Deep-Sea Research I 51 (2004) 707–719 Diel migration dynamics of an island-associated sound-scattering layer $ Kelly J. Benoit-Bird a,b, *, Whitlow W.L. Au a a Hawaii Institute of Marine Biology, P.O. Box 1106, Kailua, HI 96734, USA b Department of Zoology, Hawaii Institute of Marine Biology, University of Hawaii, 46-007 Lilpuna Road, Kaneohe, HI 96744, USA Received 25 November 2002; received in revised form 25 July 2003; accepted 26 January 2004 Abstract The Hawaiian mesopelagic boundary community, consisting of island-associated, midwater sound-scattering layers, undergoes diel migrations with both vertical and horizontal components. To understand the dynamics of the community’s migration at fine temporal scales, we utilized a bottom-mounted, 200-kHz active-acoustic mooring that transmitted 10 signals every 15 min, from dusk until dawn for 5 days. Five moorings were deployed 1.0–3.0 km from the leeward coast of Oahu in 0.5 km intervals. Two layers within the boundary community were observed to undergo simultaneous diel vertical and horizontal migration. The shallow layer came within 10 m of the surface and 1 km of the shoreline. The deeper layer remained 90 m from the surface and 2.5 km of the shoreline. Vertical migration rates were measured at 0–1.7 m min –1 while the horizontal rate averaged 1.7 km h –1 , swamping the vertical movement. The turning point of the migration pattern was observed 45 min before the midpoint between sunset and sunrise. Until the migration’s turning point, scattering strength increased relatively constantly as the animals migrated towards shore, with the highest scattering densities found in the shallowest areas at midnight. Total scattering strength measured at the leading and trailing edge of the layer support the hypothesis that increased animal densities nearshore are related to packing as mesopelagic animals avoid the surface and the bottom. We observed high levels of biomass moving rapidly, over a great distance, into shallow waters very close to shore providing insight into the significant link the mesopelagic boundary community provides between nearshore and oceanic systems. r 2004 Elsevier Ltd. All rights reserved. Keywords: Vertical migration; Horizontal migration; Temporal dynamics; Mesopelagic boundary; Scattering layers; Acoustics USA; Hawaiian Islands 1. Introduction Sound scattering layers are distributed world- wide and have been studied since World War II (Gjosaeter and Kawaguchi, 1980). Scattering layers of micronekton are important components of oceanic ecosystems; they consume a significant portion of the zooplankton produced (Clarke, 1973) and are an important link to higher trophic levels (Rasmussen and Giske, 1994). Many of the micronektonic animals making up these layers are found in different ‘vertical life-zones’ during the day and at night because of diel vertical migrations of tens to hundreds of meters (Longhurst, 1976). ARTICLE IN PRESS $ Supplementary data associated with this article can be found, in this online version, at doi:10.1016/j.dsr.2004.01.004 *Corresponding author. Department of Zoology, Hawaii Institute of Marine Biology, University of Hawaii, 46-007 Lilpuna Road, Kaneohe, HI 96744, USA. E-mail address: benoit@hawaii.edu (K.J. Benoit-Bird). 0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.01.004