Croaking gourami, native to Southeast Asia, is steadily spreading all over Bangladesh, but how it got here is shrouded in mystery
The story being told today is about a fish. And the title is no exaggeration. Its appearance is barely noticeable, as if it were non-existent during any haul. Yet this fish is one complete package of wonders.
It can produce sounds that is audible to our ears. It is exotic to our country. In fact, more than half of Bangladesh is currently known to harbour the species. But we do not know much about how this alien entered our water or how it is thriving with ease. The fish, croaking gourami, deserves a The Business Standard Earth issue.
Let us reel back a decade. It was an otherwise ordinary night in 2011. It was dead at night and the whole city was sleeping. Back then, I had four fish tanks in my room. Suddenly, I heard some strange noises, somewhat similar to frog calls.
But my apartment was on the third floor; no way available for any frog to get into. I was perplexed. Then, I tried to figure out the source. The sounds were coming from a three-foot tank. I kept a few strange specimens therein that I had collected from a river on the Dhaka outskirts. They did not match with any fish described in ichthyological books published in Bangladesh in those days.
I turned the aquarium light on. Three of them, all males, were in a territorial display, flaring at each other and jousting. It was spectacular, also remarkably noisy. The brutes were making some astonishing grunts.
I cannot remember now what had kept me awake that long. You can see that it was rewarding. It was the night I had had a practical experience on a unique fish behaviour – sound production. And, the unique and "unidentifiable" specimens were croaking gourami. I had come to know later on.
Now, fast-forward to the present Covid-19 time. Earlier this June, Palash Biswas, a student of mine, had sent me a fish photograph to identify. Now stationed at the tip of southern Bangladesh, a region subjected to strong tidal cycles, he collected the fishes from a backyard pond.
I became awe-struck as soon as I had opened the photos. The photo earmarked the fabled adaptability of the fish in an array of habitats, even in hypersaline waters. I identified it at a first glance – croaking gourami, an alien in our water. This incident was decisive. "I really should make an issue on the fish,'' I told myself.
These are the three openings I thought of while considering the story. From 2010-2020, I have had some riveting observations on the fish. As you can see, croaking gourami never ceases to surprise me.
Croaking gourami, scientifically known as Trichopsis vittata, is a small freshwater fish originated in stagnant backwaters of Southeast Asia. It has a prolonged body, more tapering at the centre, can reach seven cm at most. Its position in the gourami family makes it a cousin of our very known Lal Kholisa or dwarf gourami.
Like any other gourami, croaking gourami possesses a specialised organ with which it can breathe straight from air (Now you know why your Lal Kholisa lived so well even in a small glass jar.) If proper air ventilation is not provided, gouramis can literally choke and drown.
The colour of croaking gourami is brown-based with purple shins and blue iridescences on fin-edges. On the flanks, they have some two to four parallel stripes. They stay mostly washed out and darkish. In the norm of ornamental fish-keeping, they are not so "attractive". The only thing special in croaking gourami is its iris – rimmed with bright blue. The halo always stays evident.
Through aquarium trade and accidental release, this fish has invaded Myanmar and India with success.
It can be found in Florida, the US, halfway across the globe from its range. The US Geological Survey revealed its strong tolerance toward rough conditions. According to a study conducted in 2016, croaking gourami can sustain up to 20 ppt salinity, temperature as high as 30°C and can fight off ich, a notorious skin parasite of fish.
What is with the voice?
Well, sound production itself is a complex feat for any life under water. You may find fish that sings with its swim bladder (we call it Potka in Bangla). There is fish that vocalises by grinding bones of its body.
How croaking gourami archives so can be compared to the mechs of grasshoppers and cicada. In simple, the gourami follows a stridulation technique. The stringed sound is produced by the beating of two pectoral fins with rapid vibrates. The fish has evolved modified fins for the task. These series of pulse can reach up to 1.5 kHz.
