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).