Round-the-Clock Intelligent Monitoring Technology with UAV to Improve the Efficiency and Quality of Monitoring the Population of Phoca largha

Abstract

The spotted seal (Phoca largha), the only pinniped species capable of natural breeding in Chinese waters, serves as a flagship species for the Bohai Sea and the Yellow Sea marine ecosystems. Changes in its population numbers are an important indicator of the status of the marine ecological environment. However, when using traditional monitoring methods, such as telescope inspections and routine unmanned aerial vehicle (UAV) photography, it is difficult to accurately grasp the changes in spotted seal populations. Here, we document the use of infrared imaging with a UAV to facilitate round-the-clock monitoring of spotted seal numbers in a protected area. This approach revealed that late night and early morning (22:00–05:00) were peak times for their haul-out activity, with a maximum count of 166. This provides a new idea for the investigation and monitoring of marine mammals and the protection of marine ecosystems.

The spotted seal (Phoca largha) is an Arctic and sub-Arctic phocid species, closely related to the more widely distributed harbour seal (Phoca vitulina) across temperate zones [1,2]. It is also the only pinniped that can naturally breed in Chinese waters and is mainly distributed in the Bohai Sea and the Yellow Sea [3]. As the region’s only flagship species, the spotted seal was classified as “vulnerable” (VU) on the China Red List of Vertebrates in 2016 and was later designated a priority protected species in China in 2021 [2,3,4,5]. The Bohai-Yellow Sea spotted seal population, comprising 0.31% of the global P. largha mature individuals (n = 320,000), is a genetically independent branch [5,6,7,8,9]. Seals possess quite strong diving ability and forage in water [10]. Spotted seals are primarily ice breeders but may come ashore on to sandy beaches or rocky reefs to moult following the breeding season or to rest [10,11,12].

Monitoring flagship species is crucial for assessing ecosystem health, conserving biodiversity, and formulating targeted protection strategies. Spotted seal populations are typically estimated via vessel-based surveys, aerial surveys, Photo-ID, and mark–recapture modelling [13,14,15,16,17,18]. However, night-time monitoring remains challenging. The round-the-clock automated monitoring method using infrared thermal imaging UAV that we introduced has significantly improved the efficiency and quality of monitoring for this flagship species in the nature reserve.

The frozen area of Liaodong Bay in Bohai Sea is one of the most important breeding areas for spotted seals (Figure 1a). According to satellite beacon tracking data, Figure 1b shows the distribution of spotted seals in the Yellow Sea and Bohai Sea in March, following the breeding season [19]. The Dalian Spotted Seal National Nature Reserve, located along the Bohai Sea coast (Liaodong Bay) in China, serves as a critical breeding habitat for spotted seals. Huping Island and Ant Island within the reserve function are key haul-out sites and typical research monitoring stations (Figure 1c).

From 2022 to 2025, during the moulting period in March-April of spotted seals, population monitoring was conducted at key haul-out sites within the reserve (For detailed monitoring times, see

Table 1. Monitoring time of the spotted seal.

Year	Date	Time
2022	30 March–8 April	06:00–17:00
2023	29 March–7 April
2024	31 March–9 April
2025	1 April–10 April	06:00–17:00, 18:00–05:00

). Between 2022 and 2024, daytime monitoring (06:00–17:00) combined binocular surveys with visible-light UAV transects, with observations conducted every two hours (six times per day). In 2025, infrared thermal imaging by UAV and an intelligent observation station were deployed, featuring autonomous operation (programmed to take off and land every two hours) and real-time data transmission for uninterrupted 24 h surveillance (Figure 2). The UAV (DJI Matrice 4T) was operated at a constant altitude of 80 m, requiring wind speed below level 8 (~20 m/s) for take-off. The noise equivalent temperature difference of the thermal infrared camera is 50 millikelvins (mK), and the thermal imaging resolution is 640 × 512 (totalling approximately 328,000 pixels). The telephoto camera has a resolution of 48 million pixels.

For each year, we averaged the data at consistent daily time points (with sample sizes N = 10), calculated the standard error (SE), and derived the observed median and maxima values. Owing to the full-day monitoring, the analysis of 2025 also included nighttime data. One-way ANOVA analyzed significant differences between daytime and nighttime. Inter-year comparisons were subsequently performed. The calculations are completed under the software IBM SPSS Statistics 22.0 (SPSS Inc., Chicago, IL, USA).

