Predation by Captive Wild Brook Trout on Calcein‐Marked versus Nonmarked Atlantic Salmon Fry

Juvenile Atlantic salmon Salmo salar and other fish species marked with the fluorochrome dye calcein exhibit a green fluorescence in fin rays and other calcified structures under specific optical conditions. To determine whether brook trout Salvelinus fontinalis would preferentially prey upon calcein‐marked individuals, we introduced calcein‐marked and nonmarked Atlantic salmon fry simultaneously to captive wild brook trout in four controlled indoor raceway trials. Each trial consisted of 2 brook trout and 100 each of calcein‐marked and nonmarked Atlantic salmon fry; no individuals were used in more than one trial. At the termination of each 3‐d trial, predators were removed from raceways, and surviving Atlantic salmon fry were examined with a calcein detection device to tally numbers of marked and nonmarked individuals. In individual trials, 2 brook trout consumed between 20 and 99 Atlantic salmon fry over a 3‐d period (10–49% of available prey). Replicated goodness‐of‐fit (G‐statistic) analysis showed the number of calcein‐marked and nonmarked Atlantic salmon fry eaten by captive wild brook trout did not fit the expected 1:1 ratio among all the trials (P < 0.05). However, pooled data from the four trials showed that the numbers of marked and nonmarked Atlantic salmon fry eaten by predators were nearly equal at 139 and 133 individuals, respectively. Correspondingly, G‐statistic analysis of pooled data showed that numbers of marked and nonmarked Atlantic salmon fry eaten by brook trout did fit the expected 1:1 ratio. Overall, we found no conclusive evidence that calcein‐marked Atlantic salmon fry were preferentially preyed upon by captive wild brook trout. Our experiment also demonstrates the utility of using calcein to mass mark Atlantic salmon fry as a means of performing evaluations that were not practical with previous tagging and marking methods.