Fishing in 3 central reservoirs may have adverse ecological impact




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Mr Beng Yong Tang


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Abstract: This letter calls on the authorities to reconsider the opening up of 3 reservoirs in the central catchment area / nature reserve for fishing, and proposes alternatives which pose less of a threat to nature and the environment.

Dear Sirs/Mdms,

Firstly i appologise that this letter is so long-winded. Given the urgency of the situation, I did not want to delay and spend more time editing it.

With regards to the Straits Times article "Okay to fish at 3 more reservoirs" (ST Dec 17, 2000) I would like to suggest that the authorities seriously reconsider this.

I am an avid angler and go fishing at least twice a week, and as such, I welcome the willingness on the part of the authorities to further the sport of fishing in Singapore. Some anglers are happy that MacRitchie, Lower Peirce, and Upper Seletar reservoirs are being opened for fishing, but others, including myself, feel that this is not a good idea.

This is because the central catchment area cum nature reserve is the very last stronghold of most of our native aquatic plant and animal species, many of which are rare or endangered. Already over 40% of the native fish fauna of Singapore has become extinct1, and we need to make more effort to preserve whatever is left of our formerly much richer, but still very rich, biodiversity.

Singapore has an ever-increasing human population and an ever-shrinking area of natural habitat. Because of this the native flora and fauna are under a lot of pressure and the uncontrolled exploitation of fish stocks in the reservoirs is unlikely to be sustainable.

In Kranji and Seletar, I and other long time anglers have noticed a very drastic decline in the population of snakeheads, walking catfish, and other once abundant species including even such fast-breeding introduced species as tilapia (Note to ST Forum editor: search your database of ST news articles from the 1980's, there are some which mention the big catches in those days at Kranji and Seletar. Try also including "toman" and "tilapia" in the search). This trend cannot be totally blamed on anglers as other environmental factors are involved, but overfishing is certainly an important factor, particular for species like snakeheads, which in addition to being much sought-after by anglers, do not reproduce at a very high rate.

Furthermore, as I understand it, the central catchment area reservoirs are our main source of drinking water (most other reservoirs are used for industrial water), and if the ecology of the reservoirs is disturbed, this can have detrimental effects on water quality which may increase the cost of water treatment, and also pose a threat to public health.



Allowing fishing in the reservoirs can potentially have the following negative effects:

This is my single greatest concern. Although the use of live bait is prohibited, in practice it will be very hard to ensure 100% compliance with this and some anglers will still use live bait. Anglers may catch small fishes from other places or may purchase them from aquarium shops and bring them to the reservoirs to use as bait. Many of these fish may be species that are non-native to Singapore, and they may escape and establish breeding populations in those reservoirs. They may also carry diseases, parasites, and other organisms that may also become introduced to our reservoirs

An even more likely scenario is that, if fishing is allowed there, anglers will be motivated to introduce foreign fish species that they wish to catch into those reservoirs (it is illegal to do so but they will still do it). This has been done in many countries with disastrous ecological consequences. It has already happened in Singapore with the introduction of South American peacock bass, and it is rumoured, North American largemouth bass into some of our reservoirs. The ecological impact of these and other recent non-native fish introductions has yet to be ascertained 3.

The introduction of non-indigenous species can very seriously impact native organisms, pushing some species to extinction. It can also dramatically alter food web interactions and nutrient cycles, thus causing imbalances in the ecosystem, which can badly affect water quality in the reservoirs. Here are two examples:

Lake Victoria, Africa: Nile Perch, a fish not native to the lake, was introduced in the 1960's in an attempt to improve sport and commercial fisheries. Its population in the lake remained dormant until the 1980's when it exploded. The result was an ecological disaster.

"Lake Victoria is in danger of becoming the world's largest pool of dead water. Already half its native fish are extinct, and the 30 million people who eke out a living from its troubled waters are facing calamity" 4.

Because of the ecological damage, the lake water, which was formerly very clear, has become very murky with algae and much of it has become so devoid of oxygen due to rotting algae that no fish can live in it.

Apple snails in Southeast Asia 5: In the 1980's, apple snails (Pomacea) from South America were introduced to Taiwan to start a snail farming industry. However, the foreign snails were not popular as food, and also proved capable of carrying a local parasitic worm that infects humans. Instead of becoming a profitable aquaculture product, the snails infested paddy fields, where they posed a real threat to rice production and the environment in general. The snails rapidly spread to Indonesia, Vietnam, Laos, Cambodia, Thailand, Hong Kong, South Korea, southern China, Japan, the Philippines, Hawaii (where they are a major pest in the cultivation of taro, the staple food there), Malaysia, Papua New Guinea, and Australia. They have thus become a threat to food production in Southeast Asia.