Daytime monitoring data from 2022 to 2024 showed that the maximum seal counts at key haul-out sites within the reserve primarily occur in the morning (6:00–9:00) (

Table 2. Monitoring results of the spotted seal.

Time	Year	Average Value	Standard Error (SE)	Median Value	Maxima Value
06:00–07:00	2022	84	12	86	102
	2023	87	17	83	119
	2024	71	5	71	83
	2025	104	19	95	142
08:00–09:00	2022	72	6	72	82
	2023	43	21	40	72
	2024	71	8	72	81
	2025	82	17	80	105
10:00–11:00	2022	55	11	56	74
	2023	30	13	31	42
	2024	57	12	54	83
	2025	65	8	60	84
12:00–13:00	2022	37	8	38	47
	2023	25	6	26	35
	2024	38	8	38	53
	2025	44	14	45	66
14:00–15:00	2022	32	9	32	46
	2023	19	4	20	25
	2024	30	6	31	39
	2025	36	10	36	51
16:00–17:00	2022	21	9	21	37
	2023	20	5	20	29
	2024	20	4	20	26
	2025	36	22	32	95
18:00–19:00	2025	55	25	49	103
20:00–21:00		83	20	86	114
22:00–23:00		98	14	96	121
24:00–01:00		108	16	109	124
02:00–03:00		121	12	123	136
04:00–05:00		120	23	116	166

). The average monitoring results for the daytime (6:00–17:00) from 2022 to 2024 were 78 (±11), 65 (±29), and 71 (±6), respectively. The median daily counts recorded from 2022 to 2024 were 45, 42, and 46 individuals, respectively, while the highest daily counts for the same period were 102, 119, and 83 individuals, respectively.

There was no significant difference in the daytime monitoring results of spotted seals between 2022 and 2025. In 2025, daytime monitoring recorded an average of 61 spotted seals (±30), while nighttime monitoring averaged 98 (±30). The inclusion of nighttime monitoring data revealed significant diurnal variations in seal counts at key haul-out sites. Nocturnal numbers exceeded daytime counts by ~60%, with statistical significance (p < 0.05). In addition, the peak counts occurred between 22:00 and 05:00. The highest daily number recorded was 166 individuals.

Compared to traditional methods, this study employed UAV equipped with advanced thermal imaging and high-definition cameras for around-the-clock monitoring. This approach not only addressed the inability to monitor spotted seals at night but also revealed that late night and early morning were peak times for their haul-out activity. This nocturnal pattern possibly reflects behavioural adaptations to avoid predators and reduce human disturbance [20]. While this study has pioneered a method for monitoring leopard seal populations at night using UAV equipped with thermal infrared cameras, several limitations remain associated with this approach. Adverse weather conditions, including strong winds, freezing temperatures, or rain and snow, can significantly hinder UAV operations. Moreover, UAV are unable to detect leopard seals concealed within rock crevices when operating at altitude, nor can they monitor seals beneath the water’s surface. Additionally, when seals congregate in groups, obtaining an accurate count becomes particularly challenging. Furthermore, relying solely on UAV-based population counts cannot provide a comprehensive assessment of the leopard seals’ overall health status.

The United Nations’ Sustainable Development Goal 14 (SDG 14) calls for the protection and sustainable utilization of marine resources to advance sustainable development. In response, China released the 2021 White Paper China’s Biodiversity Conservation, formally integrating biodiversity protection into national strategy. However, marine mammals face escalating threats from habitat degradation due to climate change and human activities. The Yellow Sea Ecoregion—the only Chinese waters within the globally prioritized 43 marine ecoregions—exemplifies critical strategic importance for national ecological security [21]. Given the critical indicator role of spotted seal population fluctuations for ecological conservation and biodiversity protection in the Yellow Sea and the Bohai Sea, efforts must enhance protected area monitoring effectiveness. We should explore the application of artificial intelligence recognition algorithms, and enhance automated monitoring and statistical analysis capabilities for counting the spotted seal population. Furthermore, continuous refinement of the integrated monitoring system for spotted seals, including UAV, satellite remote sensing, surveillance video and field monitoring, is also needed. The diversified monitoring methods will achieve all-round monitoring of spotted seals, leading to a “Space-Air-Ground-Sea” monitoring system. These measures are designed to systematically build and expand a spotted seal image database, thereby enhancing biodiversity conservation efforts in the Yellow Sea and the Bohai Sea regions.