Spatial and temporal distribution patterns of acoustic backscatter in the New Zealand sector of the Southern Ocean

Mid-trophic level organisms (MTLO) of open-ocean marine ecosystems play a key role linking primary and tertiary consumers. Despite their importance, characterisation of MTLO is limited due to sampling difficulty, and is largely obtained through active acoustics. Acoustic data collected from vessels of opportunity transiting across the Southern Ocean between New Zealand and the Ross Sea provided the opportunity to study distribution and abundance of MTLO over 7 yr. Analyses were performed to identify spatial (vertical and horizontal) and temporal (annual, seasonal and diel) patterns in 28 acoustic transects collected between 2008 and 2014. Mean acoustic backscatter (s(a)) at 38 kHz varied between years, but overall was reasonably stable, being in the same order of magnitude across all transects. Backscatter consistently and significantly decreased from north to south. Although this latitudinal pattern could be related to MTLO abundance, trawl samples collected in 3 research voyages suggest that it may also reflect differences in species composition and size distribution; consequently, indices based on bulk backscatter must be interpreted with caution. Vertical distribution of backscatter showed clear diel vertical migration patterns and 4 distinct vertical bands (i.e. epipelagic, transition, mesopelagic and deep mesopelagic), with seasonal differences in concentration and behaviour. Deep mesopelagic layers stopped north of the Ross Sea, which may relate to the temperature limitation of contributing organisms. Predicted climate change effects in the Southern Ocean could modify the spatial distribution of MTLO and have impacts on top predators relying upon the mid-trophic level as their main food source.