Results
Genetics
All loci were highly polymorphic, ranging from 21 alleles atGbr 12 to 32 at Gvu 05. Population meanH o ranged from 0.64 to 0.83 (mean 0.76) andH e from 0.57 to 0.82 (mean 0.75; Table 1).
Allelic richness across all loci tended to be lower in lake-developing
populations, ranging from 2.8 in Lake Christabel to 4.9 in Lake Wanaka,
while coastal populations ranged from 3.9 to 6.3. Departures from
Hardy-Weinberg equilibrium were found in 49 out of 240 locus-population
combinations before stepwise Bonferroni correction. As 11 of these
occurred at Gbr 130, and 21 occurred at Gbr 140, these loci
were excluded. Bonferroni corrections were then applied with only 1 of
192 remaining out of equilibrium after stepwise Bonferroni corrections
(p < 0.0003). No locus was affected by null alleles in
any sample, so all remaining loci were used in the study.
All landlocked populations exhibited a higher degree of genetic
structuring on a lake by lake basis, when compared to diadromous
populations (Fig. 2a), regardless of distance from coastal populations
(Mantel p = 0.07; see Supplementary Tables A1, A2 for full
results). Landlocked systems had pairwise F STvalues of approximately 0.03–0.06 when compared to immediate downstream
sites less than 5 km away (Fig. 2a,b). Lakes located in the same
catchments (L. Rotoiti, L. Rotoroa), had lowerF ST values, resulting in a significant
relationship with increasing distance (Mantel p = 0.04).
Comparisons among coastal streams tend to have lowerF ST values, but significantly increase with
geographic distance (Mantel, p = 0.04). High structuring levels
were present in these comparisons even when geographic distance was
relatively small and sites were located in the same catchment, such as
an F ST value of 0.05 for Smokey and Stoney
Creeks, 18 km apart. Structuring within the respective tributaries of L.
Wanaka and L. Wakatipu was relatively low, comparable to that of among
coastal stream comparisons (F ST = 0.01–0.03),
and no structure was shown by STRUCTURE analysis (Supplementary Fig. A2;
Supplementary Table A3).
Patterns present in F ST scores were corroborated
by patterns identified using STRUCTURE (Fig. 3). STRUCTURE initially
split populations in the tributaries of east coast lakes (Wanaka and
Wakatipu) from all sites at K = 2. At K = 3, the
tributaries of west coast lakes were split from coastal west coast
sites. Tributaries of L. Paringa and L. Moeraki split at K = 4
with tributaries of Lake Cristabel beginning to split at K = 5.
Tributaries of L. Wanaka and L. Wakatipu are clearly split at K =
6, which was determined as the most likely population number using
Evanno’s method. At K = 9 both tributaries of both L. Moeraki and
L. Cristabel are clearly split form the other west coast lakes (Fig. 3).
Within lake STRUCTURE results did not show distinct population
clustering for tributaries of L. Wakatipu or L. Wanaka (Supplementary
material, Fig. A2).
Otoliths
The larval development period of all otoliths from juvenile G.
brevipinnis collected from lake tributaries lacked the high Sr:Ca
ratios indicative of a diadromous life-history, whereas those from
coastal populations all displayed high Sr:Ca and low Ba:Ca. That is, all
individuals in lake tributaries had developed in freshwater, while all
those caught from coastal streams had developed in the ocean. Linear
discriminant analysis suggested that (in order of importance): Rb, Ba,
Sr, Mg, Cu, Ni were useful in stock discrimination, with linear
discriminant (LD1) explaining > 67% of variation in all
systems (Table 2).
At the largest (system) spatial scale, evidence of population
structuring was clear, with coastal, Wanaka, and Wakatipu samples all
forming distinct clusters (Fig. 4a), with reclassification to system of
capture 96% successful (Table 2). At a finer scale (the a prioriregions within each lake and along the coast), regional clusters were
generally well defined (Fig. 4b), and reclassification success rates
were again high: 94% for coastal regions, 86% within Wanaka and 71%
within Wakatipu (Table 2). At the finest spatial scale, individual
sampling sites still show distinct, though less well-defined, clustering
(Fig. 4b), and reclassification success rates for coastal, Wanaka, and
Wakatipu sites were 62.5%, 78.6%, and 60% respectively (Table 2).
Reclassification success was reflected in the LDA plots, with clear
clustering present at high reclassification levels (e.g. Wanaka), and
greater overlap occurring with lower reclassification levels (e.g.
Wakatipu) (Fig. 3b).
Trawling
Trawling in the lakes collected a total of 1,991 larvae, with 95% being
collected in river plumes, 2% in nearshore areas and 3% in offshore
areas. All large river plumes had significantly higher densities of
larvae than trawls collected from offshore or nearshore sites. There
were no significant density differences among small river sites,
although the highest densities were recorded in the plumes associated
with Buckler Burn and Albert Burn (Fig. 5.) Statistical outputs relating
to larval trawling are available in Supplementary material (Appendix 1,
Table A4).