Marine Ecology Progress Series
MEPS is a leading hybrid research journal on all aspects of marine, coastal and estuarine ecology. Priority is given to outstanding research that advances our ecological understanding.
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DOI: https://doi.org/10.3354/meps09688
copiedInternal wave-mediated shading causes frequent vertical migrations in fishes
ABSTRACT:
We provide evidence that internal waves cause frequent vertical migrations (FVM) in fishes. Acoustic data from the Benguela Current revealed that pelagic scattering layers of fish below ~140 m moved in opposite phases to internal waves, ascending ~20 m towards the wave trough and descending from the wave crest. At the trough, the downward displacement of upper waters and the upward migration of fish created an overlapping zone. Near-bottom fish correspondingly left the benthic boundary zone at the wave trough, ascending into an acoustic scattering layer likely consisting of zooplankton and then descending to the benthic boundary zone at the wave crest. We suggest that this vertical fish migration is a response to fluctuations in light intensity of 3 to 4 orders of magnitude caused by shading from a turbid surface layer that had chlorophyll a values of 3 to 4 mg m−3 and varied in thickness from ~15 to 50 m at a temporal scale corresponding to the internal wave period (30 min). This migration frequency thus is much higher than that of the common and widespread light-associated diel vertical migration. Vertical movements affect prey encounters, growth, and survival. We hypothesize that FVM increase the likelihood of prey encounters and the time for safe visual foraging among planktivorous fish, thereby contributing to efficient trophic transfer in major upwelling areas.
KEYWORDS
Pelagic and demersal fishes undertake vertical migrations in response to internal waves, as these cause changes in light penetration through the turbid surface layer of the sea. Illustration: Hege Vestheim
Internal waves are ubiquitous in the oceans. Kaartvedt and co-workers show that internal waves mediate underwater light conditions: at the trough of an internal wave, the turbid surface layer becomes deeper, integrated light extinction increases, and the waters at depth become darker; at the crest, the turbid layer is shallower and light penetration increases. Pelagic and demersal fishes ascend as it becomes darker and descend when it is lighter, and the time scale of these migrations corresponds to the periodicity of the wave. The authors hypothesize that these frequent migrations increase the number of prey encounters and the time available for safe visual foraging, thus improving fish growth and survival, and trophic transfer efficiency in the system.
S. Kaartvedt (Co-author)
- King Abdullah University of Science and Technology, Red Sea Research Center, Thuwal 23955-6900, Saudi Arabia
- Department of Biology, University of Oslo, 0316 Oslo, Norway
Thor A. Klevjer (Co-author)
- King Abdullah University of Science and Technology, Red Sea Research Center, Thuwal 23955-6900, Saudi Arabia
- Department of Biology, University of Oslo, 0316 Oslo, Norway
D. L. Aksnes (Co-author)
- Department of Biology, University of Bergen, 5020 Bergen, Norway