Recolonisation of former habitat by harbour seals in southern Denmark despite intense anthropogenic activity

Abstract

Harbour seals were extensively hunted in Denmark, but have only been driven to local extinction in one larger area, the South Funen Archipelago and Little Belt, where the species has been absent throughout the 20^th century. Despite high growth rates of the Danish harbour seal populations after protection from hunting in 1976, seals have only been observed sporadically in the South Funen Archipelago and Little Belt until recent years, where recolonisation now causes conflicts with fisheries. Here, we review historical sources documenting the local extinction of harbour seals in the area during the 19th century and report the results of aerial surveys of haul-out sites during moulting seasons of 2021–2023 and pupping seasons of 2022 and 2023. Historical sources reveal that seal hunting was a common practice in southern Denmark, but catches dwindled to rarities during the 19^th century. During recent surveys, seals were detected at six of the identified potential haul-out sites. Around Aarø Island, an average of 141 (range: 92–186) harbour seals were recorded over four moulting season surveys, constituting the majority (90%) of the total counts of the surveyed area. During the pupping seasons, a total of five pups was encountered at two different haulouts. As none of the haulouts are protected during the pupping and breeding seasons, protective measures may support this recolonisation of the historic harbour seal breeding range.

Introduction

Harbour seals (Phoca vitulina) have a wide range in the northern hemisphere, occupying temperate and arctic coastal habitats (Teilmann et al., 2023). Compared to other pinnipeds, individual home ranges for harbour seals are small, with most movement distances from the key haul-out site being below 50–100 km, resulting in genetic differentiation and creation of subpopulations over relatively small distances (Liu et al., 2022). Today, the harbour seal is the most abundant seal species in Danish waters and was historically found in all parts of Denmark, where it was extensively hunted for its fur, blubber and to mitigate damages and competition with fisheries (Søndergaard et al., 1976; Olsen et al., 2018). The sympatric grey seal (Halichoerus grypus) was historically even more numerous than the harbour seal, but was driven to local extinction by a culling campaign from 1889 to 1927 and centuries of previous hunting (Søndergaard et al., 1976; Olsen et al., 2018). The Danish harbour seal populations were likewise targeted by the culling campaign and severely reduced in numbers, but survived at low abundances throughout the country from 1927 to the protection in 1976, with the exception of one larger area: the South Funen Archipelago and Little Belt, where harbour seals disappeared (Figure 1). Despite dedicated management efforts, including establishment of seal reserves from the 1950s to the 1980s, the protection of seals from hunting (grey seals in 1967 and harbour seals in 1976) and subsequent growth of the harbour seal populations in other parts of Denmark, seals have only been sporadically observed in South Funen Archipelago and Little Belt during the 20th century (Søndergaard et al., 1976).

Figure 1. Map of Denmark with harbour seal haul-out sites in the four management units shown (top) and study area (bottom) with place names from the text. In the South Funen Archipelago and Little Belt, the 13 surveyed localities are shown. Localities with seals detected during surveys have red markers, while areas surveyed without seal observations are marked in black.

Historical sources document the long-term presence of seals in the area leading up to the local extinction. An early source is a local record book describing the case of a poacher caught on Drejø (Figure 1), who was pardoned on the condition that he supplied the landowner with three seals and their skin and blubber before Whit Sunday (7 June 1663), after which he was hired as a seal hunter (Rohmann, 1861). A source from the late 18th century indicates a substantial contemporary seal abundance based on a sermon from a priest at Avernakø (Figure 1), who lamented the ‘sacrilege’ of most of his male congregation favouring seal hunting to church service on a New Year's Day (Søkilde, 1875). In the late 1930s, Frydendahl (1939) conducted an interview survey with people on Funen and in the South Funen Archipelago, who had personal memories of the historic seal hunting and/or had parents or grandparents who had been active hunters. These interviews document the gradual local extinction of seals during the 19th century: the interviewees relay that in their childhood, parents, grandparents and ‘old people’ told stories of hunts with daily catch numbers of 23, 16 and 10 seals. These numbers dwindle with time, and recollections from the interviewees' own lifetimes concern rare encounters with single or few seals, mostly from their childhood. One person tells that admissions were sold to see a seal caught around 1870. Thus, seals seem to have disappeared from the South Funen Archipelago at the initiation of the bounty campaign to cull seals in Denmark in 1889 (Olsen et al., 2018). Information from Frydendahl (1939) indicates that a substantial part of the hunted seals were taken at breathing holes in the ice during cold winters, suggesting the historic occurrence of grey seals or even ringed seals (Pusa hispida) in the area, in addition to harbour seals.

