Within-lake length adjustment improves otolith shape discrimination among lake trout Salvelinus namaycush populations

Distinguishing fish populations is a long-standing challenge in fisheries management, and otolith shape provides a low-cost marker of population structure. Fish size strongly influences otolith shape; however, within-lake size variation can obscure differences among populations. Here, a within-lake length adjustment of otolith elliptic Fourier descriptors increased the otolith shape differences detected among three populations of Manitoban lake trout Salvelinus namaycush (n = 153). The adjustment estimated and removed the relationship between each descriptor and fish total length within each population while retaining that population's mean shape. Lake of origin explained 3.2% of otolith shape variation before adjustment and 4.9% after, and all three pairs of lakes differed significantly after adjustment compared with one of three before. Classification assigned fish to the three lakes more evenly after adjustment, with correct assignment of Second Cranberry Lake fish rising from 13% to 34%. The within-lake adjustment recovered more of the shape variation among lakes than the shapeR package while keeping all 28 Fourier coefficients. Reconstructed mean outlines identified Clearwater Lake otoliths as the most distinctive, carrying a more pronounced rostrum than the two other lakes. Despite a shared stocking history among the three lakes, otolith shape still differed among them, consistent with growth environment as a source of these differences. The within-lake length adjustment provides a low-cost description of population structure for inland fisheries that rely on archived otolith material.