Once in a Blue Moon: How Does the Moon Affect Fish Behavior?

moon-rise over Pallette Lake, Wisconsin

The next time a blue moon will occur on Halloween won’t be until 2077, so if you missed it this weekend… well. Too bad. A blue moon is the second of two full moons that occur in the same month. It’s a rare astronomical event, thus the expression, but it is part of our solar system’s natural rhythm. And fish know that!

While there’s no evidence that fish know exactly what kind of full moon it is, many species change their behavior depending on the moon phase. It might seem like superstition, but there may be something to those old fisherman’s tales.

There are several hypotheses for how the moon affects fish behavior. They vary by species, and mostly have to do with two major properties of the moon: its light, and its gravity. Let’s start with its light.

Many animals rely on light to see, so it is hypothesized that during full moons, visually oriented predators feed more actively because their prey are easier to see. For example, the nocturnal foraging efficiency of largemouth bass was shown to increase with simulated moonlight (McMahon and Hoanov 1995). It could also be an indirect linkage where the predators follow prey that are more active during high moonlight conditions (Gilwicz 1986). It follows that during new moons, prey species could be more active, since they may be able to avoid predation in low light. One interesting hypothesis proposed by (Steinhart & Wurtsbaugh 1999) is that the effects of the moon on fish behavior may be dampened during winter months in high latitude lakes where ice cover might dampen the intensity of changes in moonlight. In addition to feeding behaviors, some fish species reproductive cycles are linked to the lunar cycle. More light may be better for species that exhibit parental care, but worse for species at risk of predation. Species that do not exhibit parental care, like broad cast spawners, (I call it the “spray and pray” strategy) may spawn during new moons to give their defenseless eggs a better chance at survival.

While the light of the moon can get fish moving in fresh or saltwater, the gravitational effects of the moon are usually only felt in the ocean, and they manifest as tides. Tides are the effect of the ocean “swelling” in response to daily changes in the earth’s rotation relative to the moon. Each day there are two high tides and two low tides, and these tides increase in intensity during the new and full moons, which is when the moon’s gravitational pull on the earth is the strongest because it is reinforced by the gravitational force of the sun. For a better explanation of tides check out NOAA’s helpful page.

Bonefish pre-spawning aggregation. photo by Cameron Luck

Some fish rely on the extreme tides provided by new and full moons to get access to their preferred spawning and feeding habitat. For example, the surf smelt spawns on the sand at beach levels that can only be reached at high tide (Loosanoff 1937). Other species use the tides as a dispersal mechanism. Bonefish migrate offshore to spawn during the high tides around the full moon so that stronger currents disperse their eggs widely, providing more chances for larvae to access good food resources (Danylchuck et al. 2011). Also, spawning at night may help increase survival in coral reef ecosystems that often have high predator densities (shark bait, hoohaha).

It has been shown a few times that catch rates on fishing boats improve during full and new moons, but the complexity of fish responses to the lunar cycle make predicting success very difficult (Lowry et al. 2007). Planning your fishing trip based solely on a solunar calendar may not be the best idea, but knowing the behavior and ecology of your species could give you a much better shot at catching one. Are you an angler/astronomer? Or do you only catch a fish once in a blue moon? Let us know how the moon affects your favorite fishes in the comments below!

Literature Cited

Danylchuk, A.J., Cooke, S.J., Goldberg, T.L. et al. Aggregations and offshore movements as indicators of spawning activity of bonefish (Albula vulpes) in The Bahamas. Mar Biol 158, 1981–1999 (2011). https://doi.org/10.1007/s00227-011-1707-6

Gliwicz Z.M. (1986) A lunar cycle in zooplankton. Ecology 67, 883–897.

Lowry, M., D. Williams, and Y. Metti. 2007. Lunar landings—Relationship between lunar phase and catch rates for an Australian gamefish-tournament fishery. Fisheries Research 88(1):15–23.

Loosanoff, V. L. 1937. The spawning run of the Pacific surf smelt, Hypomesus pretiosus (Girard). Internationale Revue der gesamten Hydrobiologie und Hydrographie 36:170-187

McMahon T.E. & Holanov S.H. (1995) Foraging success of largemouth bass at different light intensities: implications for time and depth of feeding. Journal of Fish Biology 46, 759–767.

Steinhart G.B. & Wurtsbaugh W.A. (1999) Underice diel vertical migrations of Oncorhynchus nerka and their zooplankton prey. Canadian Journal of Fisheries and Aquatic Sciences 56, 152–161.

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