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The Freshwater Fish Crisis

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Credit: Fera.
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A team at Exeter University used DNA barcoding techniques to uncover the true identity of what’s inside our batter, which was being passed off under false names such as “huss” and “rock eel”. The spiny dogfish, the type of shark being used, has been over fished for years as consumers have unknowingly favoured its taste and texture over other fish varieties.

Overfishing is one of the major causes of declining fish populations, and its impact extends further than just that of the dogfish. In fact, freshwater fish are threatened by a number of human activities, which can gravely impact the species and its associated food sources. 

Freshwater threats

The International Union for Conservation of Nature (IUCN) estimates that 126,000 described species rely on freshwater habitats, including fishes, invertebrates, mammals and plants. Most alarmingly, with 45 % of all fish species found in freshwater habitats, 200 of the 522 (38 %) European freshwater fish species are threatened with extinction and 12 are already extinct

Humans are hugely dependant on fish populations for food and income. In Africa alone, more than 7.5 million people rely on freshwater fish for both nutritional and monetary sustenance. Yet, it is human activity that is the biggest threat to freshwater fish populations. 

Dammed if you do

Dams have helped to irrigate farmland, prevent flooding and generate tremendous amounts of electricity. While it’s easy to see the positive impact that such construction projects have on our lives and environment, the damage they can do to freshwater fish is often ignored. 

Construction projects, such as canals, roads and dams, cause blockages that were not in place previously. In particular, weirs and other dam constructions fragment rivers, creating strings of artificial water bodies punctuated by barriers that fish are often unable to pass. This can block or delay upstream fish migration, causing populations to become isolated and more vulnerable to predation, disease and competition compared to the conditions the species has adapted to. 

Dams are also a primary driver of hydrologic change. Ecosystems are dependent on seasonal changes in river flows, which are often altered by dams. These changes alter the historical disturbance regime, rendering some biotic adaptations obsolete while favouring others. For example, reduced flow variability disturbs the way in which fish have previously adapted to seasonality, and leaves them unable to cope with such unexpected changes.

A paper published in Science in 2016, explored the unprecedented boom in construction of hydropower dams in the world's most biodiverse river basins. The Amazon, Congo and Mekong contain one third of the world's freshwater fish. While the planned construction of 450 new dams will meet the need for clean energy generation, has the impact on freshwater biodiversity been considered?

The authors of this paper propose that in order to maximise societal benefit and minimise environmental degradation, more comprehensive and rigorous impact analyses must take place prior to commissioning a new dam, not only to protect fish populations, but to lessen the effects of economic losses faced by communities. 

Invasive species

Invasive alien species (IAS) that have found their way into freshwater habitats also pose a threat to existing species. IAS are species that occur outside of their natural ranges as a result of human activity and are considered invasive because they increase in abundance at the expense of native species. The European Union (EU) has recently published its first list of invasive species of EU concern, which comprises of IAS known to pose threats to biodiversity, which includes the American bullfrog and the red swamp crayfish. 

There are currently over 14,000 IAS recorded in Europe, with more than half originating from outside EU countries. Freshwater communities are generally isolated environments, making them particularly vulnerable to the introduction of exotic species. Invasive species may displace native species due to competition for limited resources, they may prey upon species to the point of extinction or alter the natural habitat and make it impossible for native species to thrive. 

While globalisation has helped us take our goods, businesses and people to new borders, it is also responsible for the spreading of species. People, and the resources we use, travel around the world faster than ever and often carry uninvited species along with them. For example, ships can carry aquatic organisms in their ballast water, and smaller boats may carry them on their propellers. In addition, higher average temperatures and changes in rain patterns as a result of climate change have also disrupted ecosystems and caused flora and fauna to travel to new waters. 

Food sources 

Another threat to freshwater populations is pollution. This comes in many forms, including detergents, food processing waste and chemicals. However, it's not necessarily the direct exposure of pollutants to fish that can cause population depletion, but the exposure to their main food source — invertebrates. 

If invertebrate populations have been reduced or even eradicated from a habitat due to pollution, this can have detrimental effects on the entire ecosystem. Therefore, freshwater invertebrates play a direct role in fish wellbeing, and one that needs to be carefully considered.  

Under pressure

Global water demand is projected to increase by 55 % between 2000 and 2050, of which much of this demand is driven by agriculture, accounting for 70 % of global freshwater use. Why does the industry need an ever-increasing volume of water? To feed an ever-increasing population. 

As the agricultural sector aims to meet growing demand, there is a heavier reliance on plant protection products to defend plants from pests, often near to ponds, streams and other aquatic habitats. This could reduce invertebrate populations, and in turn freshwater fish populations, if the product isn't applied as per label instructions. 

Additionally, an increased use of pesticides furthers problems of resistance, meaning more products will need to be brought to market to replace redundant alternatives. But, fundamentally, the right considerations must be taken to protect freshwater populations. Pesticides must be assessed effectively, but fairly, to ensure the industry has a safe selection of products available. 

Fera, in partnership with the Centre of Crop Health and Protection (CHAP) and Innovate UK recently launched the E-Flows mesocosm, Europe's largest and most advanced, fully flow-through mesocosm. 

This outdoor experimental system allows research to take place on invertebrates (and aquatic plants) in highly realistic conditions, due to the large water volume it can support and its fully flow-through capabilities. This means the effects of pesticide exposure on invertebrates can be predicted, and, in turn, the food source of freshwater fish is protected.

While freshwater fish are the most endangered group of animals on the planet, it's important to take into account the entire ecosystem, including invertebrates, when predicting the effects of construction and pollution. As the global hydropower boom persists, climate change continues to lead to species migration and the growing population drives increased demand in agriculture, effective research will remain vital to predict effects to aquatic populations.