Drop in freshwater fish

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Freshwater communities of fish and invertebrates in rivers, swamps and floodplains are likely to experience significant declines as most species, like the Murray cod pictured, have specialised habitat, breeding and dietary requirements.

Opportunities for recreational fishing in inland NSW are likely to be reduced - especially in highland trout fisheries - if current climate change scenarios are realised.

Reduced stream flow and more concentrated fishing effort in remnant waterholes are areas of concern for recreational fishers, and a much reduced commercial carp fishery is also likely.

While aquatic life in Australia is generally well-adapted to short term climate variability, few - if any - species can cope with longer term shifts in mean climate or the more intense extreme events currently predicted.

The scenario for NSW freshwater aquatic systems is for the long-term drying of aquatic areas, more droughts and higher water temperatures with diminished water flows.

This will produce low oxygen levels and increased conductivity (salinity).

Freshwater communities of fish and invertebrates in rivers, swamps and floodplains are likely to experience significant declines as most species have specialised habitat, breeding and dietary requirements.

Compared to the open estuaries and ocean waters, freshwater rivers are geographically constrained.

They limit the migratory options for aquatic plants, invertebrates and fish.

With low or reduced flow, freshwater river systems will shift from running water to pool environments with a corresponding shift in the biological communities which may favour some exotic species.

Freshwater flows are a stimulus for breeding in many Australian freshwater fish species and thus the changes in volume and timing of spring floods are predicted to significantly reduce opportunities for spawning and dispersal migrations.

Some species undertake obligatory migrations to complete essential life history stages.

Reductions in rainfall and altered flow regimes will interrupt these processes and threaten many species.

Distributions of a number of currently-listed threatened species are limited to a small number of key areas.

These species will also have an increased risk of extinction as climate change is likely to further limit the availability of suitable habitat.

In freshwater systems, water temperature stress will be most marked for freshwater invertebrates and fish which tolerate limited ranges in temperatures.

This will especially affect the species which live at higher altitudes and will impact on the very economically valuable freshwater trout fishery.

Native species and the breeding and farming of trout and salmon in these highland regions will likely be negatively affected and the species may face local extinctions, if temperature regimes substantially change.

What drives disease outbreaks in aquatic environments is generally poorly understood, but there are three important interactions:

  • Factors associated with the pathogen (e.g. virulence, dose, abundance).
  • Factors associated with susceptibility of the host (including general health, stress levels, immunity) and
  • Environmental factors influencing host/pathogen interactions.

Where global warming provides conditions which favour a given pathogen and/or weaken the corresponding host (eg. the stress associated with changes in optimal habitat and feeding conditions), disease outbreaks are more likely to occur.

There are variable consequences for exotic and pest species. Numbers of some species, like carp, may diminish because of reduced spawning opportunities as floodplains contract.

In contrast, further increases in water temperatures and different water flow regimes may favour expansion of smaller invaders like mosquito fish, Gambusia holbrooki, and new invaders like the aquatic plants water hyacinth and the fast growing hardy weed Lippia.