An evaluation of acoustic and video methods to estimate the abundance and vertical distribution of jellyfish

Abstract

Four non-invasive methods (three acoustic and one video) were compared to estimate the abundance and vertical distribution of scyphomedusae, using the locally abundant coronate species Periphylla periphylla in Lurefjorden, north of Bergen, Norway, as the test organism. The acoustic methods included single-target quantification by, respectively, a hull-mounted 38 kHz split-beam echo sounder and a 675 kHz scanning sonar mounted on a remotely operated vehicle (ROV). Data from acoustic backscattering provided estimates of abundance and changes in the vertical position of scattering (¼ aggregation) layers. Vertical migration was also studied by acoustic target tracking, following single individuals as they ascended in the afternoon. The ROV-mounted scanning sonar missed an unidentified part in the lower size distribution of the target population. Vertical video profiles, conducted during ROV dives, provided the most detailed information on abundance and distribution, although uncertainties remained regarding the size distribution in the population.

Diel vertical migration of individual jellyfish (Periphylla periphylla)

“…results). Several previous studies established that acoustic targets deeper than 150 m will be P. periphylla (Fosså 1992;Youngbluth and Bå mstedt 2001;Bå mstedt et al 2003). The week before our cruise, extensive sampling was carried out in Lurefjorden by a different science crew, reaching the same conclusion (Ulf Bå mstedt, pers.…”

“…Lurefjorden is characterized by an extraordinary large population of this mesopelagic species (Fosså 1992;Youngbluth and Bå mstedt 2001;Bå mstedt et al 2003), occurring in concentrations several orders of magnitude greater than in the open ocean (Youngbluth and Bå mstedt 2001). The fjord is also exceptional by the virtual absence of mesopelagic fish in the basin water.…”

“…This has been related to high light extinction, which makes feeding conditions unfavorable for visual predators Aksnes et al 2004). P. periphylla carries out DVM (Fosså 1992;Youngbluth and Bå mstedt 2001;Bå mstedt et al 2003), and it has been suggested that 80-90% of the population in Lurefjorden participate in gross migrations from below 100-200 m at day, to upper layers at night (Youngbluth and Bå mstedt 2001). However, Klevjer (2006), making acoustic registrations down to 360 m, recently showed that many individuals occurred in deep water, also at night.…”

“…This is because their bodies often are fragile and easily damaged by traditional sampling with plankton nets, because available plankton nets are too small to sample large or dilutely distributed medusae, or because the medusae occur in such large numbers that they clog and burst the nets (e.g., Brierley et al 2001;Youngbluth and Bå mstedt 2001). However, P. periphylla is detected acoustically (Bå mstedt et al 2003;Klevjer 2006) and may be easily studied in Lurefjorden because of the absence of alternative targets in the deep water. Although the population density is unusually high compared with oceanic populations of this species, the overall concentrations of jellyfish are sufficiently low to facilitate resolution of individual jellyfish in acoustic studies, largely avoiding problems of multiple targets (cf.…”

Norwegian fjords: From natural history to ecosystem ecology and beyond

“…Acoustic observations of diel vertical migration were made by hull-mounted echosounders and an echosounder located on the seabed ).The behaviour of Periphylla has been studied in situ by SCUBA divers at night when the medusa are close to the surface. During daytime when the medusae are in the dark deeper waters, they have been studied by use of stillcameras and video-cameras mounted on an unmanned submarine or ROV (Remotely Operated Vehicle) (Båmstedt et al 2003). Many scientists and TV crews have been lured to Lurefjorden to study Periphylla.…”