Twelve years of acoustic monitoring reveal rising spring–autumn harbor porpoise (Phocoena phocoena) detection rates in the Pomeranian Bay (Baltic Sea), a transition zone between populations of concern

IntroductionWe analyzed long-term trends (2010–2021) in harbor porpoise (Phocoena phocoena) acoustic detections in the Pomeranian Bay (Baltic Sea) and adjacent waters east of Rügen, a transition zone between the Belt Sea and Baltic Proper populations.MethodsUsing passive acoustic monitoring (PAM) at up to 23 CPOD stations (covering an area of approximately 7,000 km²), we quantified the percentage of porpoise‑detection‑positive days (%DPD) as an index of acoustic presence. To address uneven station coverage over time, we combined (i) a Bayesian hierarchical trend reconstruction using phenologically matched stations to bridge mid‑period data gaps, and (ii) station‑consistent endpoint bootstrap contrasts between an early (2010–2013) and later period (2019–2021) based on identical stations, serving as a robustness check. Generalized additive mixed models (GAMM) were used to evaluate relationships between detections and selected variables.ResultsBootstrap contrasts provided robust evidence for strongly higher %DPD in spring and summer, and moderately higher %DPD in autumn in the later period, whereas winter patterns differed between phenology types. GAMMs indicated that potential water temperature, chlorophyll‑a, and current velocity covaried with annual detections in environment‑only models, but these associations weakened once temporal structure was modeled.DiscussionThe strong spring/summer increase in the period from May to October is consistent with increased seasonal occurrence of Belt Sea porpoises further east; however, changes in acoustic behavior or detectability cannot be excluded and the data do not directly quantify abundance. Our findings document increasing porpoise acoustic detection rates, except for winter, in a region facing multiple anthropogenic pressures, including shipping, offshore wind farm development, and pipeline construction.