Abstract
Identifying the connectivity and habitat use patterns of fish populations can enhance knowledge on the productivity and resilience of fishes. Flounders (Rhombosolea spp., pātiki) in New Zealand are estuarine-dependent marine species that rely on successful immigration and emigration from estuarine nurseries to maintain recruitment. The present thesis aimed to utilise natural tags, including morphometrics, otolith microchemistry, and stable isotopes, to determine the population structure, ontogenetic connectivity, and habitat linkages of flounder species in east Otago, New Zealand.
Morphometrics and meristics have been widely used to identify the structure and connectivity of fish populations. Intra-species variation in dorsal and anal fin rays of juvenile (age-0+) sand flounder (Rhombosolea plebeia) supported existing knowledge of two genetically distinct populations in New Zealand. Spatial and temporal variability in the eye diameter of sand flounder collected in east Otago estuaries demonstrated the influence of environmental variation and adaptive physiology on morphometric features.
Elemental signatures in otoliths and stable isotopes in muscle tissue and eye lens were used to infer linkages between juvenile flounder, their environment, and connectivity to the adult sand flounder population in east Otago. Distinct elemental signatures in otoliths and isotopic ratios (δ13C and δ15N) in muscle tissue and eye lens were found in juvenile sand flounder collected from different estuarine nursery habitats in east Otago. Shallow intertidal dominated estuaries (SIDEs) and Otago Harbour, a deep, subtidal dominated, longer residence time estuary (DSDE), had higher strontium and lithium concentrations in otoliths and higher δ13C in soft tissues compared to shallow, short residence time river and tidal river with adjoining lagoon estuaries (SSRTREs) which had higher manganese and barium concentrations in otoliths and lower δ13C in soft tissues. The region of adult (age-1+) sand flounder otoliths that corresponded to the juvenile life stage was characterised to an east Otago estuary, assigning the majority of adults to SIDEs (48/78), followed by Otago Harbour (25/78) and SSRTREs (5/78). Stable isotope analysis of the eye lens ‘core’ revealed an ontogenetic increase in the reliance on pelagic resources and trophic level in adult sand flounder from SIDEs and Otago Harbour, while no ontogenetic change was detected in adults from SSRTREs.
Trace elements in otoliths and stable isotopes in tissues of juvenile flounder were also utilised as environmental indicators, using a fish kill event in Kaikorai Lagoon as a case study. Higher otolith manganese concentrations, lower δ13C values, and higher δ15N values present in Kaikorai Lagoon compared to other estuaries in east Otago were indicative of eutrophic and hypoxic conditions. Following the fish kill event, a significant decrease in δ13C values indicates that basal resources in the estuary were altered.
The results suggest SSRTREs have reduced nursery function for juvenile sand flounder in east Otago, which may be linked to the lower proportion of benthic primary production and/or the increased severity of hypoxic events. Applying natural tags on various estuarine-dependent flounder species in New Zealand should be encouraged to reveal population structure, connectivity, and environmental linkages.