Kicking off our special features on the zooplankton symposium is Stein Kaartvedt whose talk, Echosounders: Non-intrusive observations of the pelagic, on Monday morning demonstrated how acoustic monitoring helps assessing zooplankton mortality, as well as acoustically revealed behavior and feeding patterns of macroplankton.
Echosounders can contribute significantly to our understanding of the physics of the oceans, their organisms and ecosystem functioning. Organisms and other particles, as well as physical gradients, reflect the sound submitted by the echosounder, and the depth of an acoustic target is calculated. To simplify, the larger the organism, the stronger the echo, and the more fish, the more acoustic backscatter. Sound penetrates much longer in water than light, and the lower the frequency, the longer the range. A 38 kHz echosounder normally used in fishery investigations may detect a single large fish at a range (depth) of 1000 m. However, to detect small organisms, higher frequency is needed. The implication is that studies of small zooplankton must be done at short ranges.
Acoustic mapping provides unsurpassed temporal and spatial resolution. A thousand meter water column can be profiled every second second and large geographic scales can be covered by a moving vessel. The backside is the identification of the acoustic target. For the latter, the acoustic methods are supplemented with tools like trawling and video, though also the characteristics of the acoustic signal and differences in responses to different frequencies provide clues to the identities of the targets.
Assessing the abundance, distribution and behavior of fish is important in analyzing marine ecosystems. A fish ecologist may primarily be interested in the fish itself, while for a plankton ecologist, the fish recorded by the echosounder may be the predator of the zooplankton. The abundance, distribution and behavior of fish are therefore important parameters in analyzing zooplankton ecology. For this, echosounders are powerful tools.
To take the acoustic analysis one step further, high frequency echosounders are used to observe zooplankton directly. Zooplankton can only be observed in shallow waters unless the acoustic system is submerged to depth. Submerging the system has an additional advantage relating to both fish and plankton. The closer to the target, the narrower the acoustic beam, and the better resolution of the individuals one aims at studying.
Acoustic studies have traditionally been conducted using hull-mounted transducers. Stationary, submerged echosounders can be used in telling quite new stories. The remote and inaccessible nature of deep-sea habitats has largely precluded direct observations of their inhabitants. However, today's pressure proof submersible systems can be deployed at large depths. This allows for direct observation of in situ behavior of deep-living macroplankton, fish and squid as they traverse the acoustic beam both horizontally and vertically. Autonomous systems may be battery powered or can be cabled to shore for power and transmission of data. Ecosystem processes and organisms can be studied at timescales of seconds to years, and cm-sized (and even smaller) individuals and their potential predators can be assessed in their undisturbed natural environment.
Some applications of ecosounders for assessing marine ecosystems were presented during my key-note lecture at the ICES/PICES 6th Zooplankton Production Symposium, including how acoustic monitoring helps to assess zooplankton mortality, as well as acoustically revealed behavior and feeding patterns of macroplankton. The use of echsounders has revealed both short-term and seasonal patterns in organismal behavior and ecosystem processes which hardly could have been disclosed in other ways.
Deployment of autonomous echosounder in the Red Sea.
The remote and inaccessible nature of deep-sea habitats has largely precluded
direct observations of their inhabitants. However, echosounders can be deployed
at large depths, providing information on abundance, distribution and behavior
of deep-living plankton, squid and fish. Photos: Stein Kaartvedt.