Nighttime Swimming Behavior of a Mesopelagic Fish

Abstract

Nocturnal migration of mesopelagic fish into surface waters is well-documented. Yet, although there is increasing evidence of individual-based deviations from average population migrations and of the importance of small-scale individual behavior for predator-prey interactions and energetic cycling, little is known about what mesopelagic animals do when in upper waters. Using high-resolution data from an upwardfacing, moored, split-beam echosounder we analyzed the night-time individual vertical swimming behavior of pearlsides (Maurolicus muelleri) over one winter. The population behavior is characterized by migration to the surface after sunset, "midnight-sinking" and another migration to the surface in the morning, followed by return to the daytime habitat. Focusing on individuals unveiled diverse behavioral patterns during different phases of the migration. After ascending to upper layers at dusk, M. muelleri leaves the surface waters, not by sinking, but by actively swimming in a step-wise pattern characterized by relocations alternated by pauses. Following the descent, vertical swimming is sustained at lower levels. Around midnight, the vertical swimming direction changes from predominantly downward to upward. Several hours before dawn, the fish start ascending toward the surface in a step-wise pattern. During population ascent in the afternoon and descent in the morning, some individuals at the fringes of schools migrate without intermittent pauses. This study documents the feasibility of using submerged, stationary echosounders in unveiling the individual behavior of mesopelagic fish.

Importance of the Lunar Cycle on Mesopelagic Foraging by Atlantic Bluefin Tuna in the Upwelling Area of the Strait of Messina (Central Mediterranean Sea)

“…For instance, different species have different camouflage abilities and preferences for different isolumes [ 15 , 65 ], which could affect vulnerability of different species to visual foragers such as BFT. In general, detailed knowledge of the vertical migration behavior of mesopelagic fish and squids is limited, but one recent study indicates that it can be quite complex (e.g., discontinuous and intermittent through the night [ 11 ]). Furthermore, according to Battaglia et al [ 30 ], mesopelagic fish with a lower swimming ability (e.g., some weakly migrant mesopelagic fish) are more vulnerable than others to the passive transport of upwelling currents in the Strait of Messina, consequently becoming more abundant in shallow waters when tidal currents become strong.…”

“…Most mesopelagic species carry out nycthemeral migrations toward the epipelagic zone during the night to feed on their prey and move back to deeper waters during daytime [ 9 ]. However, each species occupies a particular trophic niche and displays different vertical distribution, adaptations, migratory and feeding habits [ 10 , 11 ]. The water column illumination influences the vertical distribution and migratory behavior of each mesopelagic species, affecting their vulnerability to predation by visually foraging predators in the upper water layers.…”

“…For this reason, mesopelagic species tend to follow a preferred light intensity (or isolume) in the water column in order to maintain camouflage and hide themselves from predators; avoidance of predators is also assisted by activation of bioluminescent counterillumination strategies [ 12 , 13 , 14 , 15 ]. Interspecific differences in the amplitude of vertical distribution of mesopelagic micronekton at night [ 16 ] are then also related to the ability of each species to address camouflage and perform defensive tactics [ 4 , 11 , 17 ].…”

Three‐dimensional swimming behavior and activity of a mesopelagic fish

“…The raw dataset (Christiansen et al 2019) contained 613,003 tracks obtained from automatic target tracking after cross-filter detection (Balk 2019). These tracks had maximum ping gaps of 10 pings, depending on the distance from the echosounder (hereafter range), and were recorded at ranges between 5 and 85 m. Target tracking may introduce errors at high target densities (Handegard 2007).…”

“…While aspects of M. muelleri's individual nocturnal vertical behavior have been described previously (Christiansen et al 2019), we here provide the first quantitative assessments of the three-dimensional behavior and activity of juvenile M. muelleri. Moreover, quantitative analyses of threedimensional behavior in small mesopelagic fish are scarce and hence our study paves the way for future studies of the twilight zone inhabitants.…”