Euphausiid assemblages of the oceanographically complex north-west marine bioregion of Australia, (2010)

An area between 29S and 31S in the south-east Indian Ocean was investigated during a multidisciplinary study aboard the R.V. Southern Surveyor (voyage 05/2010) during the austral winter (1721 July 2010). At each of the three latitudes, stations were sampled at 0.5 longitudinal intervals across shelf break and oceanic waters between 111.5E and 115E (Fig. 1). All stations had a water column depth >1000 m. In total, 13 stations form the focus of this study. Depth-stratified zooplankton samples were taken using a multiple opening-closing EZ net (mesh size 355 m; mouth area 1m2; Open Seas Instruments, Nova Scotia, Canada; similar to the BIONESS described by Sameoto et al., 1980). The EZ net was towed obliquely through the water column on a conductive cable which allowed remote control for releasing nets at chosen depth intervals (Wiebe et al., 2015). The net was towed at each of the 13 stations regardless of day or night (Supplementary data, Appendix S1). Four depth strata were targeted at each station; 050 m, 50100 m, 100150 m and 150200 m. Nets were retrieved slowly at a vessel speed of 2 knots with 15 min in each depth stratum. Zooplankton samples were preserved in 5% buffered formaldehyde in seawater solution. Estimates of available potential zooplankton prey that could be consumed by euphausiids were made by pouring the zooplankton samples from each depth stratum through a 1mm sieve and measuring the settled volume of remaining zooplankton in a graduated cylinder after a 24 h settling period Mature and immature euphausiids from all stations were counted and identified to species level with the use of relevant taxonomic keys.