“I saw an unusual event today, more than 200 birds in a fish feeding frenzy.
“The above shows an overview of a small segment of birds feeding
“They were predominantly Little Egrets (Egretta garzetta garzetta, > 200), 7 Grey Herons (Ardea cinerea), and 2 Little Cormorants (Phalacrocorax niger). I could easily have missed a few Intermediate Egrets (Mesophoyx intermedia intermedia) or Great Egrets (Casmerodius albus modestus).
“The feeding was reminiscent of birds in a feeding frenzy on alate termites (see HERE).
“Above shows a composite of birds with fish.
“This large flock was picking off fish at a very fast rate from the surface of an ex-mining pool that was used for fish farming in a wetlands area. I saw Little Egrets feed on more than 6 fish within 60 seconds and Little Cormorants on 5 per minute. Most Little Egrets were hovering to catch fish (with some minor mid-air clashes) but some, including the Grey Herons, just floated on the surface and picked off fish with ease.
“I could not understand why this was happening. I am aware that this particular pond is rather deep so could not determine why the fish were on the surface. I could even see an occasional fish leaping out of the water.
“The fish were not being harvested, not being chased and the pond was not being drained to account for easy access. The fish were not dead (poisoned) and the large size and open nature of the pond did not allow for oxygen deprivation. There was no fish food or bait on the surface. It had been very wet night with intermittent rain.
“I arrived at the site around 7.30am and it was still very grey and blustery, like a UK morning, but this did not explain the phenomena. Judging from the Little Egret beaks, the fish were 4-6 cm in length. The above shows a variety of Little Egret fishing methods – floating to feed, hovering to feed and plunging in to get prey.
“The image above shows the smart floaters. I seldom see egrets or herons ‘swim’ but they can when there is a need. All the herons were fishing in this manner and about 20% of the Little Egrets. I was observing this from the only vantage point, some distance away on a very busy road and hence could only take a short video (below) and some quick images.”
Dato’ Dr Amar-Singh HSS
Ipoh, Perak, Malaysia
20th December 2016
Location: Malim Nawar, Perak, Malaysia
Habitat: Ex-mining pools, fish farming, wetlands
Lee Chiu San
A hypothesis on why the fish appeared to be distressed and had come to the surface where the birds could feed on them.
If the pond is deep, as you state, over time, an anaerobic layer could have accumulated on the bottom. This layer, of dead leaves and other organic matter, generates toxic substances.
Under normal circumstances, fish stay in the upper layers of the water and are not affected by the toxins.
But if there has been heavy rain (or for that matter any major disturbance to the water) the water will be mixed, and the toxic layer may merge with the layer in which the fish live.
This causes them distress, and drives them to the surface for more oxygen, even, as you observed, making them leap out of the water.
As a general rule, fish farmers do not construct very deep ponds. For practical reasons, most are chest-deep, to facilitate wading by workers. Even in extreme cases, ponds constructed for fish farming seldom exceed 3 meters in depth.
Of course, fish farmers may attempt to take advantage of opportunities afforded by abandoned mining pools and quarries. Reports state that without the provision of mechanical aeration to keep the water circulated, projects located in such sites are not long term. Toxins eventually build up in the depths and affect the fish yield.
Dear Chiu San,
thank you for the detailed response & possible explanation. Very valuable. These ponds are not constructed but ex-mining pools, hence quite deep. The vast majority of thse ponds have mechanical aeration systems installed.
Lee Chiu San
Mechanical aeration is an absolute must for fish farming in deep pools. And it must be kept running continuously. Once stopped, in the absence of oxygen, decomposition of organic matter takes place, and toxins are created.
Also, the aeration/circulation system must be designed to allow for full turnover of the water and not permit the formation of stagnant pockets.
Possible explanations for what caused the fish to be distressed and rise to the surface:
1) The aeration was stopped and toxins formed in the lower depths.
2) The direction of the aeration/circulation was not well designed, and stagnant pockets were allowed to form.
As I mentioned in my earlier post, fish can survive indefinitely in lakes and ponds with toxic areas so long as there is not much mixing of the good and the bad water. However, when something such as heavy rain causes unusual flow, the good and bad water mix, and fish suffer.
This happens most often after rain when there are channels such as canals or slopes that lead into the pond and which allow a large influx of water in a specific direction to stir things up.
The situation is worse if there has been a long dry period beforehand, during which toxins have had a chance to build up to critical levels.