A.L. Sutton

Euphausiid assemblages of the oceanographically complex north-west marine bioregion of Australia

The north-west marine bioregion of Australia, which includes the waters adjacent to the Kimberley and Ningaloo coasts, is influenced by both the Indian and Pacific oceans and has high tropical biodiversity, some of which is conserved in a suite of Marine Protected Areas. In the present study, the epipelagic euphausiid assemblages of this bioregion were investigated and related to the physical and biogeochemical properties of the water column, as well as food availability. Twenty-five euphausiid species were identified, including three new records for Australian waters. Pseudeuphausia latifrons was the most abundant species, dominating the shelf waters across both study areas. Stylocheiron carinatum replaced P. latifrons in the deeper waters where species richness was greater. Off Ningaloo, there were higher concentrations of euphausiids, and this may be linked to the bathymetry, the narrowness of the shelf and the resultant effects of these features on oceanography and biogeochemistry. Assemblages were primarily structured by depth, but mean seawater density, dissolved oxygen, fluorescence and mesozooplankton abundance also significantly explained some of the variation in euphausiid assemblages. The present study has confirmed that the physical and biogeochemical properties of the water column and food availability are recurrent factors affecting euphausiid assemblage variation in the eastern Indian Ocean.

Habitat associations of cetaceans and seabirds in the tropical eastern Indian Ocean

There is an increasing demand for integrated pelagic surveys to support ecosystem-based management of marine environments and their associated marine life. The Browse Basin in the tropical eastern Indian Ocean was surveyed using ship line transects to determine habitat associations of cetaceans and seabirds with submarine topography and local oceanographic conditions during winter and spring 2008. Fourteen species of cetaceans, including the data deficient pygmy blue whale, and 23 species of seabirds were encountered throughout the basin. Aggregations of both cetaceans and seabirds were observed at two significant submarine topographic features, Scott Reef and Browse Cliffs, particularly during spring when encounters and abundances of odontocetes were far greater. The attraction of cetaceans and seabirds to Scott Reef and Browse Cliffs was likely foraging-related given these features were associated with upwelling and elevated biomass of krill and fish. Sub-mesoscale sea surface temperature and chlorophyll a fronts also occurred in vicinity to Browse Cliffs and the shelf environment. The Browse Basin is an important, and potentially predictable, foraging ground for a variety of top predators, and their occurrence would have implications for the current and future management practices of oil and gas industries operating in the region.

Structuring of larval fish assemblages along a coastal-oceanic gradient in the macro-tidal, tropical Eastern Indian Ocean

Abstract In the eastern Indian Ocean, the Kimberley region off north-western Australia is characterised by a wide continental shelf, complex coastal topography and very large tides (>10 m). Diversity of fishes is high although little is known about the distribution of their larvae in this dynamic region. This study investigated spatial variation in ichthyoplankton assemblages along four coastal-oceanic transects and related the results to tidal cycles and other environmental variables. Larvae representing 92 neritic and 21 mesopelagic teleost families were collected; this far exceeds the diversity described for other comparable tropical continental shelf and oceanic systems. Distinct ichthyoplankton assemblages characterised inner shelf (Engraulidae and Gobiidae were dominant families), outer shelf / slope (mixed neritic and mesopelagic taxa) and oceanic waters (mesopelagic taxa, particularly Myctophidae, Gonostomatidae and Phosichthyidae), and were correlated with changes in environmental variables (water depth, seawater density, mixed layer depth and zooplankton). The spring-neap tidal cycle and subsequent tides (same stations sampled 12 h apart) had no significant influence on larval fish assemblages. Coastal waters were delineated by a density front and larval fish assemblages were significantly different from offshore assemblages. This study provides new insights into the pelagic ecosystem in the tropical eastern Indian Ocean and contributes to the understanding of the role of environmental variables and physical forcing in the structuring of larval fish assemblages.

Vertical structuring of epipelagic euphausiid assemblages across a thermohaline front in the south-east Indian Ocean

Oceanographic fronts are strong discontinuities in water mass properties and can result in differences in biota over short horizontal distances. The vertical structuring of epipelagic euphausiids was investigated across an ephemeral thermohaline front between Leeuwin Current Water (LCW) and Sub Tropical Surface Water (STSW) in the southeast Indian Ocean. Vertical ontogenetic layering was evident and euphausiid larvae were most abundant in the 0-50 m stratum in both water masses. Differences in assemblages between water masses were mostly driven by the numerical abundance of shared species; most species were in greater abundance in LCW. LCW was characterized by a uniform water column and no significant structuring of assemblages with depth. Structuring was, however, evident in STSW and analyses indicated this was a result of stratification of temperature, salinity and dissolved oxygen. Higher chlorophyll a concentrations and zooplankton settled volumes were evident in LCW, as well as the upper 100m of STSW. Decreases in temperature and salinity were correlated with the variation in assemblages at >100m depth in STSW. This is the first study to examine vertical structuring and environmental correlations of euphausiid life stages and assemblages across a thermohaline front in the south-east Indian Ocean.