3.2 | Genotype-environment association
The results of LFMM analysis linking environmental and phenotypic
variables to SNP variants indicated similar patterns of association with
latitude and collection date in both years (Table 4). Of the 76 SNPs
associated with these variables in 2014 and 305 in 2015, ten were shared
between years (Supplementar materials, Table 1). The loci common in both
years were significantly associated with latitude and collection date
only. However, because of the sampling being conducted in a generally
south to north direction, sampling date and latitude are collinear. This
may possibly indicate a temporal gradient of selection where less fit
individuals, those with deleterious alleles die off during their first
months of life. Therefore the fish collected at later dates may be a
subset of the fitter individuals as compared to earlier collections. Or
there may be a true latitudinal gradient, or a combination of both
factors contributing in various proportions to a selection gradient.
Chlorophyll concentration and seawater temperature did not appear to
influence loci in 2014, but were associated with 100 loci in 2015. It is
important to note the Gulf of Alaska experienced unusually warm
temperatures in 2015 (Cavole et al. 2016), marked by large sea bird
die-offs (Jones et al. 2018).
In 2015, the fish experienced poorer growing condition as compared to
2014 (Cavole et al. 2016). This was evident in their weight for a given
length when examining the condition index graphs (Figure 4). Linear
regression analysis indicated a significantly (p<0.05) lower
intercept and steeper slope in 2015 suggesting that smaller fish had
poorer condition in 2015, but larger fish appeared to be unaffected.
Whether the smaller fish died off and only larger fish survived is
uncertain, although there appears to be a genetic basis of selection
where a number of loci were identified as being associated with fish
body condition (% lipid and condition index). This was not observed for
the fish collected in 2014.