In recent years, there have been increased reports of harbour seal occurrence in the South Funen Archipelago, not least from fishermen, who experience damaged or lost catches (Milling, 2021; Printzlau, 2021). This renewed occurrence of seals at least 130 years after their local extinction inspired our initiation of our aerial surveys in 2021 for potential seal haulouts during the harbour seal pupping and moulting season in June and August, respectively. Here we report the results of the first three years of monitoring 2021–2023, documenting the return of harbour seals to the South Funen Archipelago and Little Belt in Denmark.

Materials and methods

To compile a list of potential haul-out sites in the South Funen Archipelago and Little Belt we enquired for sightings of hauled out seals in our network of people reporting stranded marine mammals in the area, as well as game wardens at the Danish Environmental Protection and Nature Agencies (Danish Ministry of Environment). These initiatives resulted in 13 localities where harbour seals had anecdotally been observed in recent years (Figure 1). These localities were surveyed once during the moulting season in August 2021 to 2023 (twice in 2023, as many haulouts were flooded during the first survey) and once around the pupping season peak estimated in Denmark at 22nd June (Seganfreddo et al., 2023) in 2022 and 2023.

Surveys were conducted between 10:00 and 16:00 local time at wind speeds below 10 m s⁻¹ and without precipitation at any locality in the preceding 6 hrs. As tidal amplitude in the survey area is negligible (10–15 cm), surveys were flown without consideration of tide. Two observers both took overlapping digital photographs from the right side of the plane of all groups of seals through an open window from a single-engine Cessna 172, flying 500–700 feet over the haul-out sites, using DSLR cameras with 70–200 mm lenses. The digital photographs were subsequently used to count the number of seals present by two independent observers. If the counts for a location differed by more than 5% between the two observers, a third (or more) count(s) was performed until there was agreement between the counts.

Results

In the August moulting season surveys, seals were detected at seven of the identified potential haul-out sites. At Aarø, an average of 141 harbour seals (min 93; max 186) was recorded over the three moulting seasons, constituting 90% of the total numbers in the South Funen Archipelago and Little Belt (Table 1). During the survey on August 9^th 2023, the haulouts of Strynø Kalv, Bredholm, Birkholm and Drejø were flooded after a period of strong westerly winds, potentially causing a low count of seals. Thus, another survey was conducted on 22nd August 2023 during which 12 animals were recorded at Strynø Kalv. During the June pupping season surveys, a total of five pups were encountered at two different haul-out sites, Bredholm and Aarø (Table 1; Figure 1). One grey seal was recorded during the surveys, on 22nd August 2023 at Aarø.

Table 1. Aerial survey counts of harbour seals from the seven haulouts where seals were detected during the seven surveys

For the pupping season, the total count of harbour seals including pups is given with the number of pups in parentheses.

^a A single grey seal was detected on 22nd August 2023 at Aarø NE.

Discussion

The harbour seal moulting and pupping season surveys conducted in 2021–2023 confirm the presence of substantial numbers of seals in the South Funen Archipelago and Little Belt after at least 130 years without regular presence in the area. The total of 667 seals (of which some may be resightings) recorded during the six surveys in 2021 to 2023 were all harbour seals, with the exception of a single grey seal. Harbour seals in Denmark have recovered from an estimated abundance of ca. 2000 individuals in the mid-1970s (Søndergaard et al., 1976) to around 9000 counted at haul-out sites during the moult throughout Denmark in recent years (Sveegaard et al., 2023). The successful recovery of the harbour seals in Denmark has been credited to protection of seals in 1976 and establishment of seal reserves at key sites with restricted access starting in 1951 (Heide-Jørgensen and Härkönen, 1988).

