Site selection for population genetics (hypotheses 1, 2)
Genetic structuring was evaluated within and among lake tributaries, and among coastal stream populations. Pairwise comparisons among lake tributary populations, and between lake tributary and coastal stream populations were expected to show the highest levels of structuring. Coastal streams, and catchments with lakes were selected along an approximately 450-km north-south range of New Zealand’s West Coast (total eight lakes and 16 coastal sites, Fig. 1). Six lakes in four West Coast river catchments (Buller River - Lakes Rotoiti, Matiri, Rotoroa; Grey River - Lake Christabel; Paringa River - Lake Paringa; Moeraki River - Lake Moeraki) were selected based on varying proximity to the coast, ranging from only a few kilometers for Paringa and Moeraki to a substantial distance inland for the others (> 80 km). To help determine the cause of structuring, lake tributaries were paired with coastal streams downstream of the lake in the same river catchment as close as possible to the lake outlet as well as open coastal sites located in different catchments lacking lakes (n = 16 coastal sites total [8 downstream of lakes, 8 open coast]; Fig. 1).
To add a further level of potential isolation, two lakes draining to the east coast of New Zealand were included (Lakes Wanaka and Wakatipu). Both lakes are located approximately 300 km from the coast and upstream of two large hydroelectric dams (both < 80 years old) creating impermeable barriers to migration from coastal populations. There is, however, no barrier to migration between Lakes Wanaka and Wakatipu. Potential within-lake structuring was assessed in Wanaka and Wakatipu because of their large size (lake area > 170 km2; max depth > 300 m) and multiple spatially isolated inflowing tributaries. To conduct within-lake comparisons, five tributaries spanning the length of both lakes were selected and also used to test connectivity across shorter timescales using otolith analyses (Fig. 1).