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

Abstract

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.