An increase in detection rates of the critically endangered Baltic Proper harbor porpoise in Swedish waters in recent years

Abstract

The Baltic Proper harbor porpoise (Phocoena phocoena) is currently listed as critically endangered (CR), with the Static Acoustic Monitoring of the Baltic Sea Harbor Porpoise (SAMBAH) project concluding that only $500 individuals remain. This population has a distribution that spans the waters of nine countries, making regular abundance estimates and management action challenging. Given the continued decline of other depleted porpoises, namely the vaquita (Phocoena sinus), the question is often raised about whether management action would even have a positive impact, or whether it is too late for population recovery. When abundance estimates are sparse over time, monitoring programs at key sites are likely to serve as the best indication of population trends, and may provide an early indication of changes at the population level. We compared passive acoustic monitoring data from 12 stations that were utilized both in the SAMBAH project (2011)(2012)(2013) and as a part of the Swedish National Monitoring Program (2017-2020) to determine trends in detection rates. There was a 29% increase in mean daily detection rate during May-October (over the breeding season) between the two study periods. At the three stations with the highest number of detections, log linear regression revealed a yearly increase of 2.4% between 2011 and 2019 (À4.4-9.6, 95% CI). This may be indicative of the beginnings of population recovery, or simply an indication that the decline has stalled. The rate of increase is still well below what is likely to be possible for porpoise populations, and unlikely to buffer against any potential increase in pressures in the future. We therefore call for urgent management action to remove threats and protect this CR population, the only resident cetacean in the Baltic region, in order to give it the best chance of recovery.

Screening for Influenza and Morbillivirus in Seals and Porpoises in the Baltic and North Sea

“…Since the protection of marine mammals in the 1970s, the Northeast Atlantic pinniped populations have experienced a rapid recovery and reclaimed habitats and haul-out sites in the Baltic Sea and North Sea. While current population estimates of harbour seals (~25,000) [ 70 , 71 ], grey seals (38,000 (counted)) [ 72 , 73 ], ringed seals (>22,800) [ 74 , 75 , 76 ] and Belt Sea and Baltic Proper harbour porpoises (~17,300 (95% CI = 11,695–25,688; CV = 0.2) and ~500, respectively) [ 18 , 77 , 78 , 79 ] from the Baltic Sea, Belt Sea and Kattegat are below the minimum threshold (>250,000–500,000 individuals) for enzootic circulation of morbilliviruses [ 80 ], recent population estimates of UK and Wadden Sea grey and harbour seal populations suggest a UK population size of 157,500 (approximate 95% CI 146,00–169,400) and 43,750 (approximate 95% CI 36,000–58,700) [ 74 ], respectively, and a Wadden Sea population size of ~39,500 harbour seals [ 81 ] and >9000 (counted) grey seals [ 82 ]. Meanwhile, the population of North Sea porpoises have been estimated to be ~350,000 (2016: 345,000, CV = 0.18) [ 83 ].…”

What the F‐POD? Comparing the F‐POD and C‐POD for monitoring of harbor porpoise (Phocoena phocoena)

“…Such a misinterpretation of increased occurrence using F‐PODs could be a considerable problem for large‐scale monitoring programs for vulnerable populations, such as the Baltic harbor porpoise with the SAMBAH project using over 300 C‐PODs for trend assessments (Amundin et al, 2022 ; SAMBAH, 2016 ). C‐PODs in this region have been successful in identifying a yearly increase in the porpoise population of 2.4% from 2011 to 2019 (Owen et al, 2021 ). In many regions fine‐scale trends are important and comparing detection rates derived from F‐PODs to existing C‐POD detection rates could negatively influence conservation measures if not appropriately interpreted.…”

Assessment of Australian Sea Lion Bycatch Mortality in a Gillnet Fishery, and Implementation and Evaluation of an Effective Mitigation Strategy

“…Although marine mammals are known to interact with most fishing gear types, interactions with gillnets are particularly pervasive and a significant source of marine mammal mortality (Read et al, 2006;Reeves et al, 2013). Gillnet fishery interactions pose the principal threat to many endangered small cetaceans (Brownell et al, 2019;Gulland et al, 2020;Cisneros-Mata et al, 2021;Owen et al, 2021), and also threaten many pinniped species (Hamer et al, 2011(Hamer et al, , 2013Kovacs et al, 2012;Cosgrove et al, 2016;Machado et al, 2016;Jounela et al, 2019;Ramos et al, 2020). Although advances have been made in mitigating interactions with many gear types (Hamilton and Baker, 2019), interactions with gillnet fisheries continue to pose some of the greatest management and conservation challenges (Read et al, 2006;Reeves et al, 2013).…”