In situ behaviour and acoustic properties of the deep living jellyfish periphylla periphylla

The importance of jellyfish in marine systems is increasingly being recognized, and in some ecosystems, jellyfish may now be considered the top predator. We studied the behaviour of individuals of the deep-water schypozoan Periphylla periphylla in one such location, the Lurefjord, Norway. The study was performed using a combination of submersible acoustics (38 kHz), video and net methods, and the focus was on variation in behaviour and vertical distribution in relation to the diel cycle. A proportion of the population underwent synchronous vertical migrations, but P. periphylla were still recorded throughout the water column both day and night. The majority of individuals were swimming (vertically) at speeds <2 cm s^-1 irrespective of the time of day. However, occasional vertical swimming events with speeds exceeding 10 cm s^-1 were recorded. Such events of elevated vertical speeds were of short duration, followed by subsequent periods of no vertical movements. Different size fractions appeared to have different patterns of vertical swimming activity, with smaller jellyfish swimming more continuously than the larger Periphylla. The echo strengths of the individual returns (target strength, TS) peaked at approximately -62 dB, and variability in TS for individuals was high, with the strongest echoes seen in deep water. The results show the feasibility of acoustic methods for studying the in situ behaviour and acoustic properties of these jellyfish, but also that acoustically weak jellyfish are only recorded close to the transducer or the acoustic axis, which will bias acoustic data on vertical size distribution and acoustic abundance estimates.