“Water clarity affects collective behavior in two cyprinid fishes” by Michael et al., 2021
Shifting environmental conditions will inevitably present challenges for the organisms living in a certain area. Naturally occurring phenomena such as heavy rains and earthquakes make navigating an aquatic system particularly difficult for predators that predominantly rely on visual cues to hunt, or prey to escape being hunted. When water becomes turbid, (or cloudy), the visibility will decrease, and the environment becomes riskier for both the hunters and the hunted. Visual predators will have lower success in hunting endeavors and prey will have less of a warning when a threat is approaching.
While a naturally shifting habitat is normal, the encroachment of human industry has disturbed aquatic systems beyond the norm. Higher accumulation of various deposits (both naturally occurring–like silt or decayed matter, for example, and of human origin–such as pharmaceutical or agricultural runoff) has the potential to affect the behavior of whole groups or populations of animals.
Michael et al. chose to study two visual-reliant freshwater minnows: the red shiner (Cyprinella lutrensis) and the sand shiner (Notropis stramineus). These two species exhibit shoaling behavior (swimming in one or multiple groups), which is a prey protection mechanism. (A large, undulating mass of prey is more difficult for a predator to focus on than a lone individual). Red shiners and sand shiners rely on vision to shoal properly. Michael et al. wanted to explore any changes in shoaling behavior that might occur when groups of C. lutrensis and N. stramineus were introduced to increasingly turbid conditions.
Using an overhead HD camera, the shoaling variables they measured (in relation to the two species themselves, the treatment (turbidity), and the time exposed) included: the distance between fish while swimming (interfish distance), the total space taken up by the group (shoal area), how often fish swam in a straight line (sinuosity), and swimming velocity in wild-caught shiners. For each trial, 5 fish in an observation pool were exposed to three increasing levels of turbidity: 0-0.05 nephelometric turbidity units (NTUs), 50-150 NTUs, and 200-350 NTUs.
Michael et al. found that the differences found in interfish distance, shoal area, and velocity were significant for both species when exposed to the treatment with the red shiners having more extreme shoaling changes (above figure). They also found differences for all variables (interfish distance, shoal area, sinuosity, and velocity) on a minute-per-minute basis (e.g. the space between fish, for example, was significantly different after each minute).
Michael et al. collected evidence supporting that C. lutrensis and N. stramineus changed their group shoaling behavior in response to increasing turbidity levels. This is compatible with previous studies that Michael et al. attempted to replicate. It is important to note that while the authors may believe that the shiners’ collective behavior changed because of a lack of visual stimuli is a reasonable hypothesis, it is not indefinitely proved here. It is a possibility that the fish could shoal in cloudy water but did not for some reason.
The study of artificial adjustments to an organism’s environment is important because it can be utilized in conservation efforts for endangered species. The elaborate video and tracking techniques Michael et al. used could also be useful in many other fields of biology and beyond.
Article citation:
Michael SC, Patman J, Lutnesky MM. Water clarity affects collective behavior in two cyprinid
fishes. Behavioral Ecology and Sociobiology. 2021 Aug;75(8):1-3.
Article hyperlink: https://doi.org/10.1007/s00265-021-03060-x
