ENRICH Voyage Report, IN2019_V01: The availability of Antarctic krill to large predators and their role in biogeochemical recycling in the Southern Ocean
Double, M.C., Bell, E., Miller, B., Kelly, N., Kawaguchi, S., Lawrence, J., Leaper, R., Olson, P., Westwood, K. et al.
In March, 2017 the project entitled, ‘The availability of Antarctic krill to large predators and their role in biogeochemical recycling in the Southern Ocean’, was awarded ship time on Australia’s Marine National Facility’s research vessel, RV Investigator. The resulting 49-day ENRICH Voyage (Euphausiids and Nutrient Recycling In Cetacean Hotspots) departed from Hobart, Tasmania on the 19 January and returned to the same port on the 5 March, 2019.
Multi-disciplinary marine science was conducted off Antarctica from 64°S to 67°S and between 138°E and 154°W. Active acoustic data were collected continuously throughout the voyage totalling over 9,000 km of effort south of 60oS. The survey of the study area included six formal broad-scale transects (approx. orientated N-S). During the 1,670km of effort on these transects 975 distinct krill swarms were detected and the acoustic density and 3D structure was recorded for each swarm using the calibrated echosounder. The open-ocean cold-water calibration of the EK60 echsounder was conducted south of 64oS on the 26th January. Krill were distributed throughout the survey region with the highest densities in the western region close to but offshore of the shelf-break whereas in the east the highest densities were further north. A total of 41 target trawls were conducted using the RMT1+8 scientific trawl net in order to determine the krill size, maturity stage composition of various swarms. Morphometric data were collected from 4,385 krill and the growth rates of 5,472 were measured in 20 Integrated Growth Rate experiments.
In total 295 sonobuoys were deployed during the voyage, which provided 574 hours of passive acoustic monitoring. Antarctic blue whales were detected most commonly with 33,435 calls on 238 sonobuoys. The geographic location of calling whales could be determined over a period of 205 hours when two or three sonobuoys provided data simultaneously.
Visual sightings effort totalled 317 hours over 4,471km. In total there were 569 sightings of 1,380 cetaceans. Sightings of humpbacks whales were most common (201) followed by fin (124), blue (26) and minke whales (23). Nineteen groups of blue whales were approached for photo-identification from which suitable imagery was collected from 25 whales. To obtained data on the surfacing and movement behaviour of whales relative to their local prey field video-tracking of blue and fin whales was conducted ion 24 occasions for a total of 18 hours.
During the voyage 134 Unmanned Aerial Vehicle (UAV or drone) flights were conducted to undertake photogrammetry, photo-identification, whale ‘blow’ sampling, surface water sampling or to collected general whale and scenic imagery. Of these flights 113 were conducted used a DJI Inspire 2 and 21 using a Phantom 4. Photogrammetry video data (8 individuals) and a single blow sample were collected from blue whales.
A total of 110 biogeochemistry deployments were conducted during the voyage including 28 CTDs (Conductivity Temperature Depth; at 22 survey and 5 process stations), 35 Trace Metal Rosettes (TMRs; at 21 stations), 37 eXpendable BathyThermographs (XBTs) and 10 drifters. Not all CTD and TMR provided water samples for experimental work but in total these deployments together with UAV water sampling produced over 3,500 samples for a diverse range of analyses examining primary production, bacterial production, dissolved organic carbon, viral abundance, eDNA/RNA, dissolved chlorophyll-a, DMSP, trace metals and organic ligands.
This voyage provided the first opportunity to conduct a detailed survey of the distribution and characteristics of krill swarms while recording physical and biological oceanographic data together with the distribution and behaviour of the largest krill predators, primarily blue, fin and humpback whales. While this was an ambitious and complex multidisciplinary voyage it stands as example of how many scientific data streams can be collected simultaneously to provide a more complete description of the dynamic physical and ecosystem processes occurring off Antarctica albeit over a relatively short period of time.
Many of the datasets generated by this voyage are large and complex and integrated analyses will take many years but they will generate novel insights into Antarctic krill, their environment and the predators that depend upon them.
We estimate the data collected during the ENRICH voyage will contribute to at least 50 peer reviewed publications and will be included in at least five PhD theses. The voyage provided training opportunities for 6 graduate students, and 6 early career researchers. Outreach from the ENRICH voyage included 5 remote classroom presentations during the voyage, and generated nearly 40 articles and stories on mainstream media.