Knowing the abundance of a population is a crucial component to assess its conservation status and develop effective conservation plans. For most cetaceans, abundance estimation is difficult given their cryptic and mobile nature, especially when the population is small and has a transnational distribution. In the Baltic Sea, the number of harbour porpoises (Phocoena phocoena) has collapsed since the mid‐20th century and the Baltic Proper harbour porpoise is listed as Critically Endangered by the IUCN and HELCOM; however, its abundance remains unknown. Here, one of the largest ever passive acoustic monitoring studies was carried out by eight Baltic Sea nations to estimate the abundance of the Baltic Proper harbour porpoise for the first time. By logging porpoise echolocation signals at 298 stations during May 2011–April 2013, calibrating the loggers’ spatial detection performance at sea, and measuring the click rate of tagged individuals, we estimated an abundance of 71–1105 individuals (95% CI, point estimate 491) during May–October within the population's proposed management border. The small abundance estimate strongly supports that the Baltic Proper harbour porpoise is facing an extremely high risk of extinction, and highlights the need for immediate and efficient conservation actions through international cooperation. It also provides a starting point in monitoring the trend of the population abundance to evaluate the effectiveness of management measures and determine its interactions with the larger neighboring Belt Sea population. Further, we offer evidence that design‐based passive acoustic monitoring can generate reliable estimates of the abundance of rare and cryptic animal populations across large spatial scales.
The mosaic archipelagoes, high water turbidity, variable benthic habitats and very long shoreline of the Northern Baltic Sea make it costly and technically challenging to collect data that is useful for decision making concerning conservation and sustainable use of the marine environment. The Finnish Inventory Programme for the Underwater Marine Environment (VELMU) has gathered information on species, communities and habitats during 2004-2016 from over 122,000 observation points. The data consists of 95,600 points made with drop-video or ROV, 23,200 with dive line points (from ca. 2000 dive sites), 1850 fish larvae sampling sites, >1000 benthos samples, 780 geological sediment samples, and 20,000 km of acoustic profiling (e.g. echosounding). In addition, satellite observations, LIDAR, aerial imaging with drones and automatic video platforms were used or tested. The majority of the data has been published in an open map service (https://paikkatieto.ymparisto.fi/velmu). We present our national sampling design, by which the majority of data was collected cost-effectively in a 5 year time span (2011-2015). We show examples of our results, such as geographical distribution maps and spatial models for species, habitats, biodiversity and environmental factors. We also explain how the VELMU data has been used in marine spatial planning in the Finnish sea area. In the planning process, areas with high nature values were identified and given a high conservation priority, while other areas were identified as more suitable for human activities, such as aquaculture, wind energy production and ecotourism. Based on our 13 years long experience we review the challenges of mapping species and habitats in complex marine environments, and provide a checklist for establishing a successful inventory programme in such conditions.
Knowing the abundance of a population is a crucial component to assess its conservation status and develop effective conservation plans. For most cetaceans, abundance estimation is difficult given their cryptic and mobile nature, especially when the population is small and has a transnational distribution. In the Baltic Sea, the number of harbour porpoises (Phocoena phocoena) has collapsed since the mid-20th century and the Baltic Proper harbour porpoise is listed as Critically Endangered by the IUCN; however, its abundance remains unknown. Here, one of the largest ever passive acoustic monitoring studies was carried out by eight Baltic Sea nations to estimate the abundance of the Baltic Proper harbour porpoise for the first time. By logging porpoise echolocation signals at 298 stations during May 2011-April 2013, calibrating the loggers’ spatial detection performance at sea, and measuring the click rate of tagged individuals, we estimated an abundance of 66-1,143 individuals (95% CI, point estimate 490) during May-October within the population’s proposed management border. The small abundance estimate strongly supports that the Baltic Proper harbour porpoise is facing an extremely high risk of extinction, and highlights the need for immediate and efficient conservation actions through international cooperation. It also provides a starting point in monitoring the trend of the population abundance to evaluate the effectiveness of management measures and determine its interactions with the larger neighbouring Belt Sea population. Further, we offer evidence that design-based passive acoustic monitoring can generate reliable estimates of the abundance of rare and cryptic animal populations across large spatial scales.
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