Front variability and surface ocean features of the presumed southern bluefin tuna spawning grounds in the tropical southeast Indian Ocean Anne-Elise Nieblas a,n,1 , Hervé Demarcq b,nn , Kyla Drushka c,2 , Bernadette Sloyan a,d , Sylvain Bonhommeau e a Commonwealth Scientific and Industrial Research Organisation (CSIRO) Wealth from Oceans Research Flagship, GPO Box 1538, Hobart 7001, Australia b Unité Mixte de Recherche Ecosystèmes Marins Exploités 212, Institut de Recherche pour le Développement, Av. J. Monnet, Sète Cedex 34203, France c Laboratoire d'Océanographie – Expérimentation et Approches Numériques, Case 100 – UPMC 4 place Jussieu, F-75252 Paris, France d CSIRO Centre for Australian Weather and Climate Research, GPO Box 1538, Hobart 7001, Australia e Unité Mixte de Recherche Ecosystèmes Marins Exploités 212, Institut Français de Recherche pour l’Exploitation de la Mer, Av. J. Monnet, Sète Cedex 34203, France article info Available online 2 December 2013 Keywords: Tropical southeast Indian Ocean Indo-Australian region Southern bluefin tuna Thunnus maccoyii Spawning grounds (101S-201S 1051E-1251E) Oceanic fronts Front detection index abstract The southern bluefin tuna (SBT, Thunnus maccoyii) is an ecologically and economically valuable fish. However, surprisingly little is known about its critical early life history, a period when mortality is several orders of magnitude higher than at any other life stage, and when larvae are highly sensitive to environmental conditions. Ocean fronts can be important in creating favourable spawning conditions, as they are a convergence of water masses with different properties that can concentrate planktonic particles and lead to enhanced productivity. In this study, we examine the front activity within the only region where SBT have been observed to spawn: the tropical southeast Indian Ocean between Indonesia and Australia (101S–201S, 1051E–1251E). We investigate front activity and its relationship to ocean dynamics and surface features of the region. Results are also presented for the entire Indian Ocean (301N–451S, 201E–1401E) to provide a background context. We use an extension of the Cayula and Cornillon algorithm to detect ocean fronts from satellite images of sea surface temperature (SST) and chlorophyll-a concen- tration (chl-a). Front occurrence represents the probability of occurrence of a front at each pixel of an image. Front intensity represents the magnitude of the difference between the two water masses that make up a front. Relative to the rest of the Indian Ocean, both SST and chl-a fronts in the offshore spawning region are persistent in occurrence and weak in intensity. Front occurrence and intensity along the Australian coast are high, with persistent and intense fronts found along the northwest and west coasts. Fronts in the tropical southeast Indian Ocean are shown to have strong annual variability and some moderate interannual variability. SST front occurrence is found to lead the Southern Oscillation Index by one year, potentially linked to warming and wind anomalies in the Indian Ocean. The surface ocean characteristics of the offshore SBT spawning region are found to be particularly stable compared to the rest of the Indian Ocean in terms of stable SST, low eddy kinetic energy, i.e., low mesoscale eddy activity, and low chl-a. However, this region has high front occurrence, but low front intensity of both SST and chl-a fronts. The potential impact of these oceanic features for SBT spawning is discussed. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The southern bluefin tuna (SBT; Thunnus maccoyii) is an economically valuable fish and an ecologically important apex predator, but despite its importance and a long history of research, knowledge of its early life history is limited. The larval stage is decisive for the renewal of marine populations as it has the highest mortality of all life stages ( 499%; Hjort 1914). Early life stages are highly sensitive to environmental conditions and fish larval survival is commonly influenced by retention of larvae in regions where the ocean circulation concentrates food sources (Bakun, 1996). However, the reproductive strategy of bluefin tuna is unusual, as they migrate from their temperate and productive feeding grounds to oligotrophic tropical and subtropical spawning grounds (Schaefer, 2001; Bakun and Broad, 2003). Bakun and Broad (2003) suggest that the early life stages of tuna are highly vulnerable to predation, and thus spawning adults seek regions Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2013.11.007 n Corresponding author. nn Corresponding author. E-mail addresses: anne.elise.nieblas@gmail.com (A.-E. Nieblas), herve.demarcq@ird.fr (H. Demarcq). 1 Present address: Unité Mixte de Recherche Ecosystèmes Marins Exploités 212, Institut Français de Recherche pour l’Exploitation de la Mer, Av. J. Monnet, Sète Cedex 34203, France. 2 Present address: Scripps Institution of Oceanography, La Jolla, CA 92093- 0230, USA. Deep-Sea Research II 107 (2014) 64–76