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Automatic click detectors and full-bandwidth sound recorders are widely used in passive acoustic monitoring of small cetaceans. Detection of these signals depends on a variety of factors, including signal to noise ratio. Passive acoustic monitoring is often used to study impact of underwater noise on small cetaceans, but as detection probability is affected by changes in signal to noise ratio, variable noise levels may affect conclusions drawn from these experiments. Therefore, we examine how different detectors and filters perform in varying ocean noise conditions. C-PODs and full-bandwidth recorders (Wildlife Acoustics, SM2M+) were deployed at two stations in an environment with fluctuating ambient noise for 42 days. Noise level and harbour porpoise (Phocoena phocoena) click trains simultaneously recorded on both loggers were compared. Overall, we found that porpoise click detections by the algorithm used to analyse full-band recorder data (Pamguard) paralleled detections by the C-POD. However, Pamguard detected significantly more clicks than the C-POD. A decrease in detections was seen for both loggers with increasing noise in the band 20-160 kHz, in particular for levels above 100 dB re 1μPa rms. We also found that the Pamguard detection function changed the least over varying noise conditions when compared to the C-POD detectors. This study sheds light on the fact that inference of animal presence/absence or density that are based on echolocation cues (here, Porpoise Positive Minutes) shall account for the acoustic environments where probability of detecting signals may be affected by variability in ambient noise levels.

Relatively little is known about the distribution and diversity of marine mammals around offshore anthropogenic structures. We present results obtained from incidental sightings of marine mammals around oil and gas installations located 200 km off the Danish coast. A total of 131 sightings corresponding to about 288 animals were reported between May 2013 and May 2016. A total of seven marine mammal species were identified, five cetaceans: harbour porpoise (Phocoena phocoena), minke whale (Balaenoptera acutorostrata), white-beaked dolphin (Lagenorhynchus albirostris), killer whale (Orcinus orca), pilot whales (Globicephala spp.) and two species of pinnipeds: harbour (Phoca vitulina) and grey seals (Halichoerus grypus). The most sighted species were harbour porpoise (41%) and minke whale (31%). Relative counts and biodiversity of marine mammals observed around installations corresponded well with the expected distribution in the central North Sea. Several taxon-specific correlations were identified between number of sightings and environmental parameters (depth and latitude) or installation characteristics (installation aerial footprint). Furthermore, 85% of sightings were made during spring and summer and it is unclear whether the pattern observed reflected a natural seasonal occurrence of marine mammals in the area or an effect of reduced effort during autumn and winter. Despite the potential caveats, results obtained during this programme provide an insight into the relationship between marine mammals and oil and gas offshore installations in the North Sea.

1. Harbour porpoises frequently alter their behaviour in response to underwater sound from shipping, seismic surveys, drilling and marine renewables. Less well understood is the response of porpoises to sounds emitted from oil and gas (O&G) platforms during routine operations. 2. The responses are not easily predicted as platforms can act simultaneously and to varying degree as a source of disturbance through noise and attraction through an artificial reef effect with increased prey abundance and diversity. 3. To investigate the presence and feeding behaviour of harbour porpoises around platforms, autonomous acoustic loggers were placed for up to 2 years, at 21 stations 0-25.6 km from the largest platform in the Danish North Sea. 4. Harbour porpoises were detected at all distances year round in two distinct seasonal activity patterns. During July-January, porpoises were attracted to the platform as indicated by high foraging activity within 800 m of the platform. Echolocation activity levels were up to twofold higher than those observed at 3.2-9.6 km from the platform. 5. Similar high echolocation activity was observed 200 m from neighbouring offshore installations located within 15 km, regardless of their size, during May-July. 6. This study shows that porpoises may be attracted to offshore O&G platforms despite confirmed elevated underwater noise and are likely exploiting higher prey abundance in the vicinity of such structures. This is possibly due to increased prey availability created by the combined effect of the artificial reef formed by the underwater structure and the local protected area around all platforms where fishery is banned. 7. Hard substrate and untouched seabed are rare and valuable habitats to many organisms in heavily trawled waters like the North Sea, and the ecological importance of these structures should be considered in the development of decommissioning strategies. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Trematodes are ubiquitous elements of coastal ecosystems that commonly modify the phenotype of their invertebrate hosts, often with ramifications to higher levels of ecological organization. In this context, trematode infections have been suggested to reduce the consumption of the herbivorous gastropod Littorina littorea (L.) (Mollusca: Gastropoda), in turn affecting the composition of the macroalgal community on which the snail grazes. Here, we examine the effect of two species of trematodes, Renicola roscovita and Himasthla elongata, on L. littorea's consumption in two outdoor microcosm experiments offering the snails two different ephemeral green algae species as a food source. Our results show that, irrespective of the species of parasite and food source, infection decreases consumption: uninfected snails consumed up to 65% more macroalgal biomass than infected snails. Aside from infection status, gender and size also influenced the snails' consumption rate significantly. The differing histopathological impacts of the two species of trematodes on the hosts' gonad–digestive gland complex (in which the parasites reside), suggests that parasitic castration is a likely mechanism for the reduced energy demand of infected periwinkles. Together with existing evidence, our investigation suggests that trematodes in general depress the grazing activity of L. littorea, and that the resulting community regulation occurs throughout the snails' distributional range.

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