The diel vertical migration patterns and individual swimming behavior of overwintering sprat Sprattus sprattus

Abstract

We addressed the behavioral patterns and DVM dynamics of sprat overwintering in a 150 m Norwegian fjord with increasing hypoxia by depth. An upward-facing echosounder deployed at the bottom and cabled to shore provided 4 months of continuous acoustic data. This enabled detailed studies of individual behavior, specifically allowing assessment of individual vertical migrations at dusk and dawn in relation to light, analysis of so-called rise-and-sink swimming, and investigation of the sprat' swimming activity and behavior in severely hypoxic waters. Field campaigns supplemented the acoustic studies. The acoustic records showed that the main habitat for sprat was the upper ~ 65 m where oxygen concentrations were ≥ 0.7 mL O 2 L-1. The sprat schooled at ~ 50 m during daytime and initiated an upward migration about 1 hour prior to sunset. While some sprat migrated to surface waters, other individuals interrupted the ascent when at ~20-30 m, and returned to deeper waters ~ 20-50 min after sunset. Sprat at depth was on average larger, yet individuals made excursions to-and from upper layers. Sprat were swimming in a "rise and sink" pattern at depth, likely related to negative buoyancy. Short-term dives into waters with less than 0.45 mL O 2 L-1 were interpreted as feeding forays for abundant overwintering Calanus spp. The deep group of sprat initiated a dawn ascent less than 1 hour before sunrise, ending at 20-30 m where they formed schools. They subsequently returned to deeper waters about ~20 min prior to sunrise. Measurements of surface light intensities indicated that the sprat experienced lower light levels in upper waters at dawn than at dusk. The vertical swimming speed varied significantly between the behavioral tasks. The mixed DVM patterns and dynamic nocturnal behavior of sprat persisted throughout winter, likely shaped by individual strategies involving optimized feeding and predator avoidance, as well as relating to temperature, hypoxia and negative buoyancy.

Trophic interactions drive the emergence of diel vertical migration patterns: a game-theoretic model of copepod communities

“…Schooling behaviours can also be adopted in complement to DVM, and often depend on the period of the day. Fish tend to disaggregate at night [58,59], sometimes in concert with an upward migration [59]. However, schooling is not always an appropriate strategy, as it can also attract predators and increase predation pressure [60].…”

Oxygen, animals and aquatic bioturbation: An updated account

“…The primary motive for this daily commute to depth is visual predation, with primary consumers seeking refuge at depth during daylight hours and returning to the surface at night to feed (Bollens & Frost, 1991;Lampert, 1989;Ohman, 1990). Migratory amplitudes of DVM vary from a few tens or hundreds of metres for "jellyfish" (Haraldsson, Båmstedt, Tiselius, Titelman, & Aksnes, 2014;Kaartvedt, Klevjer, Torgersen, Sørnes, & Røstad, 2007) to mesopelagic depths (200-1,000 m) in the case of many salps, squid and fish (Gilly et al, 2006;Houssard et al, 2017;Solberg & Kaartvedt, 2017;Watanabe, Kubodera, Moku, & Kawaguchi, 2006;Wiebe, Madin, Haury, Harbison, & Philbin, 1979). Collectively, marine DVM is by far the largest coordinated movement of biomass on the planet-a continuous standing wave of pursued and pursuing animals extending across all but the highest latitude oceans (Wallace, Cottier, Brierley, & Tarling, 2013).…”

Vertical migrations of fish schools determine overlap with a mobile tidal stream marine renewable energy device

“…Diel vertical migrations of sprat schools has been observed in fjords (Knudsen, Hawkins, McAllen, & Sand, ) and the Baltic Sea (Nilsson, ), and here in an open marine coastal system of the Holyhead Deep. Due to the mixed water column in the Holyhead Deep, the ‘antipredation window’ (Clark & Levy, ) determined by the depth variation in light irradiance would appear a key driver of these vertical migrations, as opposed to temperature or dissolved oxygen levels influencing migratory behaviour in more sheltered and stratified systems (Solberg & Kaartvedt, ). This study has shown that the magnitude of fish school DVM can change in correlation with the spring‐neap tidal cycle, due to its influence on irradiance depth in the water column.…”

“…Despite most of the Irish Sea being tidally mixed year round, resulting in homogenous vertical distributions of phytoplankton, some groups of zooplankton undertake diel vertical migrations (DVM; Irigoien, Conway, & Harris, ; Scrope‐Howe & Jones, ). Pelagic fish such as European sprat Sprattus sprattus , a zooplanktivorous forage fish, change their schooling behaviour and vertical distribution during the day, schooling deep during daylight, adopting shallower depth distributions after dawn and before dusk as they vertically migrate, and during darkness schools disperse (Nilsson, ; Solberg & Kaartvedt, ). In most species DVM is driven by light levels in the water column, Zooplanktivores need enough light to forage and maintain schools, but minimize predation risk by staying in as dark and deep water as possible (Bianchi & Mislan, ; Nilsson, ; Robison, ).…”