Understanding jellyfish ecology and roles in coastal ecosystems is challenging due to their patchy distribution. While standard net sampling or manned aircraft surveys are inefficient, Unmanned Aerial Vehicles (UAVs) or drones represent a promising alternative for data collection. In this technical report, we used pictures taken from a small drone to estimate the density of Aurelia sp. in a shallow fjord with a narrow entrance, where the population dynamic is well-known. We investigated the ability of an image processing software to count small and translucent jellyfish from the drone pictures at three locations with different environmental conditions (sun glare, waves or seagrass). Densities of Aurelia sp. estimated from semiautomated and manual counts from drone images were similar to densities estimated by netting. The semiautomated program was able to highlight the medusae from the background in order to discard false detections of items unlikely to be jellyfish. In spite of this, some objects (e.g., seagrass) were hardly distinguishable from jellyfish and resulted in a small number of false positives. This report presents a preview of the possible applications of drones to observe small and fragile jellyfishes, for which in situ sampling remains delicate. Drones may represent a noninvasive approach to monitoring jellyfish abundance over time, enabling the collection of a large amount of data in a short time. Software development may be useful for automatically measuring jellyfish size and even population biomass.

Opportunistic mobile predators can adapt their behaviour to specific foraging scenarios, allowing them to target diverse prey resources. An interesting example is the harbour porpoise (Phocoena phocoena), a marine mammal with a huge energy demand feeding on a large variety of fish, squid and shrimps. Little is known about the foraging behaviour of harbour porpoises, as observations of wild specimens are notoriously difficult to obtain. In this study, foraging was identified in almost 60% of videos from UAV recordings in Danish coastal waters during daylight hours. Observations reveal them to be flexible predators, foraging on both single fish and schools of fish, as well as individually and in groups of varying sizes. We argue that some of the observed behavioural adaptations and context‐dependent strategies for prey capture are based on information transfer and social learning. Our results provide unprecedented insights into the foraging behaviour of an opportunistic mammalian predator. Furthermore, this study highlights the importance of porpoises having access to coastal areas for energy acquisition, where they are in conflict with anthropogenic disturbances such as fisheries with the risk of bycatch.

The starry smooth-houndMustelus asteriasis a near-threatened coastal shark in Europe, whose parasitofauna is largely unknown. We studied metazoan parasites of 20 immature sharks (13 males and seven females) from the English Channel and we examined their relationships with host condition and their use as host bioindicators. All the sharks were parasitized by one to six metazoan taxa among the twelve recorded in the whole sampling (one trematode, six cestodes and two nematodes trophically-transmitted; one monogenean, one copepod and one myxosporean on gills), with a mean abundance of 30.5 ± 21.4 parasites per fish (myxosporeans not included). The three major taxa were in decreasing order: the nematodeAcanthocheilus rotundatus(prevalence: 75%, Confidence Interval 53–89%), the cestodeEutetrarhynchussp. (70%, CI 48–85%), and the monogeneanErpocotyle laevis(60%, CI 39–78%). The gill copepodKroyeria lineataand the gut nematodeProleptus obtususwere identified as significant pathogens. Parasite community differed between males and females despite their immature stage, suggesting early spatial sex-segregation, withE. laevis,Eutetrarhynchussp. andAnthobothriumsp. proposed as tags. We discuss results in terms of host fitness loss and information given by parasites on diet ecology and stock discrimination. We recommend incorporating parasitology in further research to improve shark conservation and management.