Figure 3
Light flicker can enhance the detection of underwater objects. (A) Comparison between the frequency of light flicker in downward irradiance and the Fmax of fish for dim and bright stimuli. The distribution of Fmax values across frequency corresponded well to the power spectrum of flicker across depths. Depicted power spectrum (gray shaded area) represents the envelope of flicker power across the 1 m and 10 m depth range. (B) Comparison between the cumulative power of the flicker and Fmax for dim and bright stimuli (closed circles). The indices fP50, fP90, and fP99 stand for the frequencies that correspond to 50, 90, and 99 percent of the cumulative power of flicker across frequencies, averaged across the light spectrum. Vertical and horizontal dashed lines represent the fP50, fP90, and fP99 values at 1 m depth. Gray and pink shaded areas represent the frequency range enclosed by the 99% cumulative power bounds at 1 m and 10 m depth, respectively. (See Additional file 1E-G for the cumulative power indices as a function of light wavelength.) Fmax data points typically fell above the identity (dotted) line, indicating larger Fmax values for bright stimuli. The median Fmax equaled 1.8 and 3.5 Hz, for dim and bright stimuli, respectively (open circle; bidirectional red error bars represent the 25th and 75th percentiles). (C) Summary of the number of species inhabiting the various habitats and depths examined. (D,E) Association between the Fmax of fish for bright stimuli and the habitat type (D) and water depth (E). Box specifications: mean (dashed), median (solid), 25th and 75th percentiles; whiskers: 10th and 90th percentiles. For consistency, all analyses included only fish species for which Fmax data were available for both dim/natural and bright light stimuli (75% of cases), and were obtained under dark adaptation (80% of eligible pairs; n = 35); different Fmax reports for a given species were averaged.