Multi-sensor observation of a rapidly dispersing micronekton thin layer

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Ocean midwaters—areas between the sunlit surface layers and seafloor—comprise the largest habitat on Earth but are among the least understood marine environments. Available sampling platforms (e.g. net systems, moored and shipboard sensors), are often unable to resolve the environmentally-coupled distributions of marine biota throughout the water column over the relevant scales. A deep profiling (1000 m rated) stereo camera was operated in tandem with a split-beam five channel fisheries echosounder to record midwater scattering layers in detail across the oxygen minimum zone (OMZ) offshore of Baja California. A computer vision software library was developed to batch process the collected water column imagery and the derived biological information was interpolated with environmental sensor and acoustic backscatter measurements. A large aggregation of the micronekton squat lobster Pleuroncondes planipes (red crab) was described in the imaging and acoustic data. During midday hours, the micronekton were distributed in a thin layer 1.4 m in vertical extent with a sharp lower boundary and having a maximum abundance of at least 10 individuals per m3. The thin layer distribution was tightly coupled to the 1026 kg/m3 isopycnal associated with a high-frequency internal wave. At dusk the crabs redistributed upwards suddenly as a specific mesopelagic scattering layer with a daytime settling depth of 600–800 m migrated through the P. planipes thin layer near the surface. The thin layer was not disrupted by the ascent of an earlier migrating layer suggesting a layer-selective migration response.

Publication Title, e.g., Journal

Deep-Sea Research Part I: Oceanographic Research Papers