Drone vs. kayak: Jellyfish surveys take to the sky

Jellyfish are expanding their range world-wide, sometimes with negative ecological and economic consequences. A recent AMRI study examined if drones could provide a more cost effective, time efficient and precise monitoring tool of the upside-down jellyfish (Cassiopea sp.).

Cassiopea are unusual jellyfish – they spend most of their life upside-down, with their bell resting on sediment in shallow water and their oral arms extended above them. They sit in this position so the photosynthetic algae in their oral arms, called zooxanthellae, can catch more sunlight. This photosynthesis provides the jellyfish with up to 90% of its nutritional needs. The other 10% of the diet comes from zooplankton including fish larvae, which are captured using stinging cells.

Upside-down jellyfish are typically tropical animals living in shallow, sheltered, habitats such as mangrove swamps, seagrass beds or coral reefs. In Australia, upside-down jellyfish are common in the north, including Darwin, Lizard Island and Moreton Bay. However, records of two species of upside-down jellyfish suggest they are expanding their range south along the east-coast of Australia to temperate New South Wales, including Wallis Lake (since 2009), Lake Illawarra (since 2013), and Lake Macquarie (since 2017). Cassiopea appear to be invasive, so understanding the invasion front is essential and urgent.

Like many other jellyfish, populations of Cassiopea are patchy and have a boom-and-bust nature, varying from near-absent to forming dense swarms. The abundance of these jellyfish is directly related to their ecological impact, but patchy occurrence and population variability make monitoring the invasion front a significant challenge. Therefore, a cost-effective monitoring technique that will provide precise estimates of their population is needed.

Our studies of Cassiopea initially used kayaks to monitor the jellyfish because the shallow draft and easy handling of these watercraft allows ready access to remote, muddy and shallow sites that can’t be easily traversed on foot or by boat. This method allowed us to make very accurate and detailed observations of Cassiopea populations, but was labour and time intensive. Therefore, just as remotely piloted aircraft (hereafter called drones) have recently been used successfully to monitor sharks off surf beaches, we tested drones, which can quickly cover large areas, as potential alternatives for monitoring these jellyfish.

Results of this study show that surveys detected similar densities of upside-down jellyfish when measured by a drone compared to a kayak, along transects 50 meters in length – both methods were similarly accurate. However, when upscaling transect results to estimate the number of jellyfish occurring across a whole site, they over-inflate the perceived number of jellyfish by 319% for drones and 178% for kayaks, compared to the number of jellyfish actually present as determined by mapping a grid over the full site using the drone. Nevertheless, drones make it easy to survey the entire site, reducing the need for making estimates via up-scaling individual transect results. Also, measuring a whole-site using a drone took one-third of the time as when using a kayak. Clearly, drone-based monitoring brings many advantages over kayak-based monitoring. However, drone-based methods still have limitations, such as interfering effects of water glare, the need to keep the drone in sight of the pilot and being unable to make observations too close to shore due to obstacles such as mangroves. Benefits in supplementing drone surveillance with direct observations from the kayak include the ability to check identification of questionable objects, including potential small individual jellyfish, and allowing specimens to be sampled for additional studies such as genetics, morphology, life history and physiology.

Overall, we conclude that kayaks are useful for small-scale monitoring, but drones are more cost-effective, time-efficient, and precise for large-scale and long-term monitoring of upside-down jellyfish population abundance.

Claire Rowe, PhD candidate, Marine Invertebrates, Australian Museum Research Institute and the University of Sydney.

Dr Stephen Keable, Collection Manager, Marine Invertebrates, Australian Museum Research Institute.

Prof Shane Ahyong, Principal Research Scientist and Head, Marine Invertebrates, Australian Museum Research Institute.