Freezer Fish

Artic char

Jenny Jensen

Arctic char, Salvelinus alpinus, the northernmost freshwater fish, live in harsh and often ice-covered polar lakes. The first-known, year-round investigation of free-living fish in polar waters has found that char maintain their normal season-specific activities—except for two short periods around the winter and summer solstices—even when living under ice with minimal light penetration or during long hours of daylight.

In a study reported in December, Kate L. Hawley, of the Norwegian Institute for Water Research in Oslo, and colleagues affixed acoustic tags to twenty-three individual char in Lake Ellasjøen on Bear Island, located in the Arctic Ocean between Svalbard and the Norwegian coast. The tagged fish were monitored hourly for latitude, longitude, and depth by a receiver array distributed throughout the lake, allowing researchers to estimate when the fish were feeding versus resting.

For much of the year, char were attuned to the changing levels of daylight. On a seasonal basis, they regulated the balance between their food intake and activities. Around the winter solstice, however, when Bear Island is in near total darkness, the daily transitional light changes were too small for the fish to detect. As if their internal circadian clocks were temporarily turned off, the char became sluggish and fed less, even though they are well adapted for life in cold, dark, nutrient-poor environments. Researchers noted, however, that the char’s daily cycles resumed even before twilight returned, indicating that char—perhaps, “controlled by internally timed changes in appetite regulation”—can anticipate the annual return of daylight.

Likewise, the char’s daily cycle seemed to be interrupted around the summer solstice. They became hyperactive, feeding almost nonstop. Such a feeding frenzy suggested to researchers that the char are, again, anticipating and preparing for upcoming events.

Hawley and her colleagues conclude that Arctic char may benefit from a complex internal “oscillator,” a mechanism that regulates both daily and seasonal activities. It adjusts feeding, growth, and reproduction by the calendar as well as the clock. (Biology Letters)