These foreign apple snails are also found in parts of Singapore where they eat up all the water plants. Water plants take up dissolved nutrients (nitrate, phosphate, etc) and give out oxygen, and with the water plants gone, the nutrients cause algal blooms which makes the water turbid, low in oxygen, and bad smelling. In addition the plants are vital as habitats, breeding places, and food for a whole host of aquatic animals including many species of fish, and their removal seriously damages the entire aquatic ecosystem.

The native fish populations in our reservoirs are fragile and should be safeguarded. They are already under pressure from introduced aquatic species (via competition, direct predation, and drastic food web changes) as well as environmental factors such as excessive water plant clearance. Having anglers fishing them out will add yet another source of pressure on their dwindling populations. This problem can be lessened if anglers release native fish that they catch instead of taking them home to eat. Anglers should be educated and encouraged to release native fish and keep only the introduced non-native ones.

Fishing weights commonly sold in tackle shops are mostly made of lead. Some artificial lures, notably jigs and certain spinners, also have lead components. Lead is not very soluble in water, but the water in the central catchment reservoirs is probably slightly acidic (given the geology and soils of the area and the acid water streams that feed into the reservoirs) and may dissolve small amounts of lead off sinkers and other tackle. Over time very many pieces of lead-containing tackle will be lost to snags in the reservoir (I have personally collected many lost lead sinkers from the fishing area at Lower Seletar reservoir) and the amount of lead in drinking water may increase to levels which may pose a threat to public health (especially that of children, who suffer brain damage and decreased IQ from long term exposure to even low levels of lead). In other countries, alternative tackle is available, such as split shots which do not contain lead, but to my knowledge such un-leaded tackle is not yet available in local tackle shops.
As an aside, the three SAF rifle ranges in the Nee Soon area, which is in the water catchment area of Lower Seletar reservoir, are another possible source of heavy metal contamination. In other countries, the soil from rifle ranges is treated as toxic waste because of its high lead content
6, and it is advisable for the authorities (ENV / PUB) to test the water in streams and drains near weapons ranges for lead, antimony, mercury, cadmium, zinc, and other heavy metals used in the manufacture of ammunition.

Another danger is lost or discarded nylon fishing line. It takes many years to break down, and it poses a risk to wildlife, especially when it is tangled. Animals become entangled in it and die. It is especially dangerous to wading birds, tortoises, and monitor lizards, which may get their legs or necks stuck in the loops of tangled line. This is a problem anywhere that fishing takes place, not just in the reservoirs. It can be minimised if anglers take more care to properly dispose of tangled line instead of littering. The existing anti-litter laws should be strictly enforced to deter littering by errant anglers.



Instead of allowing fishing in the central catchment area reservoirs I suggest that the authorities take the following steps to further the sport of fishing in Singapore, whilst protecting our environment and biodiversity for the sake of all Singaporeans, anglers or otherwise:

1. OPEN UP LESS ECOLOGICALLY SENSITIVE WATER BODIES AS FISHING SPOTS, instead of allowing fishing in the nature reserves. Such places include:


In 1977 Prime Minister Lee Kuan Yew said:

"It should be a way of life to keep the water clean, to keep every stream, every culvert, every rivulet, free from unnecessary pollution. The Ministry of Environment should make a target: in ten years let us have fishing in the Singapore River and fishing in the Kallang River. It can be done"7.

In spite of this, it appears that streams, canals, rivers and drains, are at present viewed and treated only as means of draining away precious freshwater as fast as possible. Their roles as potential habitats for aquatic life, as aesthetic landscape features, and as places for recreational fishing, do not appear to have been taken into account. As a result, many streams, rivers, and drains that once supported a thriving community of aquatic life have been transformed into lifeless concrete drains by "drainage improvement works."

Reducing unnecessary pollution is indeed a good and necessary thing, but fish and other aquatic organisms need more than clean water. All organisms, including humans -- though we seem to have forgotten it, need to live in a balance with the rest of nature. In addition to not-too-dirty water, fish need a functioning aquatic ecosystem. They need to be part of a working food web, they need suitable hiding, spawning and nursery habitats, and they cannot survive in drains that are usually dry, or those with very fast flowing water and no shelter from the strong current, or covered drains with no sunlight.

