A joint survey conducted by Tamil Nadu Forest Department and Suganthi Devadason Marine Research Institute (SDMRI) has recorded an increase in the reef fish species in the Gulf of Mannar (GoM) region of the Indian Ocean, that lies sandwiched between southern portion of the Coromandel coast and the western coast of Sri Lanka.
The survey was done between April and June this year, and researchers say that the increase in numbers is a result of the lockdown and the subsequent fishing ban. Average fish density was found to have increased by 22% - from 405 fish per 250 metre square area to 510 fish now in the same area in the Thoothukudi group of islands Vaan, Koswari, Kariyachalli, Vilanguchalli and Vilanguchalli Patch Reef area.
The survey explored impact of meso (5 mm to 2.5 cm in size) and macro (>2.5 cm in size) plastics on the coastal area, the quality of marine water and assessed coral bleaching. While it noted a general increase in fish density, it focused specifically on parrotfish – a species of fish that has been known as the saviour of coral reefs, the populations of which have been depleting because of fishing and its export value.
It is important to note that the annual fishing ban falls during the months of April and May, but it does not technically ban all fishing activity. "The annual fishing ban is applicable only to trawlers and not to country boats used by small-scale fishermen. So, usually, small-scale fishermen continue to engage in fishing during the annual fishing ban near the reef areas," Dr JK Patterson Edward, Director of SDMRI, explains.
While temperatures continue to rise in the ocean (31.5 to 31.8 degree Celsius), a reminder of the climate crisis, the absence of trap-fishing and other fishing activities during the lockdown period in the reef area has increased the population of reef species, he concurs.
In trap-fishing, fishermen set traps underwater in the reef areas to catch fish, causing severe damage to the corals. “Divers put the traps in between corals and tie them to nearby coral heads, directly damaging them in the process. Injured corals are prone to diseases, predation, etc. and may not recover from the injuries,” explains Dr Patterson.
The underwater study conducted between April 17 and June 1 found a considerable increase in the numbers of Scarus ghobban or blue-barred parrotfish, a herbivorous reef fish species. Its population increased by 39% — from 50 individuals per 250 metre square area in February 2020 to 82 in the same area in May.
This is a great sign of improvement for the coral reef ecosystem in the area, observes Dr Patterson. “The commercially important herbivorous reef fish Scarus ghobban (parrotfish) plays an important ecological role in the coral ecosystem. It feeds on the seaweeds and maintains the ecological balance in the reef area,” he says.
The reduction in the number of parrotfish is a big concern for the reef ecosystem and caused indirectly by trap-fishing. “Herbivorous fish like parrotfish feed on macroalgae. Corals compete with algae for space on the reef and herbivorous fish help corals by grazing on macroalgae. The exploitation of the alga-eating herbivore fishes leads to the proliferation of macroalgae, which overgrow and take the space from coral,” he says.
“When this ecological balance is altered, there can be a shift from a coral to an algal-dominated system which is called coral-algal phase shift,” he cautions.
In the Gulf of Mannar region, the coral reef cover was 38.9% and macroalgae cover was 22.1% in 2015. However, widespread coral bleaching has affected this balance since. Sustained efforts by environmentalists and awareness among fishermen will help reinstating this balance Dr Patterson shares. “As per 2019 survey, average coral cover in GoM was around 25.6% and algae cover was 29.25%. Our annual underwater survey for 2020 is in progress and will be updated by the end of year.”
Parrotfish have a higher export demand than in domestic markets. According to Dr Patterson, the fish is usually sold for Rs 1000 per kg. “Smaller and bigger fish costs comparatively lesser. Parrotfish are not endangered, but depletion in their populations has been reported from many reef areas worldwide,” he adds.
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In the Thoothukudi reef area, more than 300 fish-traps are illegally deployed every day to catch the reef fish. “The most susceptible trap fishes are Scarus, Lutjanus, Lethrinus and Siganus. Among them, Scarus and Lutjanus are the target species because of the high commercial value they command,” Dr Patterson says.
The findings of this recent survey, most importantly, will help bring about a positive change in the marine ecosystem, Dr Patterson believes. “Trap-fishing is a traditional fishing method and hence is not seen as a destructive fishing practice, but the impact of trap-fishing is yet to be ascertained. These findings should definitely be used to educate fishermen to avoid trap fishing,” he adds.
Watch how lockdown impacted the Olive Ridley Turtles along the eastern coast