Mesopelagic fish belong to the mid-trophic level functional group in the marine ecosystem, and they are a key component of oceanic food web, linking primary and tertiary consumers. Despite their ecological relevance, little is known about the distribution patterns of mesopelagic fishes (Young et al. 2001), and there is a strong demand for more research for ocean-biogeochemical-population (Lehodey 2004) and ecosystem (Kloser et al. 2009, Handegard et al. 2012) models. Fisheries acoustics provides a suitable tool for monitoring mesopelagic fish abundance and distribution (Simmonds & MacLennan 2005). Using acoustics from vessels of opportunity may provide the best chance of achieving the ocean basin scale spatial coverage required to inform ecosystem models. Introduction 1. Identify large-scale patterns of pelagic fish abundance using area backscattering coefficient (s a in m 2 m -2 ) at 38 kHz as a measure of biological abundance across the South Pacific Ocean. 2. Determine the vertical distribution of s a . 3. Explore the relationship between distribution of s a and environmental and geographic variables to determine the major factors driving pelagic fish distribution. Objectives Two return trips between New Zealand and Chile in Oct-Dec 2007 and 2010 from RV Kaharoa deploying ARGO buoys. Data collected using a Simrad ES60 echosounder with hull- mounted 38 kHz transducer (2000 W power, 1.024 ms pulse). Analysis software Sonardata Echoview 4.90 and 5.0 (Myriax). Acoustic records integrated in 10 n miles elementary distance sampling units (EDSU) along the vessel track, and in 50 m depth bins. Maximum data range up to 600 m depth. Boosted regression trees (BRT) were fitted in R (version 2.13.2), using GMB package version 1.6–3.1 plus custom code available online (Elith et al. 2008). Methods Results Conclusions The key finding was that there was a strong correlation between chl a and distribution of acoustic backscatter as shown by BRT and spatial overlap (Fig. 2). The process underlying this correlation is likely food availability. Major vertical patterns of pelagic fish distribution were diel migration and layering. Major horizontal patterns were coastal intensification and oceanic patchiness, with abundance decreasing from the coasts towards the core of the south tropical gyre. Different mesopelagic species may have different acoustic properties, so correlation between acoustic backscatter and fish abundance may not be linear. Nevertheless, acoustic information based on total backscatter still provided further valuable knowledge about patterns of distribution and productivity. There is a growing interest for developing programs focused on global ecosystem models for studying large-scale patterns of distribution and abundance of biota and their changes over long time scales. This study provides data to validate model predictions of mid-trophic functional groups. Reference Escobar-Flores, P., O’Driscoll, R., & Montgomery, J. (2013). Acoustic characterization of pelagic fish distribution across the South Pacific Ocean. Marine Ecology Progress Series, 490, 169– 183. doi:10.3354/meps10435 * Email: [email protected] Acknowledgments and contact Funding for this study was provided by the Advanced Human Capital Programme, National Commission of Scientific and Technological Research CONICYT CHILE, through a Bicentennial Becas-Chile Master scholarship. 1 School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 2 National Institute of Water and Atmospheric Research, Private Bag 14-901, Kilbirnie, Wellington 6241, New Zealand Pablo Escobar-Flores 1* , Richard L. O’Driscoll 2 , John C. Montgomery 1 Acoustic characterization of pelagic fish distribution across the South Pacific Ocean -chlorophyll (chl a in mg m -3 ) -sea surface temperature (SST in degrees Celsius) -sea surface height (SSH in m) -minimum distance from main land masses (km) -depth (m) GEOGRAPHIC PARAMETERS - time since sunrise (hours) TEMPORAL VARIABLE 1 EDSU = 10 n miles 10 n miles 20 n miles ENVIRONMENTAL VARIABLES Fig. 2. Vertically summed acoustic backscatter coefficient (s a in m 2 km −2 ) per 10 n miles elementary distance sampling unit along each transect overlying maps of chl a (mg m −3 ) for the area and period of study: (a) T1-2007, (b) T2-2007, (c) T1-2010, and (d) T2-2010. Maximum bar size = 140 m 2 km −2 . Grey bars: daytime; black bars: nighttime. Table 1: Table 1. Comparison of total acoustic area backscattering coefficient (s a ) along the 4 transects. EDSU: elementary distance sampling unit size (10 n miles). Fig. 3. Proportion of total acoustic backscatter coefficient (s a in m 2 km −2 ) summarised in 50 m depth bins for each individual transect: (a) T1-2007, (b) T2-2007, (c) T1-2010, and (d) T2-2010. Fig. 4. Longitudinal section along transect T1-2010. (a) Vertically summed acoustic backscatter coefficient (s a in m 2 km −2 ) in 10 n miles elementary distance sampling units along the transect. (b) Daytime and nighttime. (c) Mean volume backscattering strength (Sv in decibels) in each cell (red-brown= strong, blue-grey= weak). Each cell in the colour central panel is 10 n miles long by 50 m deep. (d) Bottom depth. Table 2. Summary of boosted regression trees model parameters (lr : learning rate; tc: tree complexity; nt: number of trees) and performance. Model parameters were optimised by k-fold cross-validation (CV); SE: standard error. Table 3. Summary of relative contributions (%) of the predictor variables for boosted regression trees Models 2 and 3, with cross- validation data on 2462 locations. See Table 2 for model descriptions and parameters. References Elith J, Leathwick JR, Hastie T (2008) A working guide to boosted regression trees. J Anim Ecol 77: 802−813 Handegard NO, du Buisson L, Brehmer P, Chalmers SJ and others (2012) Towards an acoustic-based coupled observation and modelling system for monitoring and predicting ecosystem dynamics of the open ocean. Fish Fish doi:10.1111/j.1467- 2979.2012.00480.x Kloser RJ, Ryan TE, Young JW, Lewis ME (2009) Acoustic observations of micronekton fish on the scale of an ocean basin: potential and challenges. ICES J Mar Sci 66: 998−1006 Lehodey P (2004) A spatial ecosystem and populations dynamics model (SEAPODYM) for tuna and associated oceanic top-predator species. II. Tuna populations and fisheries. In: 17th Meeting of the Standing Committee on Tuna and Billfish, Majuro, Marshall Islands, 5−18 August, 2004. SCTB17, Working Paper ECO-2, July 2004. Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea Simmonds EJ, MacLennan DN (2005) Fisheries acoustics: theory and practice, 2nd edn. Blackwell Science, Oxford Young JW, Lamb TD, Bradford R, Clementson L, Kloser R, Galea H (2001) Yellowfin tuna (Thunnus albacares) aggregations along the shelf break of southeastern Australia: links between inshore and offshore processes. Mar Freshw Res 52: 463−474 Fig. 1. Different species of mesopelagic fish caught with mid-water trawls over the Chatam Rise, western South Pacific Ocean.