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

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.