By using acoustics consistently, marine trophic levels and organism abundance can be monitored, to see how they change over time facing climate change and fisheries. However, the first step to detect changes in components of a marine ecosystem or even further of a whole ecosystem, is to characterise these components (in this case the Mid Trophic Level).

In that frame, data collected during different cruises by NIWA (National Institute of Water and Atmospheric, New Zealand) (Figure above) were processed to characterise the distribution and abundance of the mid-trophic level organisms in the New Zealand sector of the Southern Ocean. One of the goal was to produce explanatory and predictive models of acoustic backscatter, and also to lessen the lack of observations and provide estimates of biological density from bulk acoustic backscatter for the region.

Data are opportunistic data from toothfish vessels (single freq. 38 kHz) and RV Tangaroa (multi-freq. 18, 38, 70, 120, 200 kHz) collected between 2008 and 2014, during transect i.e. transit between New Zealand and Antarctica (or opposite direction). Data was processed according to the Integrated Marine Observing System (IMOS) CSIRO (Echoview software) recommendations. Acoustic backscatter computed at the vertical resolution of 10 m high and 1 km long bins, was used as proxy for Mid-Trophic Level organism abundance..

Analyses were conducted to identify spatial (vertical and horizontal) and temporal (annual, seasonal and diel) patterns in the 28 acoustic transects collected between 2008 and 2014. The average acoustic backscatter (sa) at 38 kHz varied from year to year, but overall it was reasonably stable, being of the same order of magnitude for all transects. However, in all of them, backscatter decreases steadily and significantly from north to south (Figure below). This apparent north-south decrease in backscatter may be related to abundance, but trawl samples collected during three research trips suggest that it may also reflect differences in species composition and size distribution. Therefore, indices based on bulk backscatter should be interpreted with caution.

Acoustic echogram of transect collected by vessel ‘San Aotea II’ in 2010 between the Southern Ocean (right) and New Zealand (left), showing volume backscattering strength (Sv) in decibels (dB) echo-integrated in 1 km long and 10 m depth bins. Source : Escobar-Flores et al submitted.

Acoustic echogram of transect collected by vessel ‘San Aotea II’ in 2010 between the Southern Ocean (right) and New Zealand (left), showing volume backscattering strength (Sv) in decibels (dB) echo-integrated in 1 km long and 10 m depth bins. Source : Escobar-Flores et al submitted.

These acoustic data were also used to obtain the first estimates of abundance of mid-trophic level organisms in the region and to develop explanatory and predictive models for acoustic backscatter, using sea surface temperature, time of day (day/night) and depth. The resulting models predict backscatter reasonably well in the Pacific sector of the Southern Ocean, and in an independent dataset in the Indian sector of the Southern Ocean. They could help inferring abundance and distribution of Mid-trophic level organisms s in other parts of the Southern Ocean.

References:

Escobar-Flores PC, O’Driscoll RL, Montgomery JC (2018a). Spatial and temporal distribution patterns of acoustic backscatter in the New Zealand sector of the Southern Ocean. Mar Ecol Prog Ser 592:19-35.

Escobar-Flores PC, O’Driscoll RL, Montgomery JC (2018b). Predicting distribution and relative abundance of mid-trophic level organisms using oceanographic parameters and acoustic backscatter. Mar Ecol Prog Ser 592:37–56.

Escobar-Flores PC, O’Driscoll RL, Montgomery JC, Ladroit Y, Jendersie S Estimates of density of mesopelagic organisms in the Southern Ocean derived from bulk acoustic data collected by ships of opportunity. Polar Biol. Submitted.