The threats described below plague many other freshwater ecosystems around the world. But in the case of the Malili lake system (specifically Lakes Matano, Mahalona and Towuti), their presence is a complete disaster, as these over a million-year-old lakes are home to dozens of species of fish and invertebrates that live nowhere else. Some of them are even endemic to a single lake, i.e. if we lose, for example, Lake Matano, the iconic shrimp Caridina dennerli, the goby Mugilogobius adeia or the crabs Parathelphusa pantherina and Syntripsa matannensis will be lost for good.
1. Flowerhorn (louhan)
We often talk about the invasive cichlid of hybrid origin (Cichlasoma sp. "Flowerhorn" or Cichlasoma cf. trimaculatum). These fish, whose ancestors come from Central America and spread around the world due to the aquarium hobby, have become established in many places in Southeast Asia, where people have released them. They do not serve as food, fishermen do not like them, because they have little meat and a lot of bones (in a normal size of up to 15 cm, they are simply not worth fishing). Their existence is therefore not welcomed even for economic reasons.
Flowerhorns reproduce well and will eat anything they can fit in their mouths – including shrimps, small snails and crabs, fish eggs. This cichlid is already completely dominant in many lakes of Indonesia, and it is no different in Lake Matano, where its occurrence was recorded for the first time in 2010. Within a few years, flowerhorns have multiplied here in the thousands and spread to other lakes of the Malili system: Mahalona and Towuti. There are fewer of them, but Matano has already turned from the original lake with a diverse community of unique invertebrates into a fishpond full of flowerhorns. Shrimps hide deep under stones, small species of snails teeter on the edge of extinction. Large species of snails age and are not replaced by a new generation, because the young are easy prey for invasive cichlids. These fish also attack crabs and we have seen them eat juveniles and even adult crabs or deprive them of most of their limbs. They disturb the normal behavior of the fish, especially gobies, with which they compete for food and eat their eggs, which gobies lay on the rocks and care for them. According to our observations, flowerhorns do not have an effect on sailfin silversides and ricefish; these fish are fast and their eggs are too small and scattered. However, flowerhorns transform the entire lake by eliminating the invertebrates. Dozens of native species of fish, snails, shrimps and crabs that live nowhere else in the world no longer have a safe home.
2. Sailfin armored plecos (sapu-sapu)
This is another invasive species of fish that has spread from South America through the aquarium trade to open waters in many parts of the world and is doing untold damage there. Here it is even more true that fishermen do not like them, there is little meat and above all it is covered with a very hard armor. These plecos are usually representatives of two species (Pterygoplichthys disjunctivus, P. pardalis) or hybrids. They grow up to 70 cm long and in most places they have no enemies at all.
Sailfin plecos also eat the eggs of gobies and, in fact, anything that passes through their mouths, including smaller snails. In addition, these fish dig burrows in the bottom, so they cause water turbidity and siltation, i.e. covering all surfaces (bottom, stones, plants) with fine dust. This changes the character of the lake and contributes to the growth of algae, so that the originally "clean" stones inhabited by endemic species of snails and shrimps disappear. Sailfin armored plecos stay mainly near human settlements, where they feed on various food remains. There are probably already thousands of them in Lake Matano, they are breeding further and their impact on the ecosystem is increasing.
3. Algae
We already mentioned why algae are a nuisance – they cover surfaces that are the habitat of many invertebrates. These snails and shrimps need hard substrates, i.e. stones, from which they graze the “aufwuchs”. In most places of Lake Matano, however, the stones are now covered with a thick layer of algae and sludge, and in other lakes such algae patches are also increasing. Algae are either in the form of spongy continuous layers or have the form of whitish brushes. Somewhere there are also green filamentous algae.
Their spread indicates an increasing amount of nutrients. The lakes used to be ultra-oligotrophic (i.e. practically without nutrients), but the influence of human activity in the surroundings must naturally show itself. An interesting question is whether the fact that the invertebrates originally "taking care of the cleanliness of the stones" are almost eliminated by the invasive flowerhorn also contributes to this condition.
4. Pollution
Unfortunately, there is no shortage of garbage on the shores of lakes. These are brought into the water by the wind from the surroundings, but also thrown in by people without embarrassment. Apparently it is "tidy up". As if the lakes could swallow everything… Plastic bottles, cans, leftover food and its packaging, tires, metals, chemical residues, etc. Of course, none of this disappears, plastics accumulate in places where the current carries them.
