The boat shook as a heap of fish—weighing well over a ton—landed on the deck. It was the summer of 2016, and the trawler was dredging along the Sindhudurg coast, with its bounty glistening in the morning sun. From mantis shrimps, prawns and crabs to rays and an array of fishes, trawlers would make a good site for a marine biodiversity class, so great was the variety of marine life on display.
In reality, trawlers create far more dire situations for the ocean, its denizens and the fishermen whose lives depend on it. Most times, close to 80 per cent of the catch is not used for consumption. Instead, this extra ‘trash’ fish is converted into poultry feed or as fertiliser for aquaculture.
A Beaked Sea Snake (Hydrophis schistosus), olive green in colour with faint white bands, slithered out of the mound of mostly dead sea creatures. We gently lifted the four-foot-long snake and placed it in a holding container. Four more are uncovered before we leave, a survey exercise we’ve been conducting on the trawler every day.
The fishermen onboard this particular trawler have been helping us study the snakes that get trapped on their daily hauls. Both of us, researchers from the Dakshin Foundation, have been studying sea snake bycatch for around three years now, to understand the species diversity and abundance of sea snakes caught in various fishing nets in the coastal town of Malvan, a busy fishing region and a tourist hotspot in Maharashtra.
However, our foray into the seas was not initially focused on sea snakes. After spending a few years in the offshore waters of Odisha studying sea turtle aggregation patterns, we decided to investigate the ecology of another marine reptile: the sea snake. While common across the Indian coast, sea snakes are largely understudied. We chose the Malvan coast of Sindhudurg as a potential region to initiate a pilot survey due to the diversity of coastal habitats in the region—lesser reef systems, rocky shores, and estuaries—as well as the variety of fishing practices.
Despite their ubiquitous presence, few studies have gone into understanding sea snake ecology along the Indian coast. Considering the paucity of existing knowledge, we wrote a guidebook based on our surveys of the common species of sea snakes encountered in the Sindhudurg region, hoping that such information would help shed light on the occurrence, distribution and abundance of sea snakes that call these waters their home.
So, what are sea snakes? Somewhere between 6-7 million years ago, a lineage of venomous snakes became completely aquatic and moved to the seas. Within these sea snakes are two broad groups: the amphibious ‘sea kraits’ found in the Andaman and Nicobar Islands and Southeast Asia, and the widespread, completely aquatic ‘true sea snakes’. These snakes evolved to hunt specific aquatic prey and diversified into more than 60-70 species, with 22-25 species reported from the Indian subcontinent. Almost all the ‘true sea snakes’ are found within the Indian Ocean, with only one species, the Yellow-bellied Sea Snake, seen in the Pacific. This speaks to the uniqueness of this region, the Indian Ocean, which is home to many spectacular hotspots of marine life.
Over millennia, sea snakes have evolved a range of adaptations to survive underwater, including flat oar-shaped tails, larger lung capacity, subcutaneous respiration, and specialised sensors to navigate through their environment. As members of the family which include cobras, kraits and coral snakes, their venom is also similar, but with a stronger mixture of neurotoxic and myotoxic venom, primarily designed to paralyse their fast-moving prey: fish. The Turtle-headed Sea Snake is an exception; their venom is non-potent as they only feed on fish eggs. These remarkable physical transformations have shaped them into successful predators of this realm, for instance, the sea kraits, which hunt large predatory Moray Eels.
Sea snakes play a dual role in the ecosystem, as predators of different marine fauna and second as prey to other larger oceanic predators. Sea snakes, wherever they occur, are frequently caught in fishing nets throughout the Indian coast. Along India’s massive 7500km coastline, ranging from Gujarat to West Bengal on the mainland as well as Lakshadweep and the Andaman and Nicobar Islands, there is a diverse range of fishing practices. Out of all these, it is the trawlers that make their presence felt the most as they sweep everything that swims, crawls or buries itself on the ocean floor in their path. Sea snakes are a part of this marine haul and often die due to drowning or physical injuries after getting trapped in nets.
Our research revealed interesting patterns in the sea snake bycatch from trawling and the abundance of two species—the Beaked Sea Snake (Hydrophis schistosus) and the Spine-bellied Sea Snake (Hydrophis curtus).
As fishing pressures over the years have increased, the number of snakes caught has changed drastically. Especially those of the Spine-bellied Sea Snake, found in much greater abundance a decade ago by Dr Aaron Lobo, a marine biologist, during his surveys in the Goan coast. Spine-bellied Sea Snakes contributed to approximately 86 per cent of the sea snakes caught during his study, as compared to ours, where we encountered less than 25 per cent, indicating a sharp decline in their population.
On various occasions during our research, we pondered the risk that fishermen take while untangling a snake from their net. There is no anti-venom available for sea snakes in India. Given that snakes have long been portrayed in a negative light—think even of Adam and Eve in Biblical times—it is no surprise that fishermen in many places consider them a nuisance. The fishermen we talked to simply referred to it as an occupational hazard. They said that if the snakes could be easily untangled, they would do so immediately and release them back into the sea.
In India, sea snakes are of no commercial importance and can be referred to as ‘true bycatch’. But in the Gulf of Thailand, sea snakes are harvested for food, and in Australia, for the commercial leather industry, although the volume of this harvest has been on the decline.
Whether as targeted species or as bycatch, the uncontrolled removal of individuals due to overfishing causes local population declines. Sea snakes face multiple threats from other anthropogenic activities too. For instance, in the case of the Turtle-headed Sea Snake (Emydocephalus annulatus), populations are showing signs of induced melanism due to heavy metal deposition from pollution. This can facilitate toxin accumulation within the food web, as these snakes are prey to larger predators such as Tiger Sharks and White-bellied Sea-Eagles. Our research also revealed that sea snakes spatially overlap with fisheries significantly and compete with them for food and space.
Sea snakes are protected in India under the Wildlife Protection Act, 1972. Still, they have not been the focus of any major conservation initiative in this country or elsewhere in Asia. However, the thorough implementation of existing Indian fisheries laws in different coastal states might help mitigate some of the threats to sea snakes. Sea snakes play an intermediary role in ecosystems; they are both prey and predators of various marine fauna. Thus, studying and conserving sea snakes is also an important way to check coastal ecosystem health.