It’s a whole different lake at night…

posted by Pete McIntyre (University of Wisconsin)

Led by our trusty colleague and field helper George Kazumbe, the team has now gone diving and snorkeling at one of our main study areas at night on two occasions—and what a difference it has made!  Whereas we had believed that zooplankton are scarce in the lake, we were amazed to find that on a nearly moonless night, the water along the coast was heaving with small shrimp, tiny copepod crustaceans, and larval sardines.  These small animals (often lumped under the term zooplankton) have long been recognized as an essential link in the food chain, but our extensive work during daylight hours always led us to believe that intensive predation by the fish plus the small amount of algae in the water was enough to limit the populations of planktonic animals.  Apparently we were wrong!

In retrospect, it makes sense that there must be loads of copepods and shrimps because these small animals are a staple in the diet of about half the fish species along the shore.  Nonetheless, the difficulty of finding them during the day had convinced us that they never achieve large population sizes.

Instead, it turns out that there are plenty of zooplankton, but they are able to hide very effectively during the day (from both fish and human researchers!).  At night, when the moon is dim, they swarm forth to feed on the tiny algae suspended in the water (phytoplankton), and on each other.

Along with this revelation about nearshore zooplankton densities, we were surprised to see both how many species of small catfish were swimming around (at least 6 species, twice the number what we had found from all previous sampling), and how few eels there were.  The nocturnal catfishes were dominated by small claroteid species (most less than 4 inches in length), along with some walking catfish (Clarias) and upside-down catfish (Synodontis).  I had expected to see more Synodontis and Clarias, but in fact we see more of both groups during the day than we did at night.

The lack of eels was intriguing.  Spiny eels are common predators along the shoreline of Lake Tanganyika (and most rivers and lakes in Africa and Asia), and I suspected they were primarily nocturnal.  As it turns out, we saw an abundance of one species that is rarely observed during the day (one with beautiful rings of brown and black along its body), but not a single individual of the other 5-6 species we have found at the site.  These observations have convinced me that the most effective way to count the eels is probably from daylight observations, rather than night-time work.  It is odd that most species of this group of predators do not take advantage of the opportunities afforded by sleeping cichlids on the bottom, which would seem to offer easy pickings for a fish-eater.  However, it does help to explain why the eels have such good vision—they are remarkably hard to catch because they can see our thin-mesh nets much better than most of the cichlids do!

Perhaps the biggest surprise of all was observing two kinds of electric fishes.  We knew there would be electric catfish, which eat fish and shrimp after stunning them with an intense electric shock.  Collecting those by SCUBA was a bit nerve-wracking, but George and I got two without any electric tingles (or jolts).  But we also were amazed to see several different groups of weakly-electric fish, the elephant fishes (mormyrids).  They were all the same species (one lacking an elephantine snout), and proved impossible to catch.  These fish are about 3-4 inches long, and they use weak electrical pulses to communicate with each other, navigate complex environments, and find their prey.  In rivers where I have caught these fish before, they eat mostly insects and worms on the bottom.  However, the Tanganyikan species seemed to be well up in the water column, suggesting that they are catching the shrimp and other zooplankton.  Interestingly, they were also right against the shoreline along intricate rock walls and crevices, so perhaps that is where they hide during the day.

While George and I chased fish, Ben and Ellen used an ultra-fine net to collect zooplankton samples.  Ben found that there were far fewer zooplankton in the water on our second trip than on the first day, but still more than we had expected originally.  He also found large quantities of a beautiful diatom that forms colonies with hair-like spikes radiating outward from a central sphere, sort of like a sea urchin but with a total diameter far less than 1/100^th of an inch!

All in all, it was marvelous to see the nocturnal life of Lake Tanganyika.  However, it’s also very exhausting after a long day of work in the water!  So we’ll stick with daylight work for the most part…