Diel vertical migration and individual behavior of nekton beyond the ocean's twilight zone

Stein Kaartvedt,Anders Røstad,Svenja Christiansen,Thor A Klevjer

doi:10.1016/j.dsr.2020.103280

Abstract

Diel vertical migration (DVM) is normally limited to the upper 1000 m. However, the use of ship-borne and stationary submerged echosounders in the Red Sea unveiled consistent formation of daytime patches extending to 1200-1300 m, the patches dissolving at night when individuals migrated shallower. The diel vertical migration was not synchronized, and in the evening 4–5 h passed from the first to the last individual leaving their continuously dark daytime habitat. The mean ascent and descent speeds were ~15 cm s−1. Individuals were active upon return to deep water in the morning, some swimming rapidly up and down at speeds of up to ~25 cm s−1, with abrupt changes in swimming direction. Patch formation was swift, in spite of dilute overall population density, suggesting remote signaling to team up with conspecifics. Vertical swimming subsided as patches became established and persisted at 2–3 cm s−1 within patches. The acoustic target strength of the individuals peaked at ~ -55 dB (38 kHz) and was constant regardless of vertical swimming directions. Based on the behavioral and acoustic evidence squids (Sthenoteuthis sp) are likely acoustic targets.

Highlights

The lower boundary of the mesopelagic- or twilight zone is defined as the depth without remaining surface light, generally set at 1000 m (Denton, 1990; Warrant and Locket, 2004)
The northern Red Sea is an oligotrophic ocean with optically clear waters, where scattering layers ascribed to mesopelagic fish extend down to 700–800 m in daytime (Klevjer et al, 2012; Dypvik and Kaartvedt, 2013)
Additional acoustic targets were migrating below the deepest SL (Fig. 3)

Summary

Introduction

The lower boundary of the mesopelagic- or twilight zone is defined as the depth without remaining surface light, generally set at 1000 m (Denton, 1990; Warrant and Locket, 2004). There are reports on vertically migrating acoustic scattering layers descending into bathy pelagic waters below 1000 m in daytime and ascending at night (Van Haren, 2007; Van Haren and Compton, 2013; Ochoa et al, 2013). We here report on additional vertically migrating acoustic targets descending past the depths of the main scattering layers and even beyond the twilight zone during daytime. We deployed sci entific echo sounders in deep waters to unveil details on their diel ver tical migration (DVM) pattern and swimming behavior in the apparently continuously dark daytime habitat below 1000 m (Denton, 1990; War rant and Locket, 2004)

Methods

Results

Conclusion