Qifeng Ye

Partial migration: growth varies between resident and migratory fish

Partial migration occurs in many taxa and ecosystems and may confer survival benefits. Here, we use otolith chemistry data to determine whether fish from a large estuarine system were resident or migratory, and then examine whether contingents display differences in modelled growth based on changes in width of otolith growth increments. Sixty-three per cent of fish were resident based on Ba : Ca of otoliths, with the remainder categorized as migratory, with both contingents distributed across most age/size classes and both sexes, suggesting population-level bet hedging. Migrant fish were in slightly better condition than resident fish based on Fultons K condition index. Migration type (resident versus migratory) was 56 times more likely to explain variation in growth than a model just incorporating year- and age-related growth trends. While average growth only varied slightly between resident and migratory fish, year-to-year variation was significant. Such dynamism in growth rates likely drives persistence of both life-history types. The complex relationships in growth between contingents suggest that management of species exhibiting partial migration is challenging, especially in a world subject to a changing climate.

Long-term patterns in estuarine fish growth across two climatically divergent regions

Long-term ecological datasets are vital for investigating how species respond to changes in their environment, yet there is a critical lack of such datasets from aquatic systems. We developed otolith growth ‘chronologies’ to reconstruct the growth history of a temperate estuarine fish species, black bream (Acanthopagrus butcheri). Chronologies represented two regions in south-east Australia: South Australia, characterised by a relatively warm, dry climate, and Tasmania, characterised by a relatively cool, wet climate. Using a mixed modelling approach, we related inter-annual growth variation to air temperature, rainfall, freshwater inflow (South Australia only), and El Niño–Southern Oscillation events. Otolith chronologies provided a continuous record of growth over a 13- and 21-year period for fish from South Australia and Tasmania, respectively. Even though fish from Tasmania were sourced across multiple estuaries, they showed higher levels of growth synchronicity across years, and greater year-to-year growth variation, than fish from South Australia, which were sourced from a single, large estuary. Growth in Tasmanian fish declined markedly over the time period studied and was negatively correlated to temperature. In contrast, growth in South Australian fish was positively correlated to both temperature and rainfall. The stark contrast between the two regions suggests that Tasmanian black bream populations are more responsive to regional scale environmental variation and may be more vulnerable to global warming. This study highlights the importance of examining species response to climate change at the intra-specific level and further validates the emerging use of growth chronologies for generating long-term ecological data in aquatic systems.

Fish productivity in the lower lakes and Coorong, Australia, during severe drought

Anthropogenic modification of catchments and river flow can significantly alter estuarine habitats, hydrology and nutrient delivery with implications for fisheries productivity. The Coorong estuary at the terminus of Australia’s River Murray supports an economically important fishery as well as being recognised internationally as a critical site for migratory birds. Salinity near the Murray Mouth varies between fresh and marine depending upon river flow, but the Coorong becomes increasingly saline along its 120 km length. Freshwater flow to the Coorong is naturally variable but has significantly reduced by extraction for irrigated agriculture and domestic use upstream. Extreme drought from 2000 to 2010 and over-allocation of water resources resulted in the cessation of freshwater flow to the Coorong, significantly increasing salinity. During this period the diversity and abundance of organisms in the Coorong declined which reduced food web complexity. During lower flows the system generally becomes less productive as evidenced by: lower nutrient concentrations and loads, lower chlorophyll and primary productivity, a decrease in the abundance of fish-prey items (zooplankton, macroinvertebrates and small fish), a decrease in fish abundance, although this is not well reflected in fishery catch data because of the concentration of fishing in available habitat. The maintenance of flow is the only management strategy that stimulates recruitment, delivers nutrient resources to the estuary and ensures maintenance of habitable area by maintaining appropriate salinity.