Mesoscale distribution of larval Euphausia similis in various water masses of the East Australian Current Matthew D. Taylor a,b,n , Thomas J. Mullaney a,b , Iain M. Suthers a,b a Sydney Institute of Marine Science, Building 22, Chowder Bay Road, Mosman, New South Wales 2088, Australia b Evolution and Ecology Research Centre, University of NSW, Sydney, New South Wales 2052, Australia article info Article history: Received 21 August 2009 Received in revised form 3 June 2010 Accepted 7 June 2010 Available online 20 June 2010 Keywords: Euphausia similis Larvae Krill predation Cold core eddy Top-down control Diel-vertical migration East Australian Current abstract Larval Euphausia similis were collected off temperate eastern Australia in spring 2004 and 2006 to evaluate the relationships between larval populations, mesoscale oceanographic variability, and the wider planktonic community. Larval E. similis were present in greater numbers in the East Australian Current (EAC) relative to productive coastal waters. Larval E. similis density was homogenous across the EAC—Tasman Sea frontal region, but larvae were smaller in the Tasman Sea. Larval E. similis density was not enhanced within a cold core eddy relative to the surrounding EAC. We observed a negative correlation between larval E. similis density and larval fish density, and a weak positive correlation with fluorescence. Evaluation of a significant fish density  fluorescence interaction term showed that the effect of fish density was reduced at high fluorescence values. Analysis of normalized biomass size spectrum (NBSS) provided evidence for potential competitive exclusion of copepods by krill. Data presented in this study suggest a predatory influence on surface E. similis populations by mesopelagic larval fish. The degree of predation appears to be dependent on food availability, potentially mediated by changes in the physiological condition of krill. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Euphausiids represent a key trophic component of oceano- graphic ecosystems, with biomass second only to copepods in some systems (Pakhomov, 2004). Euphausiids provide a direct link between photosynthetic food webs and large, long lived biogenic stores at higher trophic levels (e.g. whales, Pakhomov, 2004; de Guevara et al., 2008). The importance of euphausiids in coastal and oceanographic ecosystems is also evident in the relationships between temporal and spatial variability in euphausiid production and catches of commercial fish. These relationships can be positive, where euphausiids act as an important food source for adult fish (Young and Davis, 1992; Gomez, 1995); or negative, where euphausiids act as a predator of fish eggs and larvae (Bailey et al., 1993). Euphausia similis is a widely distributed oceanic euphausiid common to the southern ocean, the western Pacific between 251S and 551S, and the Kuroshio Current (RAMS, 2009). E. similis is a major component of zooplankton communities off Australia (Sheard, 1965; Williams et al., 2001), and a dominant species of the zooplankton community in the Subtropical Convergence Zone (Pakhomov et al., 1994). In Sagami Bay, Japan, E. similis comprises over half the total euphausiid biomass (Hirota et al., 1990), and species biomass increases in spring and summer to an average daily production of 1.33 mg C m 2 (Hirota et al., 1990). In Australia, E. similis is one of the most abundant euphausiid species in the northern Tasman Sea (Griffiths, 1979). The species is identified as an important ( 450%) component in the diets of fish such as myctophids (Williams et al., 2001), skipjack tuna Katsuwonus pelamis (Ankenbrandt, 1985), hairtail Trichiurus lepturus (Martins et al., 2005), and rough scad Trachurus lathami (Katsuragawa and Ekau, 2003). Existing information suggest that E. similis originate in subtropical latitudes and are transported to higher latitudes by ocean currents (Bartel, 1976; Hirota et al., 1984), however there are no detailed studies on this species within western boundary currents in the south Pacific basin. The broad objective of this study was to investigate E. similis off the east coast of mainland Australia, with special reference to the East Australian Current (EAC, the western boundary current of the Southern Pacific Gyre) and associated mesoscale variability. Spatial patterns in euphausiid distribution have rarely been interpreted in the context of mesoscale hydrographic features such as eddies (Bernard et al., 2007). Only limited distributional information exists for E. similis in the southern Pacific Ocean, and population differences have not been compared amongst the oceanographic features of the EAC. Climate change scenarios Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2010.06.010 n Corresponding author at: Evolution and Ecology Research Centre, University of NSW, Sydney, New South Wales 2052, Australia. Tel.: + 61 2 9385 2079; fax: + 61 2 9385 1558. E-mail address: mattytaylor@unsw.edu.au (M.D. Taylor). Deep-Sea Research I 57 (2010) 1295–1303