Herring larvae swim towards the sun

By manipulating magnetic north in a special tank, researchers at the Institute of Marine Research have previously shown that both eel and haddock larvae orient using an internal compass.

Now, they have given herring larvae the opportunity to demonstrate their sense of direction.

They swim southeast at sea

«In the magnetic chamber, the herring larvae swim at random. They show no sign of having an internal, magnetic compass», explains marine researcher Alessandro Cresci. (See facts about the MagLab.)

«However, when we put them in a transparent, drifting chamber in the sea, they show a directional behavior. Herring larvae swim towards the sun», he says.

On sunny days, the average herring larva swam south towards the average azimuth, or the angle of the sun at the horizon relative to the North.

On overcast days, they oriented less precisely towards southeast. The results are published in the Journal of Experimental Marine Biology and Ecology.

Southern herring want to avoid the coastal current, northern herring want to enter it

«The larvae are the offspring of herring collected outside of Bergen in Southern Norway. The herring here do not make the epic migrations which their northern cousins are famous for. The need to follow a Southeastern bearing, might be a mechanism to keep the larvae from getting swept away by the Norwegian coastal current, a fish highway going from south to north» Cresci speculates.

The northern herring, however, spawn and hatch in the eddies between the two lanes of the coastal current. The same mechanism could help these fish enter the current and hitch a ride further north.

«The most coastward lane has the highest velocity and will give the herring a boost towards the North, where they start their migration to nursery grounds.

At the mercy of the ocean

The larvae swim at a slower speed than the currents of the ocean. The Norwegian coastal current flows at 10-16 centimeters per second. The larvae in the experiment swam at an average of 0.36-0.40 centimeters per second. The ocean current is therefore the most important factor for the dispersion of the larvae.

«But, if the herring larvae swim diagonally towards the current, that could influence where they eventually end up», Cresci says.

«We intend to investigate the causes and the consequences of the southeastern orientation of the herring further in collaboration with IMR oceanographers.

Reference

Cresci, Alessandro, Bridie J. M. Allan, Steven D. Shema, Anne Berit Skiftesvik, and Howard I. Browman. "Orientation behavior and swimming speed of Atlantic herring larvae (Clupea harengus) in situ and in laboratory exposures to rotated artificial magnetic fields." Journal of Experimental Marine Biology and Ecology 526 (2020): 151358.
LENKE: https://doi.org/10.1016/j.jembe.2020.151358