Pathological findings in bycaught harbour porpoises (Phocoena phocoena) from the coast of Northern Norway

Due to little prior knowledge, the present study aims to investigate the health status of bycaught harbour porpoises from the northernmost Arctic Norwegian coastline. Gross, histopathological and parasitological investigations were conducted on 61 harbour porpoises (Phocoena phocoena phocoena) accidentally captured in fishing gear from February to April 2017 along the coast of Northern Norway. Most animals displayed a good nutritional status, none were emaciated. Pulmonary nematodiasis (Pseudalius inflexus, Halocercus invaginatus and Torynurus convolutus) was found in 77% and associated with severe bronchopneumonia in 33% of the animals. The majority (92%) had parasites in the stomach and intestine (Anisakis simplex sensu stricto (s. s.), Pholeter gastrophilus, Diphyllobothrium stemmacephalum, Hysterothylacium aduncum and Pseudoterranova decipiens s. s.). The prevalence of gastric nematodiasis was 69%. In the 1st stomach compartment A. simplex s. s. was found in 30% of the animals, causing severe chronic ulcerative gastritis in 23%. Campula oblonga infected the liver and pancreas of 90% and 10% of the animals, respectively, causing severe cholangitis/pericholangitis/hepatitis in 67% and moderate pancreatitis in 10% of the animals. Mesenteric and pulmonary lymphadenitis was detected in 82% and 7% of the animals, respectively. In conclusion, the major pathological findings in the investigated Arctic porpoises were parasitoses in multiple organs with associated severe lesions, particularly in the lung, liver and stomach. The animals were generally well nourished and most showed freshly ingested prey in their stomachs. The present study indicates that the harbour porpoises were able to tolerate the detected parasitic burden and associated lesions without significant health problems.

Nutritional status and prey energy density govern reproductive success in a small cetacean

A variety of mammals suppress reproduction when they experience poor physical condition or environmental harshness. In many marine mammal species, reproductive impairment has been correlated to polychlorinated biphenyls (PCBs), the most frequently measured chemical pollutants, while the relative importance of other factors remains understudied. We investigate whether reproductively active females abandon investment in their foetus when conditions are poor, exemplified using an extensively studied cetacean species; the harbour porpoise (Phocoena phocoena). Data on disease, fat and muscle mass and diet obtained from necropsies in The Netherlands were used as proxies of health and nutritional status and related to pregnancy and foetal growth. This was combined with published life history parameters for 16 other areas to correlate to parameters reflecting environmental condition: mean energy density of prey constituting diets (MEDD), cumulative human impact and PCB contamination. Maternal nutritional status had significant effects on foetal size and females in poor health had lower probabilities of being pregnant and generally did not sustain pregnancy throughout gestation. Pregnancy rates across the Northern Hemisphere were best explained by MEDD. We demonstrate the importance of having undisturbed access to prey with high energy densities in determining reproductive success and ultimately population size for small cetaceans.

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.

Directional biosonar beams allow echolocating harbour porpoises to actively discriminate and intercept closely-spaced targets

Echolocating toothed whales face the problem that high sound speeds in water mean that echoes from closely-spaced targets will arrive at time delays within their reported auditory integration time of some 264 µsec. Here we test the hypothesis that echolocating harbour porpoises cannot resolve and discriminate targets within a clutter interference zone given by their integration time. To do this, we trained two harbour porpoises (Phocoena phocoena) to actively approach and choose between two spherical targets at four varying inter-target distances (13.5, 27, 56 and 108 cm) in a two-alternative forced-choice task. The free-swimming, blindfolded porpoises were tagged with a sound and movement tag (DTAG4) to record their echoic scene and acoustic outputs. The known ranges between targets and the porpoise, combined with the sound levels received on target-mounted hydrophones revealed how they controlled their acoustic gaze. When targets were close together and the discrimination task was more difficult due to smaller echo time delays and lower echo level ratios between the targets, buzzes were longer and started from farther away, source levels were reduced at short ranges, and the porpoises clicked faster, scanned across the targets more, and delayed making their discrimination decision until closer to the target. We conclude that harbour porpoises can resolve and discriminate closely-spaced targets, suggesting a clutter rejection zone much shorter than their auditory integration time, and that such clutter rejection is greatly aided by spatial filtering with their directional biosonar beam.