Some suggestions to make water courses more multi-functional in land-scarce Singapore8:

  2. Where possible, earth-banked streams and rivers should be left earth-banked and not concretised. Excessive clearing of water plants should be avoided.


  3. If bank support is necessary to prevent erosion, then use breathable concrete slabs with holes in them, on sloping banks, like the banks of the Kallang river upstream of Kallang MRT station. The holes provide places where aquatic organisms can burrow and live.


  4. Mud and sand deposits should not be dredged away unnecessarily, as these are very important habitats for invertebrates, which are an important part of the aquatic food web. Mud banks are an indispensable physical component of the estuary ecosystem, and estuaries are vital for the marine ecosystem as well as being highly productive ecosystems in their own right.


  5. Mangrove and attap trees could be planted along suitable river and canal banks to improve the aesthetics of the river and to provide shelter and food for birds, fish, and other animals9.


  6. Vertical concrete walls should be avoided as far as possible. Vertical sided canals and drains are death traps for terrestrial animals that fall in and cannot climb out. Dogs, cats, small mammals, toads, and even people have fallen in and died because they could not climb out of vertical-sided canals.


  7. Many small concrete drains used to exist which had conditions capable of supporting at least some fish and wildlife. These conditions included permanent water flowing at a slow rate, cracked sections or holes in the concrete, accumulated sand, dead leaves, or other natural substrate. Such drains typically had populations of guppies, toad and frog tadpoles, walking catfish, swamp eels, snakeheads, dragonfly nymphs, and sometimes soft-shelled turtles. They were excellent places for children to scoop "drain fish" and thus interact and learn about nature. Unfortunately most such drains have now been renovated and covered over by "drainage improvement schemes" which often seem unnecessary to me. However, suitable small drains can be purposely designed and modified to support fish and other wildlife and to allow easy access to children.

As Mr Lee said in 1977, "it can be done".





There have recently been a few articles in the Straits Times (most recently "Hook the fish, but watch your baits," Dec 18, 2000) about people using live or dead bait -- natural bait -- in reservoirs, which is currently illegal. I shall now discuss the pros and cons of allowing natural versus artificial bait in reservoirs.

In most freshwater fishing locations, and for most species of fish, natural bait, whether live or dead, produces much higher bite and hook-up rates as fish are attracted by the smell and texture of the real thing, and will bite it and hold on instead of spitting it out immediately. Artificial bait is usually much less effective, and also much more expensive to buy. Some anglers enjoy the challenge and skill of fishing with artificial bait, but others prefer a more relaxed style of fishing and also enjoy catching more fish rather than spending the whole day casting lures, sometimes without getting even a single bite. The skill, patience, physical exertion, low catch rate, and the expensive tackle needed for lure fishing are all factors that may discourage children and novice anglers and put them off the sport of fishing. Thus, from an angler's point of view, both artificial and natural baits should be allowed as much as possible in order to cater to a wider range of fishing techniques and angler preferences.

Different types of natural bait pose different degrees of risk to the environment of a reservoir:




1. I. M. Turner gives a figure of 44% extinction of freshwater fish in Singapore, in "A first look at biodiversity in Singapore," National Council on the Environment (1994) p.50 . It is more than likely that this figure has since increased as very drastic environmental changes have taken place over the last few years. See also:
(search for "singapore" on these pages).


2. At least two new species of freshwater clam (identified by Chan Sow-Yan as Anodonta woodiana, and Pilsbryoconcha exilis) have established themselves in several of our reservoirs via parasitic larvae attached to introduced fish, probably imported from Taiwan, Thailand, or Indonesia. Freshwater clams have caused problems in a neighbouring country where their shells have clogged up pipes in water treatment plants.


3. Over the last 7 years many new species of non-indigenous fish have established themselves in Singapore waters, including:

From personal anecdotal observations, the rise in the variety, quantity, and geographical spread of many of these new introduced species has coincided with a corresponding decrease in the abundance of native or long established fish species, and in some cases a very dramatic reduction in total fish abundance (all species) and also other ecological changes like loss of water weed beds and growth of algal mats. Whether there is a causal relationship, and if so what it is, between the spread of exotics and the observed changes in our water bodies, has yet to be established by scientific studies. However I am not aware of any recent scientific studies on the impact of exotics in Singapore, and it is also a problem that most Singaporeans, including the relevant authorities, seem unaware of. Many of these species are imported for the aquarium or food trade and then released into the wild when owners get tired of them, or for religious merit-gaining purposes. I just got a pack of leaflets called a "Pet Starter Kit," published by the Primary Production Department, which has some basic advice for pet owners, but makes no mention of the problem of introduced non-indigenous animals.


