Vateritic sagitta in wild and stocked lake trout: Applicability to stock origin

Aragonite is the normal form of calcium carbonate found in teleost otoliths , but it is sometimes replaced by vaterite, an alternate crystalline struct ure. We investigated the assumption that sagittal otoliths with vaterite re placement were unique to stocked lake trout Salvelinus namaycush in the Lau rentian Great Lakes. Earlier studies had attributed these abnormalities to stocking stress, and proposed that the presence of vaterite could separate individual unmarked stocked lake trout from their wild counterparts. We exa mined and described the frequency of vateritic sagittae in two wild and thr ee stocked populations of lake trout from the Great Lakes and a wild popula tion from a remote inland lake in northern Canada. Among lake trout caught 2-12 years after being stocked, prevalence of vateritic sagittae was 66% fo r Lake Superior fish, 75% for Lake Huron fish, and 86% for Lake Ontario fis h. Among wild fish caught, vateritic sagittae were present in 37% of Lake S uperior fish, 22% of Lake Huron fish, and 49% of northern Canada fish. We a lso compared year-to-year differences in prevalence in four year-classes of fingerling lake trout reared in two U.S. national lake trout hatcheries. P rior to release, between 53 and 84% of the hatchery fish had at least one v ateritic sagitta, and prevalence increased with handling associated with ha tchery practices. Vateritic sagittae in wild fish might also indicate stres s in nature. The presence of vateritic sagittae in both wild and stocked fi sh compromises the use of this characteristic as an unequivocal indicator o f a particular fish's origin. Among-population differences in both the prev alence and the extent of vaterite replacement, however, may provide a means of differentiating between stocks of sympatric unmarked wild and stocked l ake trout.