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Abstract

During a vertical migration study of the Daphnia hyalina x galeata population in Schoehsee (Ploen), the elevation of length-carbon weight regressions and fecundity levels were used as measures of nutritive state in order to distinguish between migration and non-migration strategies and their trade-offs. About three-quarters of the population migrated down to a refuge at the top of the hypolimnion (15 m) during the day but spread themselves within the epilimnion (0-7 m) and metalimnion (9-13 m) at night. The trade-off for the migration strategy was reduced mortality but a poorer nutritive state. With the probability of low visual predation in the dark day-refuge, large numbers of larger-sized migrating individuals survived to exploit the food-rich surface layers at night but unfavorable food conditions during the long daytime hours resulted in lower carbon weights per unit length and reduced fecundities. Only 7% of the population adopted a non-migration strategy by staying at the surface (0-7 m) to exploit the richer food conditions, thus becoming heavier in carbon weight and more fecund. The negative side of the trade-off was an enhanced predation risk as shown by sparse populations of small individuals, primiparous females and very few large individuals.