Estimating the abundance of the critically endangered Baltic Proper harbour porpoise (Phocoena phocoena) population using passive acoustic monitoring

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.

Group hunting in harbour porpoises

Cooperative hunting involves individual predators relating in time and space to each other’s actions to more efficiently track down and catch prey. The evolution of advanced cognitive abilities and sociality in animals are strongly associated with cooperative hunting abilities, as has been shown in lions, chimpanzees and dolphins. Much less is known about cooperative hunting in seemingly unsocial animals, such as the harbour porpoise (Phocoena phocoena Linnaeus, 1758). Using drones, we were able to record 159 hunting sequences of porpoises, out of which 95 sequences involved more than one porpoise. To better understand if the harbour porpoises were individually attracted by the fish school or formed an organized hunting strategy, the behaviour of each individual porpoise in relation to the targeted fish school was analysed. The results indicate role specialization, which is considered the most sophisticated form of collaborative hunting and only rarely seen in animals. Our study challenges previous knowledge about harbour porpoises and opens up for the possibility of other seemingly non-social species employing sophisticated collaborative hunting methods.

Inside the Dwelling: Clay Figurines of the Jägala Jõesuu V Stone Age Settlement Site (Estonia)

Sculpted clay figurines were widespread in Stone Age Europe. Theywere common in the hunter-gatherer communities in the territoriesof Sweden, Finland, Estonia, Latvia, Western and NorthwesternRussia. In these territories they were mainly associated with theComb, Pitted and Pit-Comb Ware cultures, ca 4000–2000 yearscalBC. This paper examines clay sculptures from the Jägala JõesuuV Comb Ware culture settlement site in northern Estonia, where 91fragments of figurines were found, making it the most abundantdeposits of clay figurines and their fragments in the eastern Baltic.Among them, three different types of image were distinguished:one zoomorphic (harbour porpoise) and two anthropomorphic. Allthe figurines were fragmented intentionally in ancient times, asdetermined by microscopic and experimental research. Most of thefragments were situated in the filling of a pit-house, which indicatesthat the dwelling had a sacral as well as a habitational dimension.During the research process, Stone Age clay figurines from nine moreComb Ware culture sites of Estonia and Ingria were catalogued. Thecatalogue contains 13 previously published and 21 newly discoveredinstances and radiocarbon dates taken at the sites, some of whichare being published for the first time.

Seasonal dynamics of occurrence and group size of three species of cetaceans (Delphinidae and Phocoenidae) on the southwestern coast of the Black Sea

The spatial and temporal occurrence of cetaceans in the Black Sea and particularly along its southwestern coastline is poorly studied. Based on a total of 609 encounters, we present the analysis of monthly dynamics of (1) the numbers of cetacean sightings and (2) their group size for all three Black Sea cetacean species: the bottlenose dolphin (Tursiops truncatus), the short-beaked common dolphin (Delphinus delphis) and the harbour porpoise (Phocoena phocoena) over a continuous period of 29 months (January 2018 - May 2020) from two fixed coastal observation points near the city of Zonguldak, Turkey. Seasonal fluctuations were found in the occurrence of all three species, with peaks in April-May (spring). Similarly, the largest groups of P. phocoena and T. truncatus were recorded in the late spring and early summer period, while D. delphis had a peak in July (summer). In case of the harbour porpoise, we speculate that these variations can be explained by the annual migrations from the northwestern to southern waters of the Black Sea, while the seasonal dynamics of the two Delphinidae species might depend on fish prey availability. While our data were collected locally, the results provide better insight into the ecology of cetaceans within the entire Black Sea.

Assessing the impact of fisheries-related mortality of harbour porpoise (Phocoena phocoena) caused by incidental bycatch in the dynamic Norwegian gillnet fisheries

Harbour porpoise (Phocoena phocoena) bycatch for Norwegian commercial gillnet fisheries from 2006 to 2018 was estimated using a traditional ratio estimator and generalized additive linear mixed models, with weight of fish landed and number of gillnet hauls as proxies for fishing effort. Estimates were derived from data collected with a contracted reference fleet of small coastal vessels and scaled up to the whole fleet using data from landing statistics. Bycatch estimates exhibited large yearly variations, ranging from 1151 to 6144 porpoises per year. Bycatch estimates in 4 of the last 5 years were significantly less than in the preceding 2 years. The best ratio-based and model-based yearly bycatch estimates were 1580 porpoises [coefficient of variation, (C.V.) 0.10, 95% confidence interval (CI) 1302–1902] and 1642 porpoises (C.V. 0.15, 95% CI 1165–2142), respectively. About 75% of bycaught porpoises were taken in the cod (Gadus morhua) and monkfish (Lophius piscatorius) fisheries, while the rest were taken in a variety of different gillnet fisheries. Our results suggest that bycatch of harbour porpoise in Norwegian gillnet fisheries has been unsustainable for several of the last 13 years but are currently within international bycatch limits due to a recent reduction in monkfish fishing effort.