Twilight Zone: The Foraging Migrations of Gruntfish
by Kim Weisenborn
Coral reef ecosystems are thought to be the most bio-diverse places on earth, supporting 25% of the oceans marine life, including more than 800 hard coral species and over 4,000 species of fish [a,b]. They occupy only 1% of the ocean floor and are sometimes referred to as “the rain forests of the sea.” The diversity here is highly associated with reef structure, where reef organisms have evolved a high degree of specialized morphology and behavior associated with space, reproduction, and food [c]. Some of these behaviors involve migration to and from the reef habitat. They may be seasonal, like feeding or mating migrations in connection with climate and ocean temperature. Other migrations occur due to life history events where larval stage organisms may settle in sea grass or mangrove nursery sites for a few days, a year, or longer before joining their adult counterparts among the reef habitat.
The most persistent of these migrations are diel migrations, which occur every dawn and every dusk at the exact moment of twilight. Nocturnal and crepuscular species of coral fish migrate off their home reef and into adjacent sea grass or reef structures to forage for food, and with uncanny timing. It is known that the influence behind these migrations are driven by the intensity of predation. The question is, how do diel migrating fish determine this precise moment, and what are the cues that signal this movement to and from the reef? Not only do these reef fish gather and migrate with such proficiency, it has also been documented their use of local landmarks and a sense of distance in the areas surrounding their home reef.
Several studies have used gruntfish (Haemulidae) as a model fish for observing these types of migrating behaviors. A study in 1977 by Ogden and Ehrlich documented French and white grunts departing from the reef at twilight and following a linear route for 50 to 100 meters. They departed in groups and then fragmented into a dendritic pattern until only single individuals remained stationed at their designated locations, feeding through the night on benthic invertebrates. Just before dawn, individual gruntfish gathered into groups just off the reef structure, and returned with precision along the same initial route back to the reef. The experiment showed that grunts did not reaggregate randomly after a night of feeding, but recognized a particular schooling site and their initial home reef [c].
This high site fidelity has been documented in other studies, as well. In 2012, I helped with a study in The Great Bahamas off of Abaco Island to observe gruntfish and their amazing ability to return back to their home reef structure after a capture and release experiment. Gruntfish were captured in traps on established artificial reefs that were surrounded by natural sea grass habitat. A pigmented dye was used to distinguish gruntfish that were captured and released in alternate locations away from their homebase. Those that were recaptured had returned back to their home reef after traveling 25-200 meters. One gruntfish even found its way home after being released 1 kilometer away on the other side of the peninsula. Ogden and Ehrlich documented homing as great as 2.8 kilometers [e].
Most anglers know that some of the best times to catch fish are around dusk and dawn when fish are most active. Predation is high during these transition periods of polarized light. The ability of fish to detect ultraviolet and polarized light isn’t entirely understood, but during dusk and dawn, polarized light is most prevalent.
Several things could be occurring here that influence these migrating behaviors during this period. The silvery scales of gruntfish reflect polarized light very well, and may allow other gruntfish to better detect the proximity and orientation of the group against a diffuse background relative to neighboring fish [g]. Additionally, fish eyes are similar to terrestrial vertebrates and mammals, but are more spherical. This means that most species have rod cells for seeing in low light conditions, and cone cells for seeing in color. The ratio of cones to rods is different depending on the ecology of the fish species [f]. Fish that are most active at night or twilight (nocturnal or crepuscular) will have more rod cells to allow them to see best in low light conditions. This also holds true for some predators, which are also active during these periods of migration. The transition between cone cells and rod cells can take some time (sometimes, an hour or more), so the quicker a fish can adjust to the changes in light, the quicker it can forage for food. It is possible that the transition from cone cells to rod cells may occur quicker in gruntfish than in some species of predators, allowing for prompter migration from the reef and into the camouflaged seagrass [h].
a. Spalding, M.D., C. Ravilious, and E.P. Green. 2001. United Nations Environment Programme, World Conservation Monitoring Centre. World Atlas of Coral Reefs. University of California Press, Berkeley: 416 pp. (2001)
b. Paulay, Gustav. “Diversity and Distribution of Reef Organisms.” Life and Death of Coral Reefs. Charles Birkeland, 1997. 303–4.
c. Ogden, J.C., and T.P. Quinn. “Migrations in Coral Reef Fishes: Ecological Significance and Orientation Mechanisms.” Mechanisms of Migration in Fishes. NATO Conference Series 14: 293-308 (1984).
d. Zapata, Martha J., L.A. Yeager, and C.A. Layman. “Day–Night Patterns in Natural and Artificial Patch Reef Fish Assemblages of The Bahamas.” Caribbean Naturalist 18 (2014).
e. Ogden, J.C., and P.R. Ehrlich. “The behavior of heterotypic resting schools of juvenile grunts (Pomadasyidae).” Marine Biology 42: 273-280 (1977).
f. Helfman, G., Collette, F.D., Bowen, B.W. “The Diversity of Fishes: Biology, Evolution, and Ecology.” Wiley-Blackwell: 84-87 (2009).
g. Rowe, D.M., and E.J. Denton. “The physical basis of reflective communication between fish, with special reference to the horse mackerel, Trachurus trachurus” Phil. Trans. R. Soc. Lond. B, 352 (1353): 531–549 (1977).
h. Ross, David A. “Fish Senses.” The Fisherman’s Ocean. Stackpole Books, 2000. 299.