Nocturnal light and lunar cycle effects on diel migration of micronekton

The roles of nocturnal light and lunar phase in the diel migration of micronekton from a nearshore scattering layer were examined. Migration patterns were measured over six complete lunar cycles using moored upwardlooking echosounders while nocturnal surface irradiance was recorded. We hypothesized that animals would remain at a constant isolume at night despite changes in nocturnal illumination between nights. The scattering layer migrated closer to the surface during dark nights than during well-lit ones. However, this movement was not enough to compensate for observed changes in light, and at night animals often remained at light levels higher than they experience at depth during the day. Light and lunar cycle were not completely coupled, allowing separation of the light and lunar phases. Contrary to the initial hypothesis, lunar phase accounted for substantially more of the variability in layer migration than surface irradiance, showing strong effects on the scattering layer’s depth and animal density within the layer. Changes in layer depth and animal density were amplified a small amount by variations in light level but were minimized by the seafloor in shallow areas. The horizontal component of the scattering layer’s migration was also affected by lunar phase, with animals remaining further offshore in deeper waters during nights near and during the full moon, even when these were not the nights with the highest light levels. These results suggest that moonlight may be a cue for an endogenous lunar rhythm in the process of diel migration rather than a direct cause. Diel vertical migration of zooplankton and nekton, active movement from deep, dark waters where animals reside during the day to near-surface waters at night (Longhurst 1976), is thought to be ultimately driven by maximization of survival controlled by food, predators, and physiological costs (Zaret and Suffern 1976; Enright 1977). However, individual animals have limited means to follow predator and prey distributions in the water column. Therefore, environmental cues must be used as proxies. Light is one of the strongest environmental cues available to migrating animals and is hypothesized to serve as a proxy for the risk of predation by visual predators (see a review in Forward 1988). Light is generally accepted as playing an important role in controlling the timing of diel vertical migration since most migrations occur at sunrise and sunset (Esterley 1911; see a review in Ringelberg 1995). Two mechanisms to explain how light may trigger vertical migration have been examined: absolute light intensity and rate of light change. One hypothesis about absolute light levels suggests that animals remain at an optimum light level, following this constant level, or isolume, up and down in the water column as the surface irradiance changes (Michael 1911; Longhurst 1976). Alternatively, animals may use an absolute threshold value of light to trigger upward movement when light becomes lower than the threshold (Esterley 1911; Pearre 1973). The absolute level of light may be difficult for animals to detect, however. Instead, animals may use the rate of the change in light intensity to cue migration (reviewed in Ringelberg 1995).