Arctic researchers built a ‘Fish Disco’ to study ocean life in darkness

During the winter, the Arctic doesn’t see a sunrise for months on end. Although completely immersed in darkness, life in the ocean goes on. Diurnal animals like humans would be disoriented by the lack of daylight, having been accustomed to regular cycles of day and night.

But to scientists’ surprise, it seems that even the photosynthetic plankton—microorganisms that normally derive their energy from sunlight—have found a way through the endless night. These marine critters power the region’s ecosystem, through the winter and into the spring bloom. Even without the sun, daily patterns of animals migrating from surface to the depths and back again (called the diel vertical migration) remain unchanged.

However, scientists are concerned that artificial light could have a dramatic impact on this uniquely adapted ecosystem. The Arctic is warming fast, and the ice is getting thinner—that means there’s more ships, cruises, and coastal developments coming in, all of which can add light pollution to the underwater world. We know that artificial light is harmful to terrestrial animals and birds in flight. But its impact on ocean organisms is still poorly understood.

A research team called Deep Impact is trying to close this knowledge gap, as reported in Nature earlier this month. Doing the work, though, is no easy feat. Mainly, there’s a bit of creativity involved in conducting experiments in the darkness—researchers need to understand what’s going on without changing the behaviors of the organisms. Any illumination, even from the research ship itself, can skew their observations. This means that the team has to make good use of a range of tools that allow them to “see” where the animals are and how they’re behaving, even without light.

One such invention is a specially designed circular steel frame called a rosette, which contains a suite of optical and acoustic instruments. It is lowered into the water to survey how marine life is moving under the ship. During data collection, the ship will make one pass across an area of water without any light, followed by another pass with the deck lights on.

There are a range of different rosettes, made up of varying instrument compositions. One rosette called Frankenstein can measure light’s effect on where zooplankton and fish move to in the water column. Another, called Fish Disco, “emits sequences of multicolored flashes to measure how they affect the behavior of zooplankton,” according to Nature.

And of course, robots that can operate autonomously can come in handy for occasions like these. Similar robotic systems have already been deployed on other aquatic missions like exploring the ‘Doomsday glacier,’ scouring for environmental DNA, and listening for whales. In absence of cameras, they can use acoustic-based tech, like echosounders (a sonar system) to detect objects in the water.

In fact, without the element of sight, sound becomes a key tool for perceiving without seeing. It’s how most critters in the ocean communicate with one another. And making sense of the sound becomes an important problem to solve. To that end, a few scientists on the team are trying to see if machine learning can be used to identify what’s in the water through the pattern of the sound frequencies they reflect. So far, an algorithm currently being tested has been able to discern two species of cod.