In situ quantification and analysis of large jellyfish using a novel video profiler

Abstract

Ecological studies involving large jellyfish have been limited by the inability of oceanographers to measure the abundance and distribution patterns of these highly aggregated ani- mals at local scales. Conventional plankton nets are undesirable in these applications because they cannot sample volumes large enough to accurately determine jellyfish concentration, nor do they have adequate spatial resolution to account for the ubiquitous patchiness of most large jellies. Nets are also notorious for damaging the watery bodies of jellyfish. To overcome these problems, we have developed a video system for use in the in situ study of large jellyfish. The design of our JellyCam is easily replicated since it incorporates commercially available components within a frame designed to hold hydrographic instrumentation available at most marine laboratories. We present data sets from 2 occasions as a demonstration of the utility of the JellyCam. On one occasion, a vertical profile of medusae of Pelagia noctiluca revealed intense layering of these jellyfish at the pronounced halo- cline/pycnocline. Most jellyfish in this layer were swimming toward the surface, and it was hypothe- sized that retarded forward-swimming velocity at the halocline, due to salt retention in jellyfish, caused this accumulation. A separate 800 m long horizontal transect of Phyllorhiza punctata medusae revealed distinct concentrated bands of jellyfish associated with increased chlorophyll concentration. Concomitant hydrographic data from the JellyCam showed that accumulation of both jellyfish and chlorophyll was associated with a hydrographic front. These data sets demonstrate that this system is capable of the desired 2 m 3 resolution, which is adequate for the observation and quantification of jellyfish distributions around small-scale physical discontinuities (e.g. fronts and pycnoclines). A series of side-by-side comparisons with a conventional plankton trawl resulted in comparable mea- surements of large jellyfish (Aurelia aurita) concentrations. Though in situ videography by itself is a powerful tool for investigating jellyfish, its use in conjunction with conventional nets or other technologies, such as acoustics and self-propelled vehicles (e.g. remotely operated vehicles and submersibles), will provide the most comprehensive view of jellyfish distribution in 3 dimensions.