Social hierarchies override environmental conditions in determining body coloration of brook trout

Dynamic changes in body color are commonly used as a means of intraspecific communication and for crypsis. It is well established that subordinate fish signal to dominant fish by darkening their body color. Similarly, salmonids can adapt their body color to match their environment. What is not known is whether social interactions or environment plays a larger role in driving dynamic changes in body color, as these factors rarely occur in isolation. Experiments conducted in artificial stream channels with wild brook trout tested for the potential of color matching by varying light and dark substrates and included treatments with individuals as well paired cohorts to test for the effect of social hierarchies. Fish displayed a broad ability to adapt their coloration to match a range of substrates, with a strong preference for dark substrate. In paired trials, dominant fish matched light substrate more effectively while subordinates displayed darker coloration, resulting in subordinates being poorly matched to their surroundings. Mismatching the environment in order to send appropriate social signals could have negative consequences that further reduce fitness in subordinate fish