Behavior exceeds physical forcing in the diel horizontal migration of the midwater sound-scattering layer in Hawaiian waters

Abstract

The mesopelagic sound-scattering layer around the Hawaiian Islands undergoes diel vertical migrations. This migration also includes a significant diel horizontal onshore-offshore component. Before this study, the mechanisms underlying the horizontal movement of the mesopelagic layer had not been investigated. We took an interdisciplinary approach to evaluate relationships between the mesopelagic layer and physical oceanographic processes in the water column. The study area extended west of the leeward coast of Oahu, Hawaii. We found that the nightly inshore movement of micronekton occurs very predictably each day; onshore migration brings animals nearshore between 20:00 and 20:13 h and offshore migration commences between 03:30 and 03:45 h during the months of April and May. Simultaneous observations of nearshore physical flow and micronekton distribution indicate that the observed nightly inshore movement of micronekton in the sound-scattering layer is not directly due to physical processes in the nearshore environment. Rather, this nightly inshore movement of micronekton can be attributed primarily to micronekton swimming behavior. This behavior may be cued by changes in sunlight, but that cue is strongly mediated by a factor tied to the lunar phase, potentially moonlight. Horizontal migration velocities exceeded the maximum across-slope physical flow by 5 to 6-fold. Thus, typical across-slope flows likely represent a relatively small additional cost or benefit to onshore-offshore swimming, depending on flow direction. We suggest that energy acquisition via increased prey availability nearshore far exceeds the high energetic cost of onshore and offshore migrations. KEY WORDS: Mesopelagic layer · Diel migration · Physical processesResale or republication not permitted without written consent of the publisher

Diel variation of zooplankton distributions in Hawaiian waters favors horizontal diel migration by midwater micronekton

“…This suggests that this pattern is consistent along the leeward coast of Oahu over a significant portion of the year and from year to year and is likely to be an ecologically important pattern of resource distribution. The observed zooplankton distribution should favor the nearshore-offshore migration of micronekton observed at the same time and examined in more detail in recent work (Benoit-Bird et al 2001, Benoit-Bird & Au 2006, McManus et al 2008. The results of our study show that the density of micronekton is correlated with simultaneous measures of zooplankton abundance.…”

“…Zooplankton distribution patterns are thus consistent with horizontal migration by micronekton serving an adaptive feeding function. The micronektonic animals (2 to 10 cm in length) in these scattering layers travel distances of at least 11 km roundtrip each night (Benoit-Bird & Au 2006), often against the prevailing currents ( McManus et al 2008), to access these increased nearshore zooplankton resources. This feeding advantage is likely an important evolutionary driver of the horizontal migrations observed in other micronektonic species in areas with high bottom relief (Sasaki 1914, Omori & Ohta 1981, Bordes et al 1999.…”

“…The animals make these swims actively, often against the prevailing currents ( McManus et al 2008), highlighting the potential though unmeasured energetic costs of this behavior. We hypothesized that horizontal migration provides the micronekton with greater access to food resources.…”

Use of bottom-mounted echo sounders in exploring behavior of mesopelagic fishes

“…Such lethargic behavior concurs with observations from remotely operated vehicles (ROVs) and submersibles that show midwater fish hanging motionless in the water column (Barham 1966, 1970, Backus et al 1968), but deviates from observations of myctophids undertaking extensive nocturnal horizontal migrations onto and off Hawaiian Islands shelf regions (Benoit-Bird & Au 2006, McManus et al 2008. If B. glaciale were foraging during their tranquil state, the long duration of their trajectories, together with the intermittent, small shifts in their position, would suggest ambush feeding (cf.…”

Bottom-up regulation of a pelagic community through spatial aggregations

“…Ship-based sampling with a downwardlooking acoustics package and a high-resolution profiler instrumented with optical, acoustical and hydrographic sensors [5,7,8] was conducted close to continuously sampling instruments moored at the 25 m isobath off leeward Oahu over three, 24 h periods, as well as during four other days and nights dispersed over the study period. A moored autonomous profiler collected hydrographic and chlorophyll fluorescence data every half-hour between the bottom and the surface with less than 1 cm vertical resolution [7]. A calibrated, moored upward-looking 200, 420, 740 kHz echosounder collected acoustic backscatter once per second with a vertical resolution of 1 cm.…”