Diel vertical movements by lotic mayfly nymphs under variable predation risk

1. The diet foraging periodicities of two grazing mayfly (Ephemeroptera) ny mphs, Heptagenia dalecarlica and Baetis rhodani, under variable fish (Europ ean minnow) predation risk were examined in a series of laboratory experime nts. 2. Heptagenia dalecarlica were almost exclusively nocturnal in their use of feeding areas on stone tops. There was a sharp increase in the proportion of nymphs out of refuge at nightfall, both in the control and fish-odour tr eatments. In a treatment with freely moving fish, H. dalecarlica decreased their activity during both the day and night. In feeding trials with three freely foraging minnows, nymphs were completely safe when they had access t o refuges beneath stones, whereas almost half the nymphs were consumed when no refuges were available. 3. Baetis rhodani nymphs reduced their use of stone tops when exposed to fo ur caged minnows or a freely moving fish, but this occurred both day and ni ght. In feeding trials, B. rhodani were captured only while in the water co lumn, and their mortality risk was independent of refuge availability. 4. It is suggested that because H. dalecarlica lack efficient post-encounte r defences, they must rely on pre-encounter mechanisms to reduce the threat of fish predation. It appears that in order to dwell sympatrically with fi sh, H. dalecarlica have evolved a coexistence by hiding strategy. In contra st, Baetis are vulnerable to fish attacks only if they enter drift in dayli ght, and are thus able to remain on stone tops both during the day and at n ight.