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One and Two-individual Movements of Fish after Chemical Exposure

Movement behavior of an indicator species, zebrafish (Danio rerio), was analyzed with one- and two-individual groups before and after treatment with a toxic chemical, formaldehyde, at a low concentration (1 ppm). After the boundary area had been determined based on experimental data, intermittency was defined as the probability distributions of the shadowing time during which data were above a pre-determined threshold and were obtained from experimental time-series data on forces and the inter-distances for one and two individuals. Overall intermittencies were similar in the boundary and central areas. However, the intermittencies were remarkably different between the one- and the two-individual groups: the single line was used to fit the data for the one-individual group whereas two phases were observed with breakpoints (approximately 10 seconds in logarithm) in the exponential fitting curves for the two-individual group. A difference in the probability distributions of shadowing time was observed "before" and "after" treatment for different areas. Intermittency patterns before and after treatment were contrasted in the center for the one-individual group whereas the difference was observed in the boundary for two-individual group. The intermittencies for the inter-distances of two individuals in the boundary and central areas were markedly different before and after treatment. When the differences between the intermittencies in the boundary and the central areas and between "before" and "after" treatment are considered, the distribution patterns of the shadowing time (scaling behaviors or intermittency patterns) should be a useful means of bio-monitoring to detect contaminants in the environment.