Migration is commonplace for many marine species. Whether it be in search of food or a mate, and whether the migration takes a day or a year, the patterns exhibited by sea animals seem to pass down through generations. So what takes place below the surface that leads certain animals to the same place every year? How do they know where to go, and why?
Animals migrate for a variety of reasons. For example, female blacktip sharks return to their natal nursery grounds every few years when they are preparing to give birth. This migration increases the shark pups' likelihood of survival by providing protection from larger, open-water species and offering smaller prey for the growing newborns. Zooplankton migrate up and down through the water column every day, swimming up to the surface to feed at night when they can hide from predators, and returning to deeper water before the sun rises. Grey whales are among the greatest wanderers, swimming over 10,000 miles every year between feeding grounds and nursery areas. A recent study suggests that the tropical nursery areas may have been chosen since warmer waters help whales shed their skin. In the case of eels and salmons, the only migration they exhibit is at the beginning and end of their life.
For animals that take on small-scale migrations, they may use the light levels of the rising and setting sun as their cue, or location-specific identifiers to know where to go. Much like humans learn their drive to work by visual cues (such as turning left after the gas station), fish may remember their favorite feeding spot is just past a specific coral formation. However, learning to navigate an open ocean with no markers requires a few extra steps. Whereas humans need a compass to identify the direction of the geomagnetical field, many animals can sense this internally.
Some animal species, namely whales, stay with their young after birth and migrate together. Newborn sea turtles, on the other hand, are born in the sand and move directly toward the ocean without parental presence. Visual cues are suspected to direct the young turtles as they move toward the brightest part of the sky. Once they reach the water, their magnetic compass guides them to their "innate destination", which they are born knowing to head towards. It’s as if their parents wrote coordinates on their arm in Sharpie and left a GPS with them. Sea animals that use the magnetic field for directions are able to adjust to displacement, meaning if you were to catch a lobster and drive it a mile from where you caught it, it would angle itself to correctly reach its destination after being released.
While the direction of larger-scale migrations is guided by an internal compass, many species use seasonal temperature changes as their cue to begin the journey. However, as sea temperatures are increasing, shifts in migration patterns have started emerging; species are moving poleward and deeper toward colder waters. The EPA highlights this, stating that 180 marine species have moved more than 16 miles northward and 0.5 feet deeper from 1989 to 2019, and the trend is expected to continue as temperatures keep rising.
Hays, G. C., Cerritelli, G., Esteban, N., Rattray, A., & Luschi, P. (2020). Open ocean reorientation and challenges of island finding by sea turtles during long-distance migration. Current Biology, 30(16). https://doi.org/10.1016/j.cub.2020.05.086
Pitman, R. L., Durban, J. W., Joyce, T., Fearnbach, H., Panigada, S., & Lauriano, G. (2019). Skin in the game: Epidermal molt as a driver of long‐distance migration in whales. Marine Mammal Science, 36(2), 565–594. https://doi.org/10.1111/mms.12661