GREY HERONS FISHING – AMATEUR VS PROFESSIONAL

“On 26th March 2014, I was admiring the diversity and behaviour of shorebirds along a mudflat in Singapore. These avifauna included Whimbrels (Numenius phaeopus), Little Egrets (Egretta garzetta) and Grey Herons (Ardea cinerea), all anticipating the retreat of the tide (above).

“A few juvenile Grey Herons were wading aimlessly and sheepishly along the shallow waters and one was repeatedly striking at a propagule of a mangrove tree (Rhizophora sp.) (above). It would pick up this green ‘toy’, let it go and retrieve it again, apparently enjoying its self-invented ‘game’.

“Sometimes, a broad dead leaf may also become the focus of such ‘playful fun’ (above). This reminded me of how curious toddlers love to put everything into their mouth and perform test-bites. Curiosity aside, repeated striking at inedible objects by juvenile Grey Herons would certainly provide much needed practice before qualifying as a skillful fish predator.

“While the juveniles were entertaining themselves near the shore, the adults were engaged in some serious fishing in deeper waters. One of them had just returned with a good-sized Eel-tailed Catfish (Plotosus sp.) in its beak (above).

“The heron handled the catfish with extreme care and caution, making a conscious effort to avoid the stiff spines on its dorsal and pectoral fins (above).

“Eel-tailed Catfish are recognised and respected as venomous fishes with stings that can cause intense pain, swelling, numbness, edema, gangrene, fever, weakness, nausea, local paralysis and dizziness (Auddy et al., 1995; Fahim et al., 1996). In addition, toxins have also been found in the skin secretions of such catfish (Shiomi et al., 1988).

“With this awareness of how risky and unpalatable it can be to manipulate and swallow an Eel-tailed Catfish, we can better understand why the heron was not in a rush to ingest its prey. So this determined and experienced heron brought the fish to the shallows and slowly brushed the prey’s body against the sand (above). Thereafter, it would dip the fish in and out of the water, as if to wash and rinse it. These actions would probably have served to remove the noxious slime from its skin.

“Finally, when the heron was convinced that the catfish was safe to swallow, it did so with little hesitation, head-first (above).

“This demonstrated finesse in the successful procurement and processing of a potentially lethal meal by an adult Grey Heron truly separates the ‘men from the boys’ and it would be a matter of time and lots more practice before the young amateurs graduate to become professional predators, just like their parents.”

Dr. Leong Tzi Ming
Singapore
13th April 2014

REFERENCES:
1.
Auddy, B., D. C. Muhuri, M. I. Alam & A. Gomes, 1995. A lethal protein toxin (Toxin-PC) from the Indian Catfish (Plotosus canius, Hamilton) venom. Natural Toxins, 3(5): 363–368.
2. Fahim, F. A., E. A. Mady, S. M. Ahmed & M. A. Zaki, 1996. Biochemical studies on the effect of Plotosus lineatus crude venom (in vivo) and its effects on EAC-cells (in vitro). Advances in Experimental Medicine and Biology, 391: 343–355.
3. Shiomi, K., M. Takamiya, H. Yamanaka, T. Kikuchi & Y. Suzuki, 1988. Toxins in the skin secretion of the oriental catfish (Plotosus lineatus): immunological properties and immunocytochemical identification of producing cells. Toxicon, 26(4): 353–361.

11 Responses

  1. Liao Weijie

    Lovely account and pictures. Enjoyed it immensely.

  2. Catching up on missed posts. Lucky I didn’t miss this one. Good info on the eel-tailed catfish. About 12+ years ago I caught one while fishing in our waters near the shore. I knew it was some kind of catfish because of its mouth spines, and therefore knew it would be dangerous/toxic, but I had no idea it was THIS dangerous. Anyway I threw it back without touching it. How is it that the birds are able to tolerate the poison in its fins and mouth spines?

  3. Lee Chiu San

    To Am. This is a very general answer, and don’t try some of these things at home, but with regard to fish, snake and lizard toxins, many of them are protein based, and can be digested or destroyed by cooking. WARNING – THIS DOES NOT APPLY to the poisons in the meat of puffers, some related fish, corals and some crustaceans,which remain toxic even after cooking.

    As far as catfish are concerned, many, many species are poisonous, but almost all are considered excellent eating. Humans are capable of simply digesting the poisons. By the way, the highly-venomous scorpion fishes and stone fishes are also delicious.