As other pinnipeds, harbour seals are vulnerable to disturbance at their haul-out sites, particularly during the breeding and moulting seasons (Andersen et al., 2012). In other Danish regions, seal reserves and protected areas with restricted access to the public mitigate some of the human impacts caused by disturbance. However, there are currently no seal reserves or protected areas in the South Funen Archipelago and Little Belt; a populated area with many houses along the shores that is busy in terms of recreational traffic (e.g. motor boats, sailing boats, kayaks and paddle boards) and commercial activities (ferries and freight ships). Recolonisation is apparently occurring despite the high levels of disturbance, but on the other hand disturbance may have hindered an earlier or greater recovery of the seals. Previous studies of harbour seals have indicated that to some extent, they can habituate to anthropogenic disturbances in areas with high activity levels (Cates and Acevedo-Gutiérrez, 2017; Bankhead et al., 2023).

During the pupping surveys, we only recorded three pups in 2022 at Aarø in southern Little Belt and two pups in 2023, at Bredholm in the South Funen Archipelago. Harbour seals have strong site fidelity, particularly adults during the breeding season (Härkönen and Harding, 2001; Dietz et al., 2013). A likely consequence of this is that breeding activity lags behind other occurrence in a newly colonised area, a phenomenon that is also seen for grey seals currently recolonising previous ranges (Brasseur et al., 2015; Galatius et al., 2020; Wood et al., 2020; Galatius et al., 2024). Given the high level of human recreational activities in the area, disturbance may particularly limit future pupping activity without protective measures (Becker et al., 2011). The South Funen Archipelago and Little Belt area neighbours two genetically separated harbour seal population areas from where animals may have immigrated: the Kattegat to the north and the southwestern Baltic to the east (Olsen et al., 2014) (Figure 1). In the Little Belt, the distance by sea is shortest from the nearest Kattegat haulout at ca. 70 km from Møllegrunden to Aarø, while in the South Funen Archipelago, the distance is shortest from the southwestern Baltic haul-out site at Vresen at ca. 40 km to Strynø Kalv (Figure 1). In the Kattegat population, abundance has been relatively stable since 2010, indicating density dependence (Silva et al., 2021), potentially spurring migration to new areas, similar to harbour seal and grey seal metapopulation dynamics in other regions (Gaggiotti et al., 2002; Russell et al., 2019; Carroll et al., 2020). In this way, the South Funen Archipelago and Little Belt could potentially act as a population sink for the Kattegat source population. Counts from the harbour seal population in the southwestern Baltic, on the other hand, are still growing exponentially (Galatius et al., 2021), indicating that harbour seals here are not yet at their carrying capacity. Still, given increased competition with recovering grey seals (Galatius et al., 2020) and associated changes in haul-out preferences, these southwestern Baltic harbour seals could also be a source population. Ultimately, genetic analyses will be necessary to ascertain the population origin(s) of the seals that have immigrated to the South Funen Archipelago and Little Belt area.

The century-long period of harbour seal absence from the area, persisting also after its nation-wide protection in 1976, highlights the potentially slow recovery and recolonisation of a species with relatively slow intrinsic growth rate (average <1 pup per year) and high site fidelity, when neighbouring populations are also depleted. Even at this early stage of recolonisation, harbour seals (perhaps in combination with visiting grey seals) are starting to cause controversy with fishery due to damage to the catch in static fishing gear and a simultaneous crash in commercial fishery (Milling, 2021; Printzlau, 2021). These seal-fishery conflicts are likely to increase if the recolonisation proceeds as expected.

Recommendations

Harbour seals are listed under Annex V in the EU Habitats Directive. To achieve favourable conservation status under the Habitats Directive, the range of a species should be aligned with its expected natural range. To ensure the achievement of this goal for harbour seals, we recommend the establishment of new seal reserves and/or protected areas in the South Funen Archipelago and Little Belt to minimise human impacts on the recolonisation of harbour seals here. In this regard, establishing seal reserves around the Aarø and Strynø Kalv – Bredholm haulout sites may have the greatest impact, given the observed use of these areas for both moulting and breeding. Likewise, we recommend to conduct genetic sampling and analyses to ascertain the origin(s) of the harbour seals that have immigrated to the South Funen Archipelago and Little Belt area.