Climate change that is linked to the build up of greenhouse gases and aerosols in the atmosphere has led to increases in the earth’s surface temperatures over the last 50 years. As a result the water in the world’s rivers, estuaries and the sea are also heating up. Fish are more susceptible to changes in temperature than many land-based animals.
Because their body temperature is the same as the water around them, fish cannot maintain a constant body temperature and cannot survive in temperatures too far out of their normal range. Consequently, of all of the physical stressors associated with climate change, temperature is considered to have the most impact on coastal fish.
As water temperatures increase, the metabolism of the fish increases. They need large amounts of oxygen to fuel this high metabolism and if not enough food is available then all the fishes’ energy goes into fuelling their high metabolism. This leaves them with no energy for growth and reproduction. There may also not be enough oxygen available because the amount of oxygen dissolved in water decreases as temperature increases. As a consequence worldwide, species are moving out of their normal ranges to more favourable habitats as waters get warmer.
Recent studies have shown that surface waters along South Africa’s subtropical east coast are warming significantly and this has been linked to warming and strengthening of the Agulhas current. In contrast, sections of the country’s south and west coast are cooling seasonally as winds that favour upwelling increase.
South Africa’s coast is different
South Africa is an interesting place to study the effects of climate change on marine species. The coastline, which is roughly 3000 km, is very different on each side of the continent. The west coast of South Africa is surrounded by the cold Benguela current. The Benguela Current is a nutrient-rich upwelling current. Upwelling involves the wind-driven movement of dense, cooler and usually nutrient-rich water towards the surface. Plankton grow in these fertile waters, providing food for fish. Although few species can tolerate the cold water, the cold-water tolerant or temperate species that do occur here are found in large numbers and are the basis for South Africa’s commercial fisheries.
On the east coast, the Agulhas current brings warm water from the tropics which is not very rich in nutrients. The climate is subtropical and the fish fauna are dominated by tropical or warm-water species. Although a greater variety of species is found along this coastline, the nutrient-poor water means that they do not occur in large numbers. On the south coast, the warm Agulhas Current moves further offshore and cooler columns of water rising from the depths of the ocean, called upwelling, also occur in some areas. Here a mix of tropical and temperate species occur.
What does this mean for South Africa’s fish
In response to warming waters, changes in the distribution and abundance of tropical and temperate species have already been recorded. Studies have found an increase in tropical fish species in the East Kleinemonde Estuary. This is located on the coast in the eastern part of the country known as the Eastern Cape.
The presence of tropical species in the estuary was associated with a warming of the adjacent coastal waters. Similarly, in the Mngazana Estuary also in the Eastern Cape, changes in the proportion of tropical versus temperate species were recorded.
A long-term study, based on recreational spearfish catches of the sub-tropical reef fish community at Ballito and Scottburgh which is located in the coastal city in the eastern part of South Africa known as KwaZulu-Natal, found a general increase in the abundance of tropical species in catches as well as a change in the ratio of tropical versus temperate species represented in those catches.
Although marine species generally face fewer constraints to their movement than land-based species, climate change may pose a greater threat to species when their ability to disperse is limited or suitable habitat is unavailable. This is especially so for species in one specific area. To predict changes in the distribution of the commercially important linefish species slinger, we undertook a study involving species distribution modelling. This species is found in southern Mozambique and KwaZulu-Natal who share a border.
The models indicated that slinger will respond to changing water temperatures by contracting its southern African distribution from the north. Cooling waters to the south prevent the species migrating southwards. A recent review on the effects of climate change on estuarine fish species suggests that sudden decreases in temperature associated with increased upwelling will affect both temperate and tropical species. It may also prevent tropical species extending their ranges into temperate regions.
As climate change accelerates, there will be marked changes in the composition of coastal and estuarine fish communities. However, it is very difficult to predict how communities will change in response to climate change as each species responds differently to warming, and fish assemblages are unlikely to shift their distribution as a unit. In South Africa predicting temperature-driven change is further complicated by the number of different climatic zones found along a relatively short coastline and the contrasting changes expected in each zone.
The five main environmental threats to our oceans and coastal ecosystems are overfishing, pollution, invasive species, habitat destruction and climate change. Healthy ecosystems are more resilient and able to adapt to change but the combined effects of these environmental threats reduce the ability of species and ecosystems to adapt to change.