Innovative Sonar Techniques Help Solve Mysteries around Jellyfish Population Dynamics
The need to detect and assess jellyfish is driven by a variety of interests and concerns. Certainly most of us are aware that the sting from some species of jellyfish may cause painful wounds or even death, prompting the use of warning flag systems, barrier nets, and advanced sonar systems to protect swimmers at public beaches. Compounding this threat of physical harm, jellyfish populations in many of the world's oceans are know to dramatically and suddenly fluctuate, and often without cause or explanation. Despite the significant hazards posed by contact with jellyfish, the mechanisms controlling their movements and population dynamics are not well understood. Scientific echosounders are a proven, reliable instrument for the detection and measurement of jellyfish, and these specialized sonar systems are used by researchers worldwide for a wide range of studies. Their body structure of a jellyfish is dense enough to reflect pulses of sound (pings) emitted by sonar, and thus jellyfish are quite well-suited for the use of hydroacoustics. Over recent years, researchers around the globe have developed a wide range of interesting sonar methods and equipment for their study and this article documents just a few of these projects and case studies.
Due to shifts in ocean temperatures and currents, populations of so called “giant jellyfish” can rapidly increase or “bloom” and spread to new areas. Blooms of these massive creatures wreak havoc on commercial fishing operations by fouling nets and reducing fish catches. Predicting the movements and locations of giant jellyfish can help fishermen avoid the animals, and thereby increase fishing catch rates and efficiencies.
Dr. Kyounghoon Lee, of Chonnam National University in South Korea, has extensively studied the giant jellyfish, Nemopilema nomurai, using a split beam echosounder integrated with an acoustic camera and CTD sensor deployed from research vessels while conducting mobile surveys. Results from Dr. Lee’s work provide information about the sizing distribution and migration patterns of the animals in the Yellow Sea and East China Sea. Below you can see images of the giant jellyfish and you can read more about Dr. Kyounghoon Lee and his work here.
Another Korean researcher, Dr. Kang Do-Hyung with the Korean Institute of Ocean Science and Technology (KIOST) has done extensive work using a multi-frequency BioSonics split beam echosounder studying the acoustic target strength (TS) of several jellyfish species, including the giant jellyfish Nemopilema nomurai Kishinouye. Some of Dr. Kang’s work involved using tethered jellyfish in net cages with the echosounder transducer affixed to the top of the cage. TS data derived from this research can be used for developing acoustic scattering models, and surveying giant jellyfish distributions and biomasses. Below, you can see a diagram of Dr. Kang's experiment with tethered jellyfish and a sonar echogram showing the jellyfish. Here is a link to one of Dr. Kang’s many publications.
Jellyfish increasingly cause significant problems for power plants, and other industrial facilities that intake large amounts of cooling water, when swarms of the organisms enter the water intakes and clog intake screens. Clogging events due to jellyfish blooms require laborious clean-up efforts and have caused total shutdowns of nuclear power plants in the United States, Scotland, Sweden, Japan, and Israel. Financial losses from such unplanned, sudden shutdowns can exceed 1 million USD per day. Below you can see power plant employees working to remove tons of jellyfish from water intake structures.
As a solution for power plant operators, BioSonics Automated Monitoring System (AMS) can be specifically configured for the detection of jellyfish, either for single, larger animals or dense aggregations of smaller individuals. Similar BioSonics systems are already in use at nuclear power plants in the US and Europe. The BioSonics AMS consists of a DT-X split beam echosounder coupled with a heavy duty PC running specialized software that processes hydroacoustic data in real time. Split beam transducers are fix-mounted in a horizontal or up-looking orientation and provide an acoustic curtain or “trip wire” to detect and classify objects in the water column at ranges exceeding 500m.
Diagram of a BioSonics Automated Monitoring System (AMS) for jellyfish detection. Click here for spec sheet.
For example, Jellyfish Lake, a marine lake on an island in the western Pacific nation of Palau (Micronesia), supports an impressive population of jellyfish that have only a very mild sting and attracts tens of thousands of tourists each year. These visitors come to Palau for snorkel & diving tours where they can swim alongside and photograph clouds of the brightly colored golden jellyfish (Mastigias papua etpisoni), as shown below. This infusion of adventure-seeking tourists provides an important stimulus for the local economy.
Given the economic significance of Jellyfish Lake, traditional monitoring efforts have been undertaken for decades by Coral Reef Research Foundation (CRRF) in Palau, in collaboration with the Koror State Government, the environmental authority. Because it is difficult to quantify millions of jellyfish through these traditional methods, Dr. Megan Cimino and Dr. Eric Terrill of Scripps Institution of Oceanography traveled to Palau and worked with CRRF to use new technology to examine their status. Dr. Cimino is an expert in the use of scientific sonar, and for this project she used a BioSonics DT-X Echosounder integrated into a REMUS 100 Autonomous Underwater Vehicle (AUV). Megan and Eric chose this novel method of data collection in part because the lake is inaccessible to boats and all gear must be carried overland via a foot trail into the lake.
The AUV was programmed to fly transects approximately 8 meters below the water’s surface, below the jellyfish, with the transducer aimed upwards to scan the water column. The hydroacoustic data was then processed to determine the