Changing reproductive dynamics in harbour seals of the North and Baltic Seas

Successful Reproduction is fundamental for species survival. In harbour seals, reproductive cycle includes the embryonic diapause. This study investigates seasonal and annual variations in reproductive parameters of harbour seals from the North and Baltic Sea to assess reproductive status, sexual maturity and broader reproductive dynamics. 1.523 females were found dead, or mercy killed along Germany`s coastline between 1995 and 2024. Out of these 383 were examined and categorized into three age groups (adult = 80, juvenile = 77, neonate = 226), as well as into two timelines (1995–2015 and 2016–2024). Ovaries were cut into 2 mm slices and examined for the presence or absence of Tertiary follicles, Corpus luteum, Corpus albicans and scars. Sexual maturity was first observed at 3.5 years - six months later than previously reported. Reproductive activity declined from age 13, earlier than previously assumed. A peak in ovarian weight shifted from mid-March (1995–2015) to late December (2016–2024), suggesting earlier reproductive activation. The period of minimal ovarian activity was delayed, indicating a prolonged reproductive window. Inconsistencies in Cl presence during expected embryonic diapause periods imply alterations in reproductive timing, potentially linked to prey availability or population structure. These results suggest a complex interplay between environmental pressures and reproductive physiology, with potential consequences for fitness, genetic diversity and population resilience. This study presents basic information for assessing harbour seal reproduction along Germany`s coastline and supports the need for long-term, regionally monitoring to guide effective management strategies to better understand the impacts of ongoing changes.