Under water at night
Snorkelling in jungle rivers during the day is one thing. But doing the same at night is another. When the moon is not shining, it is so dark away from civilisation that you cannot see your hand in front of your eyes. If your torch goes out when you’re snorkelling, you lose all sense of direction.
You can't even see the shore! But it is incredibly exciting in the darkness, as many predatory fish and also shrimps come out of their hiding places. Most of us had never been snorkelling at night before, so our first attempt was in the caño behind the camp. We knew it a little so thought nothing could go wrong!
It was not to be! Only three participants from Team 1 had come night snorkelling in the river and the inevitable happened! One person’s lamp failed completely after a quarter of an hour. We had to call out through the total darkness to find them again. By then person number two’s lamp was beginning to fade and only one lamp (EOS 10 LRZ) held out without any problems. However, we had completely lost our bearings and only the direction of the river's current told us which bank we had started from. Unfortunately, from the water we couldn’t tell which area we’d started from. So snorkelling side by side we looked for a spot where the trees and plants allowed us to reach the shore. Without this third light we wouldn’t have found it!
The second night snorkelling attempt took place on the Venezuelan side of the Rio Atabapo. There, almost opposite the indigenous village, was an area where large boulders formed a part of the shore area and was sure to have a different fish fauna.
This time it was easier. There was a lamp on the boat and there were six of us. So when you switched off your own lamp, there was usually some light from another lamp and you stayed close to the shore because the flow speed of this big river was enormous. If you went too far from the bank, you were carried away by the current. But even at night it is no problem at all to swim out of a main current - as long as you know where the bank is.
Snorkelling around the rocks and the surrounding flooded trees was exciting. We sighted completely different fish species than in the caño behind the village, mostly catfish.
A new experience was diving in the blackwater at night. You leave the water surface, dive down vertically, equalise the pressure several times to relieve the pressure on your ears, and then eventually reach the bottom. There you look around for fish in the light of the torch, take one or two photos and start surfacing. But because of the darkness and the blackwater, you can't see the surface. So you ascend and simply wait to reach the surface. We could only calculate approximately how deep our descent was by the number of pressure equalisations every 3 m. 3 pressure equalisations represented a depth of between 9 and 12 metres. We had to retain enough air for this distance on the way back. Once again I was glad I had taken a freediving course with world record holder Christian Redl. "When you think you're out of air, you can hold your breath for at least another minute". Such knowledge is reassuring when you are straining in the dark towards a surface whose distance is an unknown.
This is how the photos were taken
We used a Canon EOS 7D camera with a Canon EF 35 mm Macro lens in an underwater housing made by BS Kinetics. We omitted the flash, as the glare is too great for the water particles and also its brown colouring. Instead, an underwater lamp from mares (EOS 10 LRZ) was mounted on top of the underwater housing and switched on at the lowest setting. The light cone on this lamp can be adjusted continuously from spot to wide beam. We set the camera to ISO 5000 with an exposure time of 1/80 sec. in aperture priority. This way the camera always selects the appropriate aperture for the given exposure time and ISO number, which should be as small as possible because of the depth of field.
The fairy tale of night temperature reduction
We used a waterproof temperature data logger to check whether and how far the water temperature drops at night. Do we really need to set our aquarium heating to lower the temperature at night (if our heating allows it)? In larger rivers we weren’t able to discern any changes in temperature during the day or at night. Our overnight stay at the Rio Atabapo allowed us to attach the data logger in the waters of the smaller river behind our camp. We used cable ties to attach it to a branch in shallow water about 40 cm deep. However the tree meant the spot was at least temporarily in the shade. In this extreme shallow water, the data logger displayed a maximum daytime water temperature of 27.4 °C and a minimum nighttime temperature of 25.9 °C. This 1.5 °C difference is only measurable in shallow water without strong currents. We’ll repeat the experiment on future expeditions at different locations and depths.
