Association testing
Examination of the intercorrelation of predictor variables indicated that many of the morphological variables related to body size attributes were highly correlated, however, none had a significant correlation above 0.95 (Table S3 – S8), and therefore all were retained for association analysis. Significant intercorrelations among traits were consistent across samples and using these correlations we categorized traits into the following four main groups: 1) body size, 2) female sexual maturity, 3) larval growth, and 4) swimming ability. The “Body size” category included the body metrics of length, weight, girth, and interdorsal, which were all significantly positively correlated across datasets (Table S3 – S8). This trait category also included migration distance which was positively correlated with increasing body size metrics, and migration timing which was negatively correlated with these body size metrics. The male gonad mass was intended to serve as a proxy for male sexual maturity, but it also was significantly positively correlated with other body size metrics. However, the female gonad metric was not correlated with the other body size metrics and was potentially an accurate proxy for “female sexual maturity”. The “larval growth” category was populated by the only two measures of growth in the common garden experiment. The “swimming ability” category contained the three metrics of swimming performance which were all significantly positively correlated, and included migration day, which was positively correlated.
We examined the relative strength of associations of the total 302 SNPs with the traits within each of the four trait categories (Table S9), but were primarily interested in associations of SNPs on the four chromosomes chr01, chr02, chr04 and chr22, where evidence for range-wide adaptive divergence was concentrated.