Abstract
Tagging and stable isotopic studies suggest that blue cod (Parapercis colias) in Fiordland form discrete sub-populations at the scale of tens of kilometres with a high degree of residency, particularly in the inner fjord basins. Observations of a bias in movements towards the heads of the fjords has led to the hypothesis that inner fjord sub-populations function as sinks, into which a small but non-trivial proportion of the more mobile and interactive outer fjord and outer coast populations drain. Limited movement between fjord basins suggests that populations could be better managed at a smaller spatial scale than the current management regime dictates.
Here, multiple lines of evidence are used to investigate population structure and connectivity in the southern fjords. A source–sink metapopulation structure hypothesis is tested by comparing demography, trophic ecology and otolith microchemistry between paired inner and outer fjord sites. Age and size structure, growth rates and sex ratios were consistently different between inner and outer fjord habitats; inner fjord sub-populations were typically slower growing, female-dominated and included a higher proportion of older, larger individuals, while in the outer fjords the sex ratio was closer to 50:50 or male-dominated and fish were on average smaller and younger. Stomach content analysis revealed consistent differences in diet composition, diversity and energy equivalent between habitats; inner fjord diets tended to be benthic in nature, more diverse and nutritionally poorer than in the outer fjords, where abundant, energetically rich pelagic food resources predominated. Stable isotopic ratios reflected sustained trophic uncoupling between habitats over a period of months to years. Despite this spatial pattern in diet, asynchronous growth rates between habitats were largely associated with sex and not trophic ecology.
Otolith trace elemental composition differed between paired inner and outer fjord habitats in Bradshaw–Thompson, Breaksea and Dusky Sounds; three discrete groups of fingerprints within Dusky Sound corresponded to inner, mid and outer fjord signatures, implying limited mixing between sub-populations over the timescale of months to years. The association between microchemical signature and local environmental conditions was confirmed by translocation studies for both blue cod otolith and mussel shell tissue. Temporal variability in elemental incorporation into otoliths prevented comparison across years; ontogenetic trends in the incorporation of certain elements necessitated demography to be considered when comparing between sub-populations.
Fecundity was related to maternal characteristics, in particular female age; demography exerted a stronger influence on the reproductive potential of a population than did population size. The higher reproductive potential of inner fjord populations counters the sink functioning implied by tagging and stable isotopic studies. Although the hypothesised source–sink structure cannot be confirmed or refuted without better understanding of spawning behaviour and pelagic dispersal trajectories, these findings highlight the importance of including demographic as well as biomass information in fisheries models and the potential for closed areas to function as net exporters of reproductive potential.
These results help further our understanding of the structure, trophic ecology and connectivity of blue cod populations in Fiordland and have important ramifications for the effective management of blue cod stocks in enclosed waterways.