News & Insights

The Global Decline of Freshwater Fish

Freshwater habitats only occupy about 1% of the earth’s surface but provide habitat for over half of the known fish species

Freshwater fish populations are very important for maintaining ecosystem functions such as grazing problem algae, controlling populations of invertebrates or smaller fish, or transporting nutrients between different habitats such as the ocean and headwater streams.

Unfortunately, the unprecedented level of environmental pollution and habitat modification are having significant adverse effects on freshwater fish populations. This is especially pronounced in fish species that migrate, as their lifecycle strategies are highly dependent on several aquatic habitat types across large areas, all of which may have been modified.

A recent global assessment shows that migratory freshwater fish populations have declined by 76% in the last half-century. This rate of decline is greater than that of migratory marine or terrestrial species. The main causes are thought to be climate change, habitat reduction and pollution, invasive species, disease and overfishing. This global study showed that within Oceania nearly 58% of the threats to migratory freshwater fish are due to water quality degradation, followed by change and loss of habitats, and another 25% is due to the threats caused by exploitation. This study included two New Zealand species of eel and banded kokopu.

There are still major gaps in understanding, including the potential widespread impact of climate change on migratory freshwater fish within our region. More work is needed to understand the effects on fish populations and to develop pragmatic solutions to protect them. Priority actions for protecting migratory freshwater fish populations include improved monitoring systems, protection and enhancement of river connectivity, protection and restoration of habitat, increasing the public and political interest in the protection of freshwater fish, and ongoing conservation programs.

At Cardno, we can help you assess which fish species occur within your region or site, whether they are migratory species, potential risks to these species, and develop pragmatic solutions to help protect or increase populations of freshwater fish.


Bram Mulling

Freshwater Ecologist

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Bram has conducted a wide range of ecological freshwater studies in New Zealand and The Netherlands. Bram has been involved in academic research, resource management, compliance monitoring and environmental awareness programs throughout his career. His research interests are in ecosystem functioning, population dynamics, ecological engineering and constructed wetlands.