Sound production in gourami, as in other fish practicing the same ritual, stands as a means of communication. Sounds declare a territory, often ensure a mate.
It was 2012. The fish had caught the first attention of the fish scientists of Bangladesh. Professor Dr Mostafa Ali Reza Hossain, an eminent specialist in aquatic biodiversity from Bangladesh Agricultural University, documented its presences in the River Meghna and led two consecutive researches. In 2017, another team led by ichthyologist Dr Md Mizanur Rahman, assistant professor of the University of Dhaka, ran DNA studies on the species and observed it from the Central Bangladesh, including a few outskirts of Dhaka city.
I summed up these studies with my casual observations. It led to one conclusion: The fish has been invading our waters. I teamed up with Professor Hossain and noted all the occurrence records. The 2018 work confirmed another fascinating fact: the gourami made establishment beyond the great Meghna estuary, both north- and south-ward. It now has known populations in the Baikka Beel, a globally recognised wetland in Sylhet as well as in the drainages of Chattogram.
In 2019, two more pertinent studies popped up noting observations from the River Feni. And, in 2020, we can see it lives as far as in the remote coastal district of Pirojpur.
You may find the steady spread of croaking gourami highly fascinating. Well, further astonishment waits for you. Introduction pathway in Bangladesh and catalysts behind its successful invasion are still obscure. And, there are three conjectures out in the market.
One says the species gets into Bangladesh through aquarium fish trade. The second hypothesis reasons with an introduction as a means of biological control of mosquitoes. The third one indicates to a contaminated import, meaning the gourami has its way undetected, with commercially farmed fishes.
In contrast, for its dull appearance, the fish is valued the least and is often ignored in fish-keeping hobby. Moreover, there is no formal document available describing its usages as biological control. A speculation says it had been introduced in the region in the fifties. Perhaps, it is true.
Yet, indeed, it is a sheer enigma thinking how the fish did skip detection for 70 years and spread simultaneously at the same time.
Sigh for a second before we go into deep considerations. The impact of the fish in our country is still unassessed. However, something is going on for sure. Let me resort to another anecdote.
When I approached KH Newaz MD Sarafat, a young fish enthusiast, I was told an eerie phenomenon. Newaz regularly explores the wetlands of peri-urban Dhaka. He has observed a gradual decline in our native gouramis (Lal Kholisa, Boro Kholisa and Ranga Kholisa) from waterbodies wherever he saw the croaking congeners – first a few, then in loads after a couple of months as if the water was completely stacked with them.
Introducing an alien is one risky stake. The stake has caused havoc in endless places, pushed fish and other aquatic lifeforms toward extinction. We have around 20-25 exotic fishes that we have intentionally or accidentally brought into our waters. We have achieved some superior milestone in food security by practicing so.
But have we assessed what an African tilapia can do to our indigenous fish communities? Do we know the extent to which foreign carps can compete with our native carps? What are our means to curb the disastrous spread of the South American plecos? Or, is there any attempt to check risky handling of newer exotics – pacu, piranha, Mekong catfishes – any further?
I would like to recall Rachel Carson. Through her Silent Spring, we see how unabated, aggressive food production can cook a void, lifeless realm.
Let us end with hope in this dismal time.
We hold the largest deltaic plain of the planet. Our documented freshwater fish inventory hovers around the 300-mark. And, there are wonders in our waters waiting to be discovered, understood.
The courteous amount of hospitality they deserve is a respite from the alien exotics.
Croaking gourami in Bangladesh reminds us of the truth.
More on fish vocals in Bangladesh
Well, our native fish can sing too. They do it in a myriad of ways.
The drums and croakers (Poa in Bangla) do this with their high muscular swim bladder (potka). We have about 20 species.
The country has toadfishes that hoots and grunts by a specially-forked swim bladder. We have one reported species, grunting toadfish, in the Sundarbans.
We have many catfishes that are known for sound production by uniquely rubbing their pectoral spine.