Status and trend of mesopelagic ecosystems using vessel acoustics and a profiling lagrangian acoustic optical probe

Open ocean mesopelagic communities are a key ocean resource and play an important role in active carbon sequestration and could potentially contribute to global food security. To better understand the present structure, function and change of mesopelagic ecosystems, we need improved estimates of the species (groups) biomass and trophic linkages and associated long term time series. Acoustic data from the IMOS Bio-Acoustic Ships of Opportunity Program are used here to investigate the status and trend of mesopelagic micronekton in the Southern Ocean. The decade long bioacoustic time series highlight a significant increasing trend in the acoustic backscatter (400–800m) for the Tasman Sea and associated Southern Ocean region. An interpretation of this trend is an increase in the number of acoustic dominant organisms (between 40–105%) over the time frame studied. If proven and validated this could increase the active process of carbon sequestration and could be a region for increased interest in potential harvesting. Acoustic and optical probes can be used to interpret vessel acoustic data and provide an independent view of the mesopelagic realm. A recent probe development has been the profiling lagrangian acoustic optical system (PLAOS) that can in principle provide a census of the less mobile organisms through the water column through acoustic and visual means. We provide a summary of its use to index a range of biota and an interpretation of the dominant acoustic species and their behaviour using the available acoustic spectrum, still and video imagery with and without active lighting. We postulate that the echo counting method from probes may prove to be a simple and reliable method to monitor changes in deep sea ecosystems but do not put away the nets.