Diurnal vertical migration: Adaptive significance and timing. Part 2. Test of the model: Details of timing 1

The hypothesis that vertical migration has evolved primarily as a means of avoiding visually orienting predators is based on the common impression that grazers migrate upward and begin feeding in darkness, after sunset. An alternative hypothesis predicts that grazers might gain significant metabolic advantages if they began to feed a couple of hours before sunset. In three field sampling programs each of 3‐days duration undertaken to distinguish between these alternative behavioral patterns, duplicate samples were taken at 30‐min intervals during late afternoon and evening. The data for the abundant herbivorous copepod Calanus helgolandicus ( pacificus ) have been analyzed in detail. In one of the sampling programs, in early summer and at lowest herbivore abundance, migration began only after sunset, for both adults and immatures, suggesting that avoidance of predators was of overriding importance under those circumstances. In another sampling program, in late spring and at highest herbivore abundance, the onset of migration for adults as well as immatures began consistently an hour or two before sunset, suggesting that predator avoidance was relatively unimportant under that set of circumstances. Migration of immatures in the third sampling program, in midspring, began an hour or two before sunset but that of adults began only after sunset, suggesting that the appropriate trade off between improved survivorship by predator avoidance and possible metabolic advantage may depend on the developmental stage of the animal.