David Bierbach

Fish Behavior and Biorobotics

Current research Projects

Decision-making and leadership in animal groups  
Live animals, especially when in a group of conspecifics, can hardly be forced to change their behaviors as intended by the experimenter. Hence, experimental manipulations are difficult and often lacking to underpin proposed theoretical assumptions on collective movement, decision-making or leadership. One elegant solution is to replace live individuals in a group by biomimetic robots. In a multidisciplinary group, we thus developed an interactive fish robot – the so-called RoboFish. Through the interactive behaviors based on real time tracking of the fish shoal, our system can be adjusted to imitate different individual characteristics of live fish and is accepted as a conspecific by live guppies (Poecilia reticulata). This allows us to use RoboFish to investigate many different questions regarding collective decision-making.

Click HERE to view our Robofish website for more information.


Social behavior of extremophile mollies from Mexico

When animals synchronize their individual behavior, extremely coordinated group performances become possible. One example of those collective miracles are fright waves of the endemic sulfur molly (Poecilia sulphuraria) from the South of Mexico. Millions of these small livebearers inhabit springs that contain highly toxic hydrogen-sulfide (H2S). H2S inhibits aerobic respiration due to its interference with mitochondrial respiration and blood oxygen transport, but also leads to extreme hypoxia in the water. When exposed to H2S and hypoxia, livebearing fishes resort to aquatic surface respiration (ASR) and, thus, exploit the more oxygen-rich air-water interface. In natural populations, sulfur mollies have been observed to spend up to 84% of their time performing ASR. However, time spent at the water surface clearly trades off against the time fish can spend foraging as well as the risk to fall victim to preying birds. Along these sulfur creeks and springs, bird predation rates are more than 20fold increased. When at risk, sulfur mollies thus exhibit synchronized and repeated dives that result in multiple small water waves. A very similar pattern can be observed in human football arenas, the so-called “Mexican Wave” or “La Ola”. We believe these waves can distort an attacking bird. However, we currently investigate how these repeated fright waves are triggered and how information on synchronized dives spreads through the large shoals of mollies.
Invasive species in thermally altered aquatic systems in Germany  
Alien species are among the major drivers of species extinctions and, thus, loss of biodiversity, especially in freshwater ecosystems. In the European Union alone, 12,122 non-native species have been reported so far (DAISIE European Invasive Alien Species Gateway 2013). While most species introductions are accidental, several active introductions have also been documented. An example is the introduction of Nile perch (Lates niloticus) into Lake Victoria, with its disastrous consequences for the endemic fish fauna. Live-bearing fishes of the family Poeciliidae have been widely introduced to tropical and subtropical countries for malaria prophylaxis, i.e., to help control vector (mosquito) populations. In addition, some poeciliids like guppies (Poecilia reticulata), swordtails and platyfish (Xiphophorus spp.), and mollies (Poecilia (Mollienesia) spp.) are among the most popular ornamental fishes, and many introductions may have been the result of occasional releases from home aquaria. The ability to store sperm from multiple mates for several months secures several consecutive broods and allows a single gravid poeciliid female to found an entire new population. In cold-temperate areas like the northern parts of Europe, America and Asia, those tropical neozoons often sustain in waters that are heated by either natural or anthropogenic sources such as cooling wastewaters of power plants and open cast mining out drains. Thermally altered aquatic systems with temperatures above climate change projections offer a great opportunity to study the interrelatedness of climate change and biological invasions, as they may represent semi-natural “laboratories”. In close cooperation with other international scientists, I focus on how individuals, populations and communities are affected by both anthropogenic increased water temperature and the accompanied invasion of tropical species. As a study site of interest, we discovered the artificially upper Gillbach (NRW, Germany), that is verifiably inhabited by a guppy population since the mid-1970s and receives heated cooling water from a power plant. Besides guppies and native European species, also cichlids and catfish as well as tropical shrimps are regularly found to reproduce in this unique ecosystem.