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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