Site selection for otoliths (hypotheses 3,4)
To determine short-term population structure, and allow comparisons with
genetic results, within-lake and coast patterns were evaluated by
examining otolith trace element signatures from juveniles returning to
streams sampled from a subset of sites used in the genetics study.
Otolith trace element signatures provide a measure of potential mixing
or separation of populations from a given tributary (Campana, 1999). As
returning juveniles are all from the same cohort, and otolith trace
element signatures are influenced by a large number of factors
(Elsdon & Gillanders, 2005;
Grammer et al., 2017; Jaecks, Bond, & Quinn, 2016; Thomas, Ganio,
Roberts, & Swearer, 2017), the observation of a similar otolith trace
element signature in all juveniles from a given location suggests a
common environment and therefore implies recruitment from a single
larval pool (Campana, 1999), most likely indicating restricted dispersal
from the natal stream. Wide variation in juvenile otolith trace element
signatures suggests variation in larval environment and therefore larval
recruitment from a diverse range of sources (Gillanders, 2005).
Comparison of trace element signatures in juveniles collected from
different streams can thus indicate whether larvae developed in one
common pool or many (Campana, 1999; Carlson, Fincel, & Graeb, 2017;
Gillanders, 2005; Grammer et al. 2017). However, as water chemistry is
influenced by geology, regions with similar geologies may be difficult
to distinguish. To allow comparison with genetic structuring, the five
tributaries of Wanaka and Wakatipu used in the genetic portion of this
study were again selected as sampling sites for otolith trace element
analyses. Due to logistic and financial constraints, only five coastal
streams grouped into three broader regions (Region one: Breccia Creek,
Grave Creek; Region two: Thirteen Mile Creek, Kararoa Creek; Region
three: Fox River) were selected from the 16 sites used in the genetics
portion of this study. These coastal streams were selected to have
similar catchment size to the tributaries selected within Wanaka and
Wakatipu, and to span as much of the north-south range as possible (Fig.
1).