Locally, untreated water from households or agriculture enters the lakes. In the town of Sorowako, a stinking whitish sewage flows into Lake Matano. Yes, right into the lake, which is world famous for its unique invertebrates…
There is a huge nickel mine in the hills above lakes Matano and Mahalona. It cleans and monitors its wastewater, but its operation certainly has an effect on the chemical composition of the lakes. No negative impact is documented, but at least an indirect effect (due to traffic and number of employees) has to be taken into account.
Water pollution has two forms: excessive amounts of nutrients and possible toxic substances. Acute poisoning is, as far as we know, only a potential risk (and hopefully it will stay that way and there will never be any disasters and/or mass deaths). However, excessive nutrients are evident in lakes; we already mentioned algae, in some places the bottom is covered with cyanobacteria. This signals an imbalance of nutrients and, together with scattered garbage, is proof that even if everything looks nice on the surface, down in the depths pollution takes its toll.
5. Deforestation
Unfortunately, on the steep slopes above the lakes, there are more and more places where the forest has given way to plantations. People grow pepper or cloves there. Instead of dense green, the red bare soil shines in the distance – and it gets into the lakes during the rains. Soil erosion thus directly leads to the aforementioned siltation, i.e. clouding of water with fine soil particles and their settling on all surfaces, as well as water pollution. Excessive nutrients, residues of fertilizers and pesticides enter the lakes from the plantations.
6. Invasively spreading plant Hydrilla verticillata
Although hydrilla (waterthyme) is native to Asia (as well as Africa and parts of Australia), it does not belong in the ultra-oligotrophic lakes of Sulawesi. Unfortunately, it got there and thanks to the increased supply of nutrients near human settlements, it is spreading. It will soon be a problem for ships, as its thick growth reaches the surface in some locations and can wind up on ship's propellers. At the same time, any mechanical disturbance of the stands leads to further spread, as the plant is fragile and breaks into many pieces that continue to grow. Hydrilla also spreads on the bottom and the rate of expansion of each "clump" is really fast. It outgrows native plants, including the endemic Ottelia mesenterium. Because ottelia grows only in a few suitable places, certainly not on large areas, it is directly threatened by the spread of hydrilla.
7. Disruption of water level fluctuations
Unregulated lakes with natural drainage were characterized by fluctuations in water level, so that several meters of the shore were always exposed for part of the year. Today, due to the operation of a nickel mine and several hydroelectric plants on the outflow at the end of the lake cascade, this natural fluctuation is gone. The shores in Lake Matano are not exposed, which may contribute to the fact that algae flourishes on the rocks in the shallow zones. Annual drying would prevent the formation of a thick carpet of algae.
8. Unregulated shrimp collecting
The factors described above show how complicated life is for small snails and shrimps today. And we aquarists shouldn't pretend that it doesn't concern us. Small snails are not commercially interesting and there is no demand for them, but the opposite is true for shrimps. While their collecting and export to aquariums around the world in the case of the vast and so far well-functioning Lake Towuti is not a current threat (with the exception of micro-endemics such as Caridina spongicola), the most sought-after shrimp Caridina dennerli only inhabits Lake Matano. There, it faces enormous pressure from flowerhorns, and its living space has been drastically reduced due to the algae growth. There are only a few places where there are still at least partially clean stones. And there are even fewer places where the pressure of flowerhorns is tolerable at the same time. Finding shrimps anywhere other than deep under rocks is almost a miracle, as we show in this video and describe in more detail here.
Local collectors build simple piles of stones, where the shrimps retreat and from where it is then easy to collect them (by rearranging the stones) in larger quantities. This is both very effective, but also problematic: you offer critically endangered creatures a safe haven and then conveniently collect them there. You are weakening the population, which even without collecting is at an all-time low and headed for extinction. Collecting is not regulated in any way and there is no question of its sustainability in Lake Matano.
At the same time, this practice of building safe shelters for shrimps by rearranging the rocks, while stripping them of thick layers of algae, could help shrimp populations. Solving other threats should go hand in hand, i.e. efforts to minimize the effects of pollution and invasive fish.
However, the will to support the transition from unregulated shrimp collecting towards the care of protected populations (perhaps in the regime of some "semi-natural" breeding) has already been declared. Of course, we try to support local citizens and institutions in this direction. If we notice any tangible progress in this area, we will be very happy to report about it.
Markéta Rejlková