    As an aside, some snake charmers have been known to demonstrate the stunt of milking the venom from a snake, and drinking it. According to medical experts, provided the person has no mouth ulcers or stomach ulcers that would allow the poison to enter the bloodstream, human digestive juices quickly break down the protein-based poisons and render them harmless.

    As for the heron, as long as it did not get spiked by the spines, it would have no problem digesting the catfish, stings, poisons and all.

    • One main reason why birds swallow fishes head-on is to ensure that the spines do not damage the throat.

    • Thanks for the excellent answer!

      I know pufferfish poison can’t be broken down by cooking and digestion (as evidenced by the occasional deaths that occur from improperly-prepared fugu in Japan) and I’ve always found it interesting why & how some species evolved to be so highly toxic while others have not. Obviously, remaining toxic even after digestion is a much better defence mechanism as it prevents you from being eaten at all, so why have catfish and snakes, for example, not developed this defence mechanism?

      Also is it only certain sea creatures that remain toxic even after cooking? Are there any land animals that meet this criteria too? Interesting to note how more sea-dwellers seem to have more potent poisons compared to land-dwellers?

  4. Lee Chiu San

    I will hazard a guess pertaining to the different natures of toxins. Obviously, puffer fish, some other colourful marine organisms and certain insects such as butterflies of the Danidae family (Monarchs, our local Glassy Tigers, etc) became poisonous as a defence against being eaten. As such, their poisons remain toxic even after cooking. I add the caveat that what is said about the butterflies is a wild guess, since I am not aware of any chef trying to cook any.

    Snake and lizard venoms are adapted salivary liquids. They were primarily evolved to assist in the capture of prey, and perhaps took on a defensive role only at a later stage.

    Simply put, some (not all) snake and lizard venoms can be described as super digestive juices that, when injected, kill the victims by breaking down blood cells and vital organs.

    As they are meant to aid digestion, even though human and reptilian stomachs may be different, such venom should be capable of being swallowed without too many ill effects.

    The digestive effects of viper venom have been studied. Rats killed by vipers (including those killed by pit-vipers) display considerable breakdown of their internal organs, consistent with their tissues being digested. And human survivors of viper bites often suffer necrosis, the death of large areas of flesh around the area of the bite.

    But I cannot provide you with an answer as to why catfish, stonefish, lionfish and many other fish poisons can be rendered harmless by cooking.

    • Thanks for the very informative and interesting answer! I think there should be a whole post on animal poisons and toxins. Are there any birds that are toxic by the way, or have poison defences? If none, it is interesting to speculate on why this is so!

  5. Lee Chiu San

    Don’t know about the birds, but definitely there are at least two poisonous mammal groups – the shrews and the platypus. Shrews have poisonous saliva, while the platypus has venom glands connected to spikes on its hind legs.

    To digress further on poisons (that kill you when eaten) and venoms (that are injected when the animal bites you) let’s continue the discussion on fish, frogs, nudibranches and insects.

    Some poisonous species are not born poisonous. They get that way from what they eat. You may ask, how can they swallow poison and survive? The answer is because different species have different sensitivity to poisons. Nicotine is only mildly poisonous to human beings. But it is deadly to reptiles. A method of killing museum specimens of snakes and lizards for preservation without damage is to put tobacco into their mouths. Death follows in minutes.

    On the other hand, many reptiles have good resistance to strychnine and arsenic. These chemicals are used in baits to control populations of wolves, jackals, foxes and wild dogs. Monitor lizards eat the poisoned meat without noticeable ill effects.

    So, poison arrow frogs gain their poisons from the ants they eat. Monarch butterflies do the same from the milkweed that their caterpillars feed on. And, experienced fishermen will tell you, it is safe to eat barracuda less than 60 cm long, but avoid the big ones. Why? Because the big barracuda are large enough to swallow puffer fish and some other poisonous species. The poison does not harm them, but it stays in their flesh, and you die if you eat it.

    It used to be considered an old wives’ tale in this region that some lizards were thought of as venomous. But the latest research shows that the Komodo dragon definitely has venom glands. And the question is, do other monitor lizards? Or, for that matter, lizards from other families.

    • Immensely interesting! Thanks to Chiu San for the reply! I would love to see a post on any one of the topics brought up in your comment.

      (As an aside, I too have always wondered why certain animals are immune to certain poisons & others are not; I guess the answer may only be found in our evolutionary histories, where, at some point, certain circumstances or behaviours warranted the need for such immunity to develop?)

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