Evidence for light-controlled migration amplitude of a sound scattering layer in the Norwegian Sea

DL Aksnes,TA Klevjer,E Norheim

doi:10.3354/meps11731

Abstract

The oceanic sound scattering layer (SL) is an ubiquitous acoustic signature of meso- pelagic fishes and invertebrates that are important food sources for the oceanic macrofauna and players in the biological carbon pump. We investigated the relationship between SL migration amplitude and light in the Norwegian Sea. Incoming surface irradiance was measured continu- ously during night and day. The increasing latitude of the cruise track facilitated a field experi- ment wherein night light increased 2 to 3 orders of magnitude during the study period. Concur- rent with increased night light, the diel vertical migration amplitude of the SL decreased several hundred meters. The variations in irradiance at the sea surface spanned 6 orders of magnitude during the study period, while the ambient irradiance of the SL mean depth was relatively con- stant during both day- and nighttime. For the deepest penetrating wavelength (486 nm), the average ambient light was estimated to be 2 × 10 �6 mW m �2 nm �1 , which corresponded to a total irradiance of 1.9 × 10 �7 µmol quanta m �2 s �1 . The observed variation in migration amplitude is consistent with a behavior whereby the SL organisms keep their ambient light intensities within a particular range. We also observed that SL total backscattering, a proxy for biomass, decreased along with the decrease in migration amplitude. We speculate whether this decrease might, in part, relate to a previously proposed 'photoperiod constraint hypothesis' suggesting that high night light intensities in summer at high latitudes limit options for safe foraging in upper layers at night.

Highlights

Studies by Kaartvedt et al (2012) and Irigoien et al (2014) have suggested that the mesopelagic sound scattering layers (SL) of the world oceans contain much more biomass than previously assumed
Field investigations do not offer the same degree of control as laboratory experiments, and there is no way to exclude the possibility that the observed association between migration amplitude and light is due to correlation with another factor, e.g. temperature
We are not aware of any hypotheses that connect migration amplitude with temperature changes as observed here. This is in sharp contrast to light, for which several testable hypotheses have been proposed during the last century

Summary

Introduction

Studies by Kaartvedt et al (2012) and Irigoien et al (2014) have suggested that the mesopelagic sound scattering layers (SL) of the world oceans contain much more biomass than previously assumed. Mar Ecol Prog Ser 551: 45–52, 2016 the DVM of locally observed scattering layers was known long before (see references in Fornshell & Tesei 2013), the study of Dickson (1972) appears to be the first to suggest that there might be a common light-forced SL behavior at the ocean basin scale. Evidence for such optical control was provided by Kaartvedt et al (1996), who reported that observed SL shoaling off the Norwegian coast was likely due to increased shading from phytoplankton. In well oxygenated areas such as the Norwegian Sea, dissolved oxygen is not a likely constraint to the SL organisms

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Conclusion