Sampling
Tubesnout and threespine samples were collected between May - August 2017 from the West Coast of North America using dip netting and minnow traps. These fish were euthanised in the field using a mixture of 0.5g/L MS222 (Ethyl 3-aminobenzoate methanesulfonate) in sea water, the carcases were then preserved in 95% ethanol which was replaced after 24 hours. The northern populations of both species and all ninespine samples were donated by collectors. Between 30-52 fish were collected per population (Table 1), the specific details of sampling locations are included in Table S1, and population labels are described in Figure 1.