Monitoring A Bangus (Milkfish) Pond

By | September 24, 2022

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Monitoring A Bangus (Milkfish) Pond

Monitoring “Tangok” (Oxygen Starvation-Related Fish Stress or Kill)

There were ominous signs.

In the week before emergency harvest, we saw signs of fish stress in bangus in the grow-out pond. It was the middle of the dry season (December to May) and there was hardly any rain (7 consecutive weeks of dry weather). Water levels were dropping. And the pond kept getting saltier due to evaporation.

Unfortunately, our ponds (being closed system) were not visited by tidal waters which come and go. We’ve already let in the seawater and were just relying on rainwater the rest of the year.

From my personal research, we learned that the saltier the water and the warmer it is, the less oxygen (more accurately, dissolved oxygen or DO) it can hold.  

We had about 4,500 pieces of bangus for grow out in our Main Pond (about .6 hectares). We noticed, to our dismay, that the pond contained quite a number of tilapia too (which accidentally flowed in due to the past floods). The tilapia competed not only for DO, but also for nutrients and pond space, thereby increasing pond density. Pond color was brownish green.

My farm overseer had been observing clusters of bangus swimming at the water surface, gulping air during the early morning hours for about a week prior to my visit. At the feeding area, bangus seemed to be feeding less and were a bit sluggish.  

We were aware that lethal oxygen depletions begin with similar signs. The ones we observed, however, were not alarming at the time. Fish dived back to deeper waters as soon as the early morning sun shone and they stayed underwater. The clustering of bangus was also not as extensive.

Bangus starved of oxygen and swimming near the surface in clusters (phenomenon called “tangok” in Pangasinense); if pond condition worsens, could lead to fish kill
  
We therefore started tracking the water quality of our pond, especially the DO content. Too bad, we had no DO monitoring device (we just rely on occasional visits of the technician from the feed supplier) and couldn’t measure if the DO was within acceptable range (i.e. above 3 ppm). So we had to depend on visual checks at the early morning hours – 5 a.m. thereabouts.

On the following days, pond conditions started to worsen. Clusters of bangus swimming near the surface were now seen in several parts of the pond (before, we saw only a few, isolated clusters). The fish tended to ignore human and predatory bird presence and continued gulping air at the surface. The gulping fish stayed at the surface longer even as the sun rose.  

We also noticed the onset of milky colored streaks at the water edge and below. We thought we may have a fish kill in our hands pretty soon.
 

After four days, we decided to take action. Our plan was to do an emergency harvest and move all fish out of the Main pond. We intended to grow my bangus to the marketable size of 250 to 300 grams per piece but then had no choice. 

It was either we harvest and sell slightly undersized bangus or soon find most of them dead in the water and sell at almost give-away prices.

Nature of “tangok” and its main causes

“Tangok” (fish starved of dissolved oxygen and swimming to the surface, leading to stress/fish kill) to me, is one of the biggest threats in bangus farming. When “tangok” becomes severe and bangus start dying in large numbers due to increasingly poor pond water condition, there is no choice but to do emergency harvest to cut losses.

As a result, bangus need to be taken out undersized, or worse – under very severe conditions – dead. This means less quantity, less number of kilos, lower total sales, and lower profit for the cropping season. Under the worst scenario, a pond owner may sustain massive losses due to large-scale fish kill.

What are the causes of severe “tangok” and eventual fish kill?

Based on our experience, research, inputs from our technician, and interviews with neighbors, it’s a combination of things going wrong at the same time. As a result, DO in the pond gets depleted because it is being consumed much faster than oxygen is being added to or absorbed in the pond.

What are the factors or conditions that contribute to “tangok” in bangus farms as we’ve experienced it?

1. Weather

· Hot weather during dry months (especially February to April or May) – pond water has less DO because warm water is less able to hold DO compared to cool water.

· Cloudy days – sunlight is blocked thus stifling the oxygen-making ability of plant organisms in the pond.

