JOIN USOil spills in seas and oceans like the one that occurred near the Chennai coast earlier this year can distort the behaviour of fish species that inhabit coral reefs, finds a recent study. The incident that occurred off Chennai’s coast ended covering up 34-km-long stretch of the beach with toxic oil slick, affecting marine life and seashore species. Incidents such as this have everlasting effects and leave behind their imprints on the environment. In a recent study, scientists from University of Texas, USA, have investigated how exposure to oil over long periods of time impairs the learning ability and changes the behaviour of juvenile coral reef fish. This affects their choice of selection of surroundings, making survival a risky affair. The study has uncovered that, even the presence of oil in low concentrations — equivalent to that of two drops in a large swimming pool, can bring about behavioural changes in the fish.
Larvae of some common coral reef fish species like black-axil chromis, speckled damsel, lemon damselfish, ambon damsel and emperor angelfish, were exposed to oil concentrations similar to that found in seas and oceans around the world and their behaviour patterns were carefully monitored. Coral reef fish embryo and larvae are sensitive to their surroundings and generally have mortality rates as high as 90%. With predation being the main cause of mortality, only the fittest of them survive.
Ecosystem endangered
To avoid predators, coral reef fish larvae stick together and form large congregations called shoals. They also move about the reef under the cover of darkness and take refuge inside corals during day. The selection of a safe hiding place is a critical anti-predatory trait essential for their survival. On exposure to oil-based pollutants, the fish larvae failed to develop these anti-predatory behaviours and instead exhibited risk-taking behaviours. A significant difference in shoal size and movement patterns were also noticed. The shoal size of fish and their swimming speed decreased, whereas their response time to predators increased, making them more vulnerable to predation.
Over six million metric tonnes of oil and oil-based products from industrial discharge and shipping operations find their way into seas and oceans every year, causing detrimental effects on coral reef ecosystems. Exposure of coral reef fish species to oil during early stages of development has long-term consequences such as stunted growth and a shorter lifespan. The chemicals are also carcinogenic in nature and are known to cause genetic mutations and physical deformities in many marine species. Even small concentrations of oil can alter gene expression leading to degeneration of nerve cells, thereby impairing their decision-making and learning abilities.
Coral reefs are one of the most diverse and delicate of all ecosystems. They play the role of nurseries to young juvenile fish, while also helping to renew the ocean’s fish stocks. Corals are in turn benefited as the fish feed on algae and help control their growth. In their absence, the algae can otherwise grow abundantly, choking the entire reef. “Over the past 35 years almost 1/5th of the world’s coral reefs have been lost and half of the remaining is expected to disappear in the coming decades,” says Jacob Johansen, the study’s principal investigator. Anthropogenic activities like overfishing and rise in sea temperatures are responsible for the fast degradation of coral reef habitats. In 2014, many reefs across the world, including large parts of the Great Barrier Reef off the coast of Australia, underwent bleaching due to rise in sea temperatures.
With the global demand for energy and petroleum products on the rise, more and more crude oil is being transported in large tankers across continents. Off-shore and deep-sea explorations have also increased, thus increasing the threats to coral reefs and pushing them closer to the brink. Coral reefs close to oil rigs are the ones to suffer the most as they are constantly exposed to pollutants. Oil explorations and sea transport have resulted in over 300 major oil spill incidents in the last 40 years with over 3,900 million metric tonnes of oil entering the sea.
Scientists suspect that pollution of seas and oceans would further weaken the resilience of the coral reefs and therefore strongly recommend limiting the activities of oil-based industries away from coasts, especially in areas close to coral reefs.