Figure 1

Silver Carp prefer shade structures during daylight hours. (A,B) Density maps of two fish positions from two different trials and ponds. Orange hues indicate positions recorded during daylight hours, while purple hues represent positions collected at night. The gray solid lines represent the pond limits and the thick green squares indicate the actual location of the shade structure in the particular trials and ponds shown in the top left of the panels. The dashed squares indicated by lowercase letters (from ‘a’ to ‘e’) represent all possible shade positions. We observe that the fish in (A) has an extreme preference for the shaded area during daylight hours, while the same preference is absent for the fish in (B). (C,D) Differences in shade selection as a function of the hour of day. The circles show the fraction of all fish positions recorded under the shade structures within a 1-h time window for two trials and ponds. The dashed line represents the expected fraction of positions under shade if fish would move randomly over the ponds, and the background colors indicate daytime (orange) and nighttime (purple). Fish from pond #2 remained under shade during nearly all daylight hours along the two observation days in trial #1 and avoided shade at night. Fish from pond #4 selected the shade structure much more rarely during daylight hours (particularly in the mornings) in trial #6. (E) Probability increment of finding a position under a shade structure during daylight hours, as estimated via logistic regression for each trial and pond. The relationship between a fish selecting shade or not and the binary independent variable of day or night varies substantially but is nonetheless significant for all trials and ponds (p-values \(<0.001\)).