Biology

Salmon burn the midnight oil

It’s two o’clock in the morning, pitch dark along the Sainte Marguerite River in the Saguenay. You’d have to have night vision to see the tips of two snorkels slowly making their way upstream and stopping at some rocks. Connected to these snorkels are two frog-like humans, armed with underwater flashlights and waterproof notepads. They are Université de Montréal biologists who have been studying the habits of the Atlantic salmon (Salmo salar), a species that has been in decline for the last 10 years throughout eastern North America.

For the entire summer, night after night, these researchers have donned their insulated wet suits and entered the icy waters to track baby salmon. But what has possessed them to embark on this work at night? “The reason is that young salmon are much more visible at night,” explains Marie-Ève Bédard, who led a scientific team last year that documents the nocturnal habits of alevins and parr (salmon under three years of age that have not yet left freshwater for the ocean).

Ms Bédard’s research for her bachelor’s thesis in biology helped to lift the veil on the nocturnal activities of the salmonid family. The results of her study, to be published this summer in the Journal of Fish Biology, showed that young fish are most active between 10 p.m. and 2 a.m. She observed that they are 50 percent more active during these four hours than before or after this period.

For biologist Daniel Boisclair, this result added a piece to the Atlantic salmon puzzle he has been trying to solve for the past 10 years. “We need to find out as much as possible about the living conditions of fish in order to better control the factors that affect their survival,” explains the cofounder of a multi-university research centre called the Centre interuniversitaire de recherche sur le saumon atlantique (CIRSA). “Since the discovery that alevins and parr are more active at night than during the day, we only dive at night. We get up at noon, and our real workday begins at about 7:30 at night, until three or four in the morning.” Young salmon are more active in the middle of the night because there is less risk of them running into a predator at that time. Young salmon eat insects and larvae that drift in the current. This requires them to leave their cover to catch their food, thereby exposing themselves to danger.

Two years ago, Daniel Boisclair set out with post-doctoral student Istvan Imre to determine if moonlight had an impact on fish behaviour. “We compared our observations on moonless nights with those on nights with a full moon. There was no significant difference.” A few years earlier, Daniel Boisclair created quite a stir in the scientific community with his discovery that the moon could influence lake fish. The explanation was actually very simple. “ Lake fish eat algae, phytoplankton and zooplankton, all of which are very light-sensitive. On nights with a full moon, these food sources stay very close to the surface and, therefore, so do the fish.” The situation is different in a river, however, where fish do not rely on these organisms, but rather on fish, larvae and invertebrates carried along by the current. Consequently, the researchers were not all that surprised by the study results. “In rivers, alevins and parr seem to respond to different external factors. For example, we observe eight times more fish on cloudy days than on sunny days.”