· Calm, still, windless days – oxygen is believed to be added to the pond surface through the action of wind. When it’s windy, pond is ok, no “tangok”. In absence of wind, risk of “tangok” is higher. 

 · Stormy days – one of the effects of tropical typhoons is the strong winds which defoliate trees along the edge of ponds where leaves are blown off and dumped into the pond. The effect can bring about “tangok” about a week after as a result of the rotting of organic leaves at the bottom of the pond (see Item 6 below). 
 
2. Season

· El Nino – During the onset of the El Nino, temperature is extraordinarily high and pond water levels go down. El Nino, according to our Philippine weather experts, refers to a seasonal warming of the Pacific Ocean. It upsets normal weather patterns, (giving rise to short rainy season or causing droughts especially in Northern Philippines, including Pangasinan). It’s our experience that during El Nino, DO level in pond waters dip dangerously. In fact we suffered large mortality of fingerlings in the past, mainly due to lack of experience with El Nino when we were just starting our bangus farm.

· Dry Months – period from December to May are dry months when no rain is expected. March, April, and May are the equivalent of summer in the Philippines when pond water levels in Class C ponds go down and water temperature rises (though not as severe as during El Nino). Class A and B ponds continue to have water from river sources due to the action of tides.

3. Disturbance of pond water – natural and man-made

· We also observed “tangok” in the pond during certain weather events (e.g., short sudden thunderstorm after clear sunny days).

According to experts, the upper layer of water cools down and sinks. While the bottom layer of water rises to the surface. The result is the unwanted mixing of layers of water. The bottom layer is said to be normally oxygen-depleted, stagnant, and decaying. When this layer rises, the oxygen consumption of the bottom materials increases in the process; bangus is deprived of DO, thus causing “tangok”.

· Disturbance of water layers also occur when poachers wade into the pond to illegally catch bangus using gill nets. If this happens, especially when pond bottom is “hot”, silt level is thick, and a foul smell is detected, then the unsettling of water can trigger “tangok”.

4. Overstocking of fish due to unexpected presence of competitor fish (usually tilapia)

· A low DO level is made worse by overstocking of fish beyond the optimum carrying capacity of the pond. But how did this happen when the stocking quantity of bangus was carefully followed (i.e. 8,000 -10,000 pieces per hectare)?

a) unexpected presence of tilapia in large numbers. Maybe the result of poor pest eradication during pond preparation – not all the small tilapia fish and tilapia eggs may have been killed.

b) incursion of water from an adjacent fresh water pond that has tilapia and/or gourami fish in it, especially during rainy season. This happens when there are low lying dikes that are flood prone.

c) also there may be tunnels or burrows in the dike caused by burrowing rats through which tilapia or gourami can enter. Or hollow pockets in the dike due to rotting roots of dead tree that used to grow in the dike.

Being fast breeders, tilapia quickly populate the pond and compete with the bangus for feeds and dissolved oxygen.


5. Thick growth of algae and/or rooted submerged aquatic plants (locally called “tariktik”)

· They use up DO and give out carbon dioxide at night. Although they are a source of DO during photosynthesis, these plants do consume a lot of DO during the so-called respiration phase when they become too dense or too thick. During cloudy days, when sunlight is blocked, less oxygen is produced for the fish to consume.

6. Piling up of rotting organic materials and wastes in pond bottom.

· The pond may have substantial organic stuff that settled at the bottom and later decayed or decomposed, thus starving the pond bottom of oxygen. Examples are:

a. Submerged aquatic plants rooted in the bottom soil (“tariktik”) that started to sink and decay.

b. Pond algae that earlier proliferated (pond is colored green) but later died, sank, and rotted at the bottom (pond color turned grey, brown, or became clear).

c. Excessive organic materials such as chicken manure, which some pond operators apply in addition to commercial feeds. When this organic matter decomposes through the action of anaerobic bacteria, according to our technician, it produces toxic gas like ammonia and hydrogen sulfide, thus making the condition of the pond much worse. When this happens, you can detect the foul smell that the pond emits. 
Much like the smell of rotten chicken eggs.