4. TED Case Studies: Lake Victoria
On the effects of Nile perch introduction on Lake Victoria, see also:

TED Case Studies: Nile Perch, Trade and Environment

The Effects of Nile Perch Introduction into Lake Victoria


5. On introduced apple snails as pests in Southeast Asia, see:
Apple Snails
(search for "pest" on the page)

Keeping predator snails at bay

Socio-Economic Aspects of Biological Invasion, A Case Study: the Golden Apple Snail

Apple Snails in Wetland Taro Production

Some Hawaii pests arrived by invitation


6. On rifle ranges as sources of heavy metal pollution, see for example:
Heavy Metal Contaminates on Ranges

Army aims for eco-friendly ammunition
Mentions the threat that lead bullets on training ranges pose to drinking water. Gets much public relations milage out of a new lead-free tungsten bullet. I suspect that the real reason for the new bullet is its improved armour penetration capabilities and not the army's concern for the environment.


7. The then Prime Minister said that at the opening of Upper Pierce Reservoir, see:


8. Designing waterways to have more functions than just draining away water is not a new idea in Singapore, it has been mooted in various publications including a government publication, The Singapore Green Plan (published by the Ministry of the Environment, 1992). The Green Plan states on page 11: "We have developed a comprehensive drainage system to stamp out ubiquitous flooding. A ten-year Clean River Campaign, completed at a cost of $200 million has brought back aquatic life to the Singapore River and Kallang Basin. The open drains and canals will however, no longer just be functional infrastructure. They will be modified and upgraded to aesthetic waterways to allow for recreational activities in some of the larger streams." However, the things I see going on make me wonder if the government has since changed its mind and decided not to fully implement the Green Plan. See also:

Singapore: Singapore Green Plan: towards a model green city, 1992
The abstract mentions that part of the plan is to "convert open drains and canals into recreational waterways." However I see little evidence of this having been done.

Natural Resource Aspects of Sustainable Development in Singapore
This Local Agenda 21 report mentions a "Waterbodies Design Panel: Members from various agencies work to enhance the aesthetic and recreational potential of waterways to turn canals running through urbanised areas into richly landscaped rivers, enhancing the residential developments along them." If such a panel really exists, I can see little evidence of their work so far. Natural water courses continue to be turned into unsightly vertical-sided concrete drains or even completely covered over, and existing old concrete drains which have been sufficiently weathered and worn so that they can support diverse fish life continue to be "upgraded" which in practice means that they are degraded ecologically.

Green Plan aims to make Singapore model environment city by 2000
This 1991 article mentions that one aspect of the Green Plan is "Upgrading canals and rivers and improving the quality of water in them to encourage aquatic life and recreational activities... ... Modify and upgrade larger streams into 'aesthetic' water bodies with high quality water". However, it is now the year 2000, and there are very much fewer rivers and canals capable of supporting aquatic life and recreational activities like fishing, than in 1991, and this is due in no small part to the so-called "upgrading" in "drainage improvement works".


9. The Green Plan calls for waterways to be made aesthetic, and tropical coastal rivers (all the rivers in Singapore are coastal) without mangroves look about as beautiful as parks without trees. Mangroves are of tremendous ecological importance as up to 75% of commercially caught sea fish depend on mangroves directly or indirectly at some stage in their life cycle. Planting back just a few mangroves along our riverbanks may not drastically increase our supply of seafood, but it will make rivers look nicer and attract more fish, crabs, birds, and other wildlife to those places.

Many tidal canals used to be mangrove-lined rivers before they were concretised. Some of these could support a few mangrove trees and nypah (attap) palms planted along their banks, if the vertical concrete banks were made into breathable sloping banks; or alternatively, mangroves could grow in accumulated mud banks inside the canals.

As for the species to plant, Rhizophora and Bruguiera propagules do best as they are bigger and hardier to begin with. Avicennia propagules and seedlings tend to get broken and washed away by strong currents during storms and should only be planted in more sheltered situations. However even the stick-like Rhizophora seedlings are easily bent and damaged by snagging jetsam in the current, of which the worst offenders are plastic bags. For this reason, it is best to use PVC planting tubes as detailed in







The End

This document dated: 20 December 2000.