Climate change threatens fish extinction across the world

By Nyasha Rugwete and Milton Tinashe Makumbe
Climate Change will result in many amphibians, mammals, birds and fish migrating to favourable habitats outside their normal ranges that is, if they can find the space.


The biggest concern may be the chances of survival for aquatic organisms as they live in a confined environment. The warming rates being predicted by the Intergovernmental Panel on Climate Change (IPCC) may result in some fish species going extinct.

Sensitivity to temperature changes varies greatly between ocean-dwelling and freshwater fish.Fish are not spared from having to cope with Climate Change. They have the unique challenge of either making rapid movements to track their temperature requirements or adapting quickly. There has been a pole ward extension of marine plankton species and fish species.

In the English Channel, most of the cod fish has moved north to cooler waters and there is an influx of other marine life from elsewhere. Sharks generally do not like cold waters but Bluefin tuna, Triggerfish and Thresher sharks have been spotted in the English Channel around Hampshire, the Isle of Wight, and Dorset.

Marine scientists are further predicting a large increase in their numbers in the next 20 years. There is no doubt the English Channel has warmed up. Fish in the tropical oceans are already living in warmer waters that are within the upper range of their tolerance. Fish species living in these habitats may not have much room to adapt when temperatures increase slightly.

The effects of Climate Change are by contrast, acutely felt in high latitudes by fish species that are accustomed to cooler water temperatures, with fish in those streams that have a smaller window for survivable temperatures.

With increasing temperatures, fish will either have to migrate or adapt. Failure to migrate or adapt will result in extinction of some species. Water temperature is key in moderation of fish body temperature. Important biological processes such as growth, reproduction, swimming ability and fish behaviour can be adversely affected by a continued rise in water temperature.

Climate Change will have an impact on fish population dynamics since sex of fish is affected by temperature. Climate Change will ultimately result in altered phenology, abundances, productivity, community compositions, distributions and migratory patterns. Oceans are the biggest sinks of carbon dioxide (CO2), absorbing roughly 25% of the CO2contributed by to the atmosphere by humans.

Carbon dioxide will form CO2 carbonic acid when dissolved in water. Increased CO2 emissions increases the amount of CO2dissolved in water bodies resulting in water acidification. A change in ocean chemistry through increased water acidification negatively impacts fish reproductive processes.

Fish population may decline due to acidification as fish eggs are more sensitive to pH changes than adult fish. A productive and economically significant catch may be at risk because of acidosis as a result of ocean acidification. For example, the United States relies heavily on the shellfish and fish catches from the North Pacific.

The decrease in pH of ocean waters is affecting species such as mussels with carbonate shells as well as coral reefs. Coral reefs provide a habitat for a diverse population of aquatic organisms. However, coral reefs are very sensitive to acidity. An increase in water acidity is resulting in coral bleaching greatly reducing species diversity.

Ocean acidification leads to abundance of carbon in the food chain impacting on fish species through a loss of habitat and reduced food availability. Climate Change brings with it increased occurrences of extreme events such as flash floods. This may result in nutrient rich water from agricultural lands, dump sites and sewage works reaching our lakes and rivers.

Eutrophication in these lakes leads to algal blooms which reduce oxygen availability giving rise to fish deaths. Algal blooms also reduce water visibility. Reproduction of the Lake Victoria cichlids fish has been greatly affected due to reduced visibility. These cichlids choose their mates depending on colour patterns.

The reduced visibility has been identified as the cause of interbreeding within the species to the extent of endangering speciation in Lake Victoria, reducing the lake’s biodiversity. Water is a poor heat conductor and air circulation is needed to spread the heat to the deeper parts of lakes and oceans.

Wind circulation forces the lighter and warmer air below the colder and denser water. During the cold months, as the upper layer loses heat it becomes cold and dense causing an overturn.

This process ensures distribution of nutrients in the water body. As temperatures continue to rise due to Climate Change, there is complete stratification, meaning there is no nutrient distribution. Fish such as the mackerel depend on these thermocline circulations for their feed requirements.

Longer stratification periods may result in warmer waters at the bottoms of lakes and oceans. The bottom layers may become richer in nutrients as organisms that die sink to the bottom releasing the nitrogen and phosphorus acquired through life processes. This eventually increases oxygen depletion creating dead zones at these bottom layers.

Climate Change has been attributed to the decline in kapenta catches in Lake Kariba located on the Zambezi River which flows between Zimbabwe and Zambia. The diet of kapenta is dominated by tiny microscopic animals called zooplankton which graze on palatable green algae. Green algae thrive well at temperatures below 25 degrees Celsius.

Lake Kariba temperatures have been increasing favouring the dominance of blue-green algae which is not palatable. The dominance of blue-green algae leaves zooplankton with less food which is reflected in low kapenta catches currently being experienced in Lake Kariba.

Kapenta is a fish species of economic importance that supports household food and nutrition security as well as employment such that any factor that affects their catches affects many livelihoods. Many closed basin lakes dry up periodically. The frequency however, has increased due to Climate Change with some lakes reducing in size and depth.

Lake Chilwa in Malawi is an endorheic lake with no obvious outlet. The lake is highly productive with fish species such as tilapia, Shiranus chilwae andImberi, providing around 20% of all Malawi’s fish requirement. Up to 1.5 million inhabitants from southern Malawi districts benefit directly from the 2,400km2lake through agriculture and natural resource goods and services that generate an estimated US$21 million per year.

This lake has been known to dry up with the most recent being in 1995 following a drought. The biodiversity in the lake as well as livelihoods for households relying on the lake have been greatly affected. Resolving the effects of Climate Change on fish populations is complicated due to the fact that it affects different environmental factors.

These factors in turn affect various processes at different levels of biological organisation. Climate Change effects will also vary between the oceans and coastal waters. Habitats that are not overfished are likely to be more resilient to Climate Change impacts.

There is therefore need for monitoring, control and surveillance so as to improve the adaptive capacity of aquatic habitats to Climate Change. Improved weather information and storm warnings are some strategies needed to aid in disaster preparedness and response planning.

Adaptive measures also include species selection, selective breeding, and genetic modification.

About the writers: Milton Tinashe Makumbe is the chief livestock research officer and acting head of Henderson Research Institute in Zimbabwe while Nyasha Rugwete is the principal livestock research officer at the Institute.



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