Sacks of chicken manure – normally used during pond preparation as organic fertilizer


d. Tree leaves, coconut fronds, rotten coconut husks, dike grass cuttings (we have lots of these, they’re all around our ponds, not to mention mango and other trees). These can fall into the ponds through natural defoliation of old leaves or blown away and dumped into the ponds by strong winds that come with a tropical typhoon.

Taking out coconut fronds and leaves from ponds and piling them up for drying and  cutting into firewood pieces

 
e. Aquatic droppings, silt, and other residues settling on the pond bottom.

What to Do

1. Invest in a portable dissolved oxygen meter to be able to accurately measure the DO content of the pond and to provide enough time for corrective action. Monitor DO levels daily.

2. In the absence of a pond aerator, use a portable water pump to suck in water from the bottom and discharge it in air. Harmful gases are released and oxygen from the atmosphere is expected to be absorbed.

3. Take out the excessive pond weeds (“tariktik“) from the pond to prevent its decay at the bottom (we do this using a bamboo raft on which pond weeds are stacked). Pile up the “tariktik” along the dikes of the pond and let it dry out and decay outside the pond. Later on, this layer of decayed “tariktik” can be returned to the pond as organic feeds for the bangus.

4. Take out other rotting debris from the bottom of the pond BEFORE they rot. Examples are coconut leaves, rotting coconut fruits, tree leaves and branches, etc.

5. In case of mild “tangok”, temporarily stop feeding for the day since the bangus are stressed out and don’t have normal appetite to eat. Commercial floater feeds applied to the pond will just eventually sink at the bottom and rot, adding to the depleted DO condition of the water. If DO is at the stress level (below 3 ppm), postpone feeding.

6. In the case of excessive feeding, such as use of both commercial feeds and chicken manure, discontinue the application of chicken manure. Most bangus growers (especially in Class C ponds like ours) stop feeding commercial feeds altogether, and instead shift to the use of “lumut” (filamentous grass-green algae) or even stale bread as feeds.

Tossing “lumut” into the pond as alternative feeds

 7. When foul smell is detected in the pond, apply 16-20-0 (mono-ammonium phosphate) at a rate of 50 k per hectare to cool the heat at the pond bottom and let out toxic gases into the air.

8. Do partial harvest of bangus to reduce the pond’s fish population. During the harvest, take out as much tilapia as possible, which compete with the bangus for both feeds and dissolved oxygen. Assign one or two of your ponds exclusively for tilapia growing. During partial harvesting of your bangus ponds, when you take out young tilapia, transfer them to such designated ponds.




9. In the case of Class A ponds, allow the change of pond water by draining out a third of the existing pond water during low tide and replenishing it each day for two to three days during high tide. This dilutes the decomposition of unused food and other organic matters. It also lets in water with adequate oxygen levels into the pond. 

Caution should be observed, however, to make sure that the prevailing water condition in the river is of acceptable quality. When it’s not high tide you need to use water pump to pump in replacement water.

10. Review effectiveness of pond preparation steps taken. See to it that pest eradication is done properly, using correct type and quantity of organic and/or inorganic pesticides, to completely eliminate competitor fish such as tilapia and predator fish such as dalag. Check surrounding dikes carefully for hidden tunnels or burrows through which tilapia from adjoining tilapia ponds can enter. Do land fill work for dikes which are low-lying and prone to incursion of flood waters during heavy rains.

11. Be on the look out for the onset of the El Nino several months before it happens. This will allow you to plan and program your bangus growing for the next several months. And also to develop contingency plans. The idea is to ensure that when El Nino does come and when pond water levels drastically drop as a result of the abnormally hot weather, then the ponds would have been emptied of bangus to avoid high mortality or fish kills. 

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