Measles, or the thing about freshwater megafauna

In Water Matters we have repeatedly pointed out the growing problems with the relatively large animals of marshes, lakes and rivers. There has been mention of sizable fish as a kind of climate rescuers and indicators of the good condition (in this case: connectivity) of rivers; we have sounded the alarm about the extinction of more species of freshwater dolphins and fish; we have shown the multiple effects of beaver-builder management. The largest representatives of freshwater fauna are becoming more numerous and more frequently seen, but unfortunately not in their original habitat, but mainly in aquacultures and zoos. Work is underway to domesticate a number of species, from giant salamanders to caimans – not to mention fish!

Not only on land and not only in the ice age

The term megafauna (from the Greek for giant animals) is used almost exclusively at the intersection of archaeology and paleontology to refer to the largest land animals. Most of them – from mammoths and saber-toothed tigers to sack lions and varanasi-megalans – became extinct 100-40,000 years ago due to climate change, human expansion and a few other species (e.g.: the roseate crocodile in Australia and Oceania). Only a few – such as giraffes and bison – have survived to our time. Even fewer are left whose populations are recovering (e.g.: moose, beaver), or at least are stable (e.g.: Baikal seal).

In global science and zootechnics, the term megafauna is increasingly being used more broadly to refer to the heaviest marine, migratory and freshwater animals. Representatives of the oceanic megafauna will therefore be whales, giant squid, the most magnificent turtles, tunas and sharks. The inhabitants of fresh and brackish waters are much smaller. In the latter group, a body weight of 30 kg or more is considered the limit for belonging to the megafauna. This indicator usually refers to vertebrates other than birds, which, due to their pneumatic bones and air sacs in their lungs, remain noticeably lighter than mammals or fish of the same body length. A lower weight criterion would be appropriate for Aves. Maybe only 2 kg (the weight of many heron species)? Maybe 5 kg (the weight of a mute swan)? [3, 7, 8, 9]

Wetters wanted! In the river, lake, soup and canned food

Freshwater megafauna is receiving more and more attention. Why? Because it is a promising source of food for humans, while providing a number of ecosystem services to wildlife and the human economy. In popular science podcasts or newspaper articles, many representatives of this group are jokingly referred to as murrelets. In fact, their influence on the dynamics of river flows, the extent of water bodies, the physical structure and chemistry of bottom sediments, and the course of the shoreline consists in digging burrows, spreading nests, erecting dams, etc., thus, figuratively and literally, in stirring up water and the bottom [3, 7, 8, 9].

Within the megafauna, we find both apex predators and the largest plant- and herbivores. They are linked to other organisms by a series of indirect interactions, known scientifically as indirect trophic cascades and paratrophic cascades. Protecting megafauna can mitigate climate change, as well as save smaller and lesser-known species. Despite growing environmental awareness, more species of the heaviest fish, reptiles or mammals are becoming extinct before our eyes. From 1946 to 2006, for example, populations of the Ganges gavial declined by 96-98 percent (from some 5,000-10,000 individuals to fewer than 250) [Lang et al. 2019]. We summarized the most recent losses among freshwater dolphins and fish (including particularly impressive ones like the Yangtze sturgeon and Chinese paddlefish) in Aquatic Affairs [11].

Ludoist or heroine?

However, not everyone is looking for the murrelets to get to know them better and save them for future generations. The more people settle on the water’ s edge, the more frequent and the fiercer the conflicts between Homo sapiens and animals become. In Water Matters we have already written more than once about real and imaginary problems with the beaver and cormorant in Poland. There are growing reports of fatal attacks on humans by hippos. River dolphins, especially the pink ones, are blissful on a computer screen, but in reality they can severely injure.

In the United States, the presence of crocodile destroyfish is becoming a pressing socio-political issue, dividing society as much as attitudes toward tigers in India or wolves in the EU. Some see these fish as a deadly threat to bathing children and hard-working fishermen (as well as pests that tear up fishing nets, gnawing up the catch, not to mention poisonous eggs for humans). For the second, the crocodile destroyer is an irreplaceable regulator of the abundance of other species, a pillar of the entire ecosystem, and a symbol of river and wetland conservation. For thirds, it is a forward-looking slaughter species, the breeding of which will put less of a strain on the environment than the aquacultures of other predatory fish that lack the ability to breathe atmospheric air [1, 10].

Holes in embankments, gaps in knowledge

River and lake megafauna are generally declining, while scientific papers about them are increasing. Unfortunately, the literature documenting the ecology of this group is as out of balance as a river after a golden algae bloom. There is no shortage of works on animals crucial to wildlife, drought prevention or flood safety in highly developed countries, such as beavers, sturgeons and noble salmon. Much attention is paid to flagship species of national parks and zoos that will probably become extinct in our lifetime despite millions of dollars spent on their protection, like Javan rhinos, or became extinct a few decades ago, like the Thai barasinga [3, 4, 7, 8, 9].

Instead, there is a lack of works on megarabs (weighing more than 30 kg), mega turtles (e.g.: arrau) and crocodiles of the Global South. Little attention has been paid to a synthetic assessment of the impacts of all megafauna on biotopes (non-living nature). A reliable assessment of the impact of the heaviest animals on ecosystems, climate and economics is hampered by the shrinking ranges of megafauna, the decline in numbers, and the loss of record-breaking large (usually, therefore, the most prolific) individuals [3, 4, 9].

Sometimes the oldest fishermen no longer remember what species were in their waters just a few hundred years ago, or how huge the fish, “mud devils” or caimans still caught today, as long as they could reach maturity. (We, too, were amazed, looking at some of the catfish and pike spawners that died during blooms in the Oder River…). Filling these gaps in the knowledge of scientists, ranchers, and environmental engineers, and then whole societies, will improve the protection of human life and property.

It should also make a significant contribution to improving food security, as well as mitigating the undesirable effects of climate change. Even with us in the European Union, we have a lot to explore! And to implement! Methods to mitigate beaver-human conflicts, such as nets to protect dikes and dykes of fish ponds, as well as repellents and electric fencing to protect fish ponds from otters, are waiting to be developed [3, 4, 7, 8, 9].

That’s the biggest embarrassment to make two want to do it at once!

Dozens of representatives of freshwater megafauna are much more numerous today than they used to be, as well as entering new areas. However, this is true of human-reared species – from sturgeon, salmon and carp, to giant salamanders and many alligators, to water buffalo and Canadian beavers [4].

Some have become extinct within their former homelands, while others are well on their way to extinction. The effects of their domestication on native wildlife, the economy and human health have still not been fully studied. In the case of certain taxa, like the Amazonian arapaima and the giant salamanders of the Far East, only domestication has shown that we are dealing with several distinct species. Although they look and taste identical from a human perspective, reproductive isolation separates them.

Offspring of different evolutionary lines will be infertile or will not arise at all. Careless introductions or tolerating escapes from aquacultures of reproductively isolated forms, while occupying the same ecological niches, brings deplorable consequences for native populations of their relatives. A prime example is the giant salamanders of Japan, increasingly displaced by their cousins from China. Although the importation and release of these giant Chinese amphibians in the Land of the Cherry Blossom is absolutely prohibited by law, it is nevertheless quite common [6, 12]. In South America, on the other hand, the selection of new varieties of arapaim encountered unexpected difficulties when specimens from distant river basins proved mutually infertile [2, 5].

Fish bite best in murky water. Especially in a new place

While coarse fish are declining in their original homelands, some are doing surprisingly well in new places. Nearly half of the 134 freshwater megrims that have survived to this day have been deliberately used to stock remote river basins where they have never been found before. Some 70 percent of these acclimatizations have been permanently successful – self-renewing populations have been established, capable of continuing to survive without human assistance and even despite increasing human pressure. As you can easily guess, this was not good news for native species of fish, invertebrates and seaweeds.

Scientists have identified about 10 different mechanisms by which alien megarabs devastate established native wildlife. The most readily apparent are predation, herbivory and competition. In addition, there are interspecies interactions that are more difficult to study, such as the spread of diseases and parasites, loss of spawning and wintering grounds, disruption of nutrient cycling necessary for other species, and finally hybridization leading to infertile or poorly adapted to local conditions hybrids [3, 4, 7, 8, 9].

In the article, I used, among other things. z:

1. Alfaro, R., Gonzales, C., Ferrara, A. (2008). Gar biology and culture: status and prospects. Aquaculture Research, 39, 748-763.
2. Carvajal-Vallejos, F. M., Carolsfeld, J., Montellano, S., Koop, B., & Nelson, J. (2023). Looking for a home in foreign waters: population genetic structure of the introduced Arapaima gigas in Bolivia. Neotropical Hydrobiology and Aquatic Conservation, 4(1), 27-44.
3. Chen, X., Jähnig, S. C., Jeschke, J. M., Evans, T. G., & He, F. (2023). Do alien species affect native freshwater megafauna? Freshwater Biology, 68(6), 903-914.
4. Chen, X. (2024). Freshwater megafauna in a changing world: An alien species perspective (Doctoral dissertation) https://refubium.fu-berlin.de/bitstream/handle/fub188/43408/PhD%20thesis_Xing%20Chen_30.04.2024.pdf?sequence=1 [dostęp 5.10.2024]
5. Du, K., Wuertz, S., Adolfi, M., Kneitz, S., Stöck, M., Oliveira, M., … & Schartl, M. (2019). The genