Abstract
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.
Highlights
Jellyfish (JF) blooms are reported more frequently and are often linked to humaninduced environmental changes ([1,2], but see [3])
We focused on the Aurelia sp. due to their wide distribution and abundance in the fjord, as well as the color and shape of the medusae that make them recognizable from above with the drone
We did not make the distinction of the different species in the field and we refer to the genus Aurelia sp. in this report
Summary
Jellyfish (JF) blooms are reported more frequently and are often linked to humaninduced environmental changes ([1,2], but see [3]). Jellyfish have been studied for years for their important ecological role and impact on marine ecosystem processes [5,6], as populations can rapidly grow to large aggregations and form seasonal blooms [7]. Jellyfish blooms can have a significant impact on zooplankton abundance and thereby influence the entire ecosystem of a region [8,9,10]. Besides observation from boats or from the shore (e.g., [11,12]), nets have been conventionally used in long-term studies to obtain consistent estimations of jellyfish biomass and abundance (e.g., [13,14])
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