The Effect of Turbidity on the Behavior of Bleak (Alburnus alburnus)

In group-living species, social interactions with conspecifics play a crucial role in group formation and the ability to make consensus decisions, with far-reaching consequences for ecological and evolutionary processes in natural populations. Individual recognition and partner preferences based on social familiarity are important mechanisms driving a range of interactions between individual fish and social structure in fish populations. However, the social interactions of gregarious species are also influenced by the ecological environment experienced by individuals. This study aimed to determine how fish shoals’ structure is shaped by increased turbidity, a typical environmental constraint in anthropogenically impacted rivers. A freshwater, shoal-forming, visually orientated pelagic fish—bleak (Alburnus alburnus)—was used as the model organism. The behavior of 40 individuals at three different turbidity levels (0 NTU, 30 NTU, 60 NTU) was tested in the laboratory experiment. Specifically, the aim was to determine if the turbidity reduces between individual distances in response to the deteriorated visual conditions. The results showed that bleaks increased the compactness of the shoal even at the medium turbidity level (30 NTU), and compactness further increased with turbidity. Such results indicate that turbidity is an important phenomenon influencing the structure of shoals and ultimately an ecological process in natural fish populations in ecosystems affected by increasing turbidity.