Partial diel vertical migrations in pelagic fish.pdf

Partial diel vertical migrations in pelagic fish ThomasMehner1,* and Peter Kasprzak2 1Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Mu?ggelseedamm310, 12587 Berlin, Germany; and 2Department of Limnology of Stratified Lakes, Leibniz-Institute of Fresh- water Ecology and Inland Fisheries, Alte Fischerhu?tte 2, OTNeuglobsow, 16775 Stechlin, Germany Summary 1. Field studies on diel vertical migration (DVM) usually report uniform behaviour with popula- tion-wide ascents and descents during crepuscular periods. This contrasts partial seasonal migra- tions of many animal populations, where individuals choose either the resident or the migrant strategy depending on population density, feeding opportunity and predation risk in the resident andmigrant habitats. 2. We tested whether DVM of freshwater zooplanktivorous fish (Coregonus spp.) resembles par- tial migrations. Twenty-eight hydroacoustic surveys were performed in the deep Lake Stechlin (Germany) between 2000 and 2010, with samplings encompassing all months between March and December. Zooplankton samples were simultaneously taken in epilimnetic and hypolimnetic lay- ers. Fish obtained from depth-stratified samplings by a midwater trawl were used to test for indi- vidual differences between residents andmigrants. 3. We show for the first time that DVMs of freshwater fish resemble patterns of partial migrations often found in seasonal environments. Across all samplings, 7–33% of fish did not ascend at dusk, but exhibited the resident strategy. The proportion or residents increased at low zooplankton feed- ing rates in the daytime habitat and during months when the temperature difference between day- time and night-time habitats wasminor. 4. Slightly larger size and higher caloric density of migrants over residents in one of the coexisting Coregonus species suggested that individual differences contributed to the migration strategy per- formed. However, these re