichester Harbour, with a total of 68 harbour and 8 grey seals identified. These data also show evidence of site fidelity of harbour seals in this area, with almost a quarter of animals resighted within the past three years. Overall, this long‐term study indicates an increasing number of both harbour and grey seals within the Solent. However, more research is required to identify the drivers of this trend.

| 16525 CASTLES ET AL. SCOS, 2020). However, there has been a notable reduction in the growth rates of some grey seal subpopulations in recent years (Thomas et al., 2019). Therefore, it is not sufficient to merely examine the total size of a population; long-term monitoring of regional and local subpopulations is required to fully understand overall population trends and rates of change. This is particularly true in the case of new or small subpopulations. In the UK, such monitoring is important for meeting various conservation legislation and management requirements. However, in a more applied sense, monitoring at various spatial scales is also important for the mitigation of mounting anthropogenic threats in the form of fisheries by-catch, vessel traffic, marine pollution, underwater noise, marine renewable energy, tourism, and climate change (Alexander et al., 2016;Boitani et al., 2011;Coomber et al., 2016;Erbe et al., 2019;Hines et al., 2020;Papageorgiou, 2016;Partelow et al., 2015;Payne et al., 2016). Understanding seal abundance at a national, regional, and local level is necessary to support effective management and conservation at a range of scales.
Within the UK, the harbour seal population is subdivided into 14 seal management units (SMUs), which were originally defined on the basis of the spatial distribution of haul-out sites and knowledge of harbour seal ecology but also appear to be in agreement with results from genetic population structure studies (Olsen et al., 2014;Thompson et al., 2019). A detailed analysis of long-term trends for several SMUs is presented in Thompson et al. (2019) and summarized in SCOS (2020), demonstrating contrasting dynamics between subpopulations on the English east coast (increasing year on year), east coast of Scotland and in the Northern Isles (declines of varying intensity), and the western UK (stable or increasing).
However, there is no clear explanation for these differing dynamics . Unfortunately, SMUs #10-13 in south England, southwest England, Wales, and northwest England could not be included in these analyses due to limited information, as previous reports suggest harbour seals to be effectively absent from this stretch of coastline (SCOS, 2017). Thus, these SMUs have not been regularly surveyed for harbour seals , but this risks overlooking important changes at the local and regional level.
Such a change appears to have occurred in SMU #10 (south England), where harbour and grey seals have reportedly formed new haul-outs in the Solent region. In 1994, three harbour seals were recorded using Chichester Harbour, followed by an increasing number of sightings over time. A dedicated local research project in 2009 collated existing information on seals in the area and deployed satellite tags on five harbour seals to investigate residency and movement patterns (Chesworth et al., 2010).
This identified two significant haul-out sites, one in Chichester Harbour and one in neighboring Langstone Harbour, with approximately 24 harbour seals reported across both sites. The tagged harbour seals appeared to be resident in the area, remaining almost entirely within the eastern Solent and repeatedly visiting both the Chichester and Langstone Harbour haul-out sites. In July 2008, the first official sighting of a grey seal was also reported at the haul-out site in Chichester Harbour (Chesworth et al., 2010).
The report did not hypothesize the drivers behind this increase in seal abundance (i.e., reproduction vs. immigration), although anecdotal evidence exists of harbour and grey seals being present in the area for several decades prior and their numbers may also have been supplemented by the introduction of rehabilitated animals.
Regardless of where these seals have come from, their relatively low numbers and proximity to human activities raise considerable concern. The Solent has many anthropogenic uses, including recreational activities, commercial shipping from Portsmouth and Southampton harbours, fisheries and marine aggregate extraction, and utilization by the Royal Navy (Conway, 2007). In addition, the surrounding land area is highly urbanized. However, there is a paucity of literature regarding the current size of the Solent seal haulout, whether pups are being produced at this site, or the potential impact of anthropogenic activities. Obtaining updated estimates of animal numbers and potential trends is a fundamental first step to managing seals within this SMU.
Our study aimed to assess the seal population trends in Outcomes from this research will provide managers with a muchneeded update on seal abundance and site use within the Solent, and will also be relevant for guiding future monitoring of the south England SMU.

| Study area
The Solent is a sheltered channel system, separated from the English Channel by the Isle of Wight and stretching along the coastline of Hampshire and West Sussex ( Figure 2). It includes numerous estuaries and natural harbours, the largest of which are Portsmouth, Langstone, and Chichester Harbours, which form a series of connected basins comprised of extensive intertidal mud and sandbanks. Previous studies have confirmed that harbour seals utilize haul-outs in both Chichester and Langstone Harbours, moving between these sites via both the sea and a network of tidal channels (Chesworth et al., 2010). Grey seals have previously only been reported in a single sighting located within Chichester Harbour (Chesworth et al., 2010). Pre-2015, data were collected on a monthly basis using a combination of boat-and land-based surveys and did not include estimates of seal age. Post-2015, field protocols at both sites were standardized to ensure the same methodology was consistently applied. This included the following: moving surveys to an entirely boat-based platform; conducting surveys concurrently at both harbours to remove the chance of individuals being counted twice; undertaking surveys at least once a month between May and September to capture the harbour seal pupping season (with additional months surveyed on an opportunistic basis); and recording the approximate age of individuals (adult, subadult/juvenile, or pup) where possible. It can be challenging to assign an age cohort from observation alone (except for newborn pups), especially for harbour seals; despite females having lower overall lengths at maturity, they display higher early growth rates than males (Hall et al., 2019). Hence, subadults and juveniles were combined into one category. Regardlessly, age/sex data were not used in the present analyses beyond observation of pup numbers.

Note that it was not possible to collect count data in Langstone
Harbour during 2018.
Additionally, boat-based photo-ID using a digital SLR camera and appropriate zoom lens was undertaken in Chichester Harbour from 2016 to 2018. Photograph processing followed the standard protocols (Cordes & Thompson, 2015;Cunningham et al., 2009;Hastings et al., 2008;Mackey et al., 2007;Yochem et al., 1990). In brief, photographs were sorted by survey and allocated ratings based on their quality (i.e., clear focus, good lighting, and head visible) for identifying pelage markings: Grade 1 (good quality), Grade 2 (sufficient), or Grade 3 (poor). Only Grade 1 and 2 photographs were retained.
Four independent reviewers manually compared images to identify individual seals based on unique pelage markings. "New" individuals were allocated a sequential identification number and added to the catalogue. When individuals were resighted, the date was noted in a separate spreadsheet.

| Data analysis
Seal count data were analyzed in R (R Core Team, 2019) for each species and site using generalized additive models (GAMs, mgcv package; Wood, 2011) to allow smooth functions to be fitted to temporal covariates (Month and Year) based on results from exploratory analyses. Cyclic cubic splines were used for month to ensure there was no discontinuity between January and December. To account for overdispersion, GAMs were fitted with a Tweedie distribution (Miller et al., 2013). Restricted maximum likelihood (REML) was used to minimize overfitting (Wood, 2011). The best model was selected using Akaike's information criterion for small sample sizes (AICc; MuMIn package; Barton, 2020). Model assumptions were checked by producing standard residual diagnostic plots, and an acf function was used to check for temporal autocorrelation (Zuur & Ieno, 2016).
See "Data Availability" section for a link to code used in these analyses.

| Seal counts
In Chichester Harbour, there has been a significant increase in the number of harbour and grey seals between 1999 and 2019 (p < .001; Table 1). During this time, the mean number of harbour seals has increased from 5.3 (±2.1 SD) to 30.5 (±7.5 SD) (Figure 3). Similarly, the mean number of grey seals has increased from 0 to 12.0 (±3.9 SD) (Figure 4). The most recent Chichester Harbour counts indicate a maximum of 43 harbour seals and 19 grey seals observed in 2019.
There were also significant monthly trends in seal counts at this site for both species (both p < .01; Table 1). Peak numbers occurred in August for both harbour (mean 19.0 ± 12.0 SD) and grey (mean 3.2 ± 5.8 SD) seals, with harbour seals also showing a smaller peak in March (mean 12.7 ± 0.7 SD).
In Langstone Harbour, there has been a small but significant

| Photo-ID
Over three years of photo-ID data collection in Chichester Harbour, a total of 68 harbour and eight grey seals were identified (Appendix 2; see "Data Availabilty" section for a link to the full photo-ID catalogue). Note that it was not always possible to photograph every seal present and not all photographs were of high enough quality for individual identification. Therefore, these data likely present an underestimate of the true number of individual seals present.
For harbour seals, 29 new individuals were discovered in 2016, 10 in 2017, and 28 in 2018 ( Table 2). The fact that high numbers of "new" animals were still being identified also supports the suggestion that these data are an underestimate of the number of seals using this harbour. However, there is still some evidence of site fidelity; of the 68 individuals, 16 (23.9%) were resighted within the study period (Table 3). The shortest resighting period was one month, while the longest resighting period was three years. The highest number of resightings occurred in September 2019 (i.e., the final survey), which may be due to the photo-ID catalogue now being of a sufficient size to facilitate resightings. There were no apparent seasonal patterns in resightings, which likely reflects the short study duration; similarly, insufficient sex/age data were available to investigate potential patterns according to these factors.
For grey seals, one new individual was discovered in 2016, two in 2017, and five in 2018 (Table 2). One animal (an adult female) was resighted two months apart in 2018 (Table 3).

| D ISCUSS I ON
Our study revealed an overall increase in the number of harbour and grey seals within the Solent across the 20-year study period. Both species were consistently more abundant in Chichester Harbour than Langstone Harbour, with the former area also experiencing some seasonality in peak counts and acting as a pupping site for harbour seals. We recorded preliminary evidence of site fidelity, with approximately 25% of the 68 individually identified harbour seals resighted within Chichester Harbour. However, of the eight grey seals identified, only one individual has been resighted so far. Applying the same scaling in the present study produces an estimate of 60 (95% CI: 49-80) harbour seals, which closely matches the results of the photo-ID data (68 individuals). Note that there is currently no equivalent scaling factor for grey seals, so the proportion "missed" in the present surveys is unknown. show significant increases in harbour and grey seal numbers (Vincent et al., 2017). However, French telemetry studies indicate that harbour seals remained highly coastal (within 20 km from shore), close to their haul-out site (within 100 km of their capture site), and did not visit other colonies (Vincent et al., 2017). This aligns with findings from other tracking studies, including one in Chichester Harbour, which indicate tight concentrations of harbour seals around the coastline adjacent to their haul-out sites (Carter et al., 2020;Chesworth et al., 2010;Cunningham et al., 2009). In comparison, grey seals typically move much greater distances and regularly utilize offshore foraging areas (Carter et al., 2020;McConnell et al., 1999). Telemetry studies in France show grey seals moving up to 1,200 km from their capture site, including frequent trips across the English Channel (Vincent et al., 2017

| Spatiotemporal patterns within the Solent
Within the Solent, we observed discrepancies in site use between Langstone and Chichester Harbours. The drivers behind confines. Chichester Harbour is an area of outstanding natural beauty (AONB), with excellent water quality ratings and primarily recreational vessel traffic. By contrast, Langstone Harbour is highly urbanized due to its proximity to Portsmouth City, experiences both recreational and commercial vessel traffic due to its two marine aggregate wharves, and also has a sewage treatment plant located at the northern end of the harbour. These characteristics may also account for why harbour seal pupping only occurs in the former site. Yet, both harbours still support a range of habitats, maintain high levels of biodiversity with regard to wading and seabirds, fish, and marine invertebrates, and are designated as bass nursery areas (CHC, 2019; LHB, 2019). A previous telemetry study recorded harbour seals regularly foraging in Langstone and Chichester Harbour, although dive locations did switch over time, suggesting that foraging patterns were related to prey availability (Chesworth et al., 2010). However, it is worth noting that Langstone Harbour has a small number of seals overall, which may accentuate small fluctuations. Further monitoring will be beneficial to establishing whether seal numbers are indeed changing at this site. It would also be interesting to explore site use within and between the harbours with regard to age/sex factors, or even individual preferences.
The present study identified significant monthly trends in seal counts, with both harbour and grey seal numbers peaking in August within Chichester Harbour. This coincides with the annual harbour seal molt, when animals spend longer periods of time ashore for thermoregulation (Paterson et al., 2012). The higher number of grey seals at this time may represent animals traveling through the area on route to breeding sites ahead of the reproductive season.
It is worth noting that the majority of the study period did not experience coordinated counts for both harbours. As earlier counts were not conducted at the same time, it is possible that individual seals could move between the harbours and be doublecounted. Coordinated counts have been undertaken from 2015 along with standardized data collection methods, providing the basis for a robust long-term dataset. As well as monthly counts, it would also be beneficial to explore additional data collection options. Here, monthly trends in seal counts were observed within Chichester Harbour, but in the future, more frequent counts could occur over a shorter study period (e.g., several hours a day over a period of a few weeks from a land-based station). This would facilitate research into haul-out patterns in relation to temporal and environmental variables (Cordes et al., 2011); aspects of harbour seal pupping (Cordes & Thompson, 2013;Reijnders et al., 2010); and the frequency and extent of human disturbance events (Andersen et al., 2012). Given the high levels of anthropogenic activity within the Solent, the latter is of particular concern. Differing patterns of environmental conditions or human use may also go some way to explaining the differential site use between Chichester and Langstone Harbours. Additionally, in the current study, the pelage of many seals was obscured due to clinging sediment resulting from animals hauling out on intertidal mudflats. This was particularly true for the stomach and chest regions, but less so for the back. Thus, it may be worth exploring other techniques for the collection of photographic data, such as the use of drones.

| Site fidelity
In future years, it would be beneficial to not only continue the collection of photo-ID data in Chichester Harbour, but also expand it to include Langstone Harbour. This would allow further investigation into local site use and fidelity (Cordes & Thompson, 2015), as well as providing long-term datasets for examination of population dynamics (Cordes & Thompson, 2014;Koivuniemi et al., 2016), reproductive patterns (Cordes & Thompson, 2013;Thompson & Wheeler, 2008), and behavior (Neumann, 1999;Wilson & Jones, 2018).

| CON CLUS ION
This long-term study shows that numbers of harbour and grey seals are increasing in the Solent. This is contrary to several other popula- use, behavior, and disturbance of the Solent seals will also be beneficial for managers in terms of both seal conservation and human activities.

ACK N OWLED G M ENTS
The authors would like to thank Megan Strachan and Edward Smith for their assistance with reviewing photo-ID data. Phil Bouchet and Nick Riddoch provided helpful feedback on earlier manuscript drafts. We also extend our thanks to the two anonymous reviewers who provided helpful and considerate comments, which considerably improved the final paper.

CO N FLI C T O F I NTE R E S T
The authors declare no competing interests.

O PE N R E S E A RCH BA D G E S
This article has been awarded Open Data, Open Materials Badges.
All materials and data are publicly accessible via the Open Science Framework at https://github.com/samar ley86/ Solen tSeal Counts.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data and code used in this project are freely available from: https://doi.org/10.5061/dryad.5tb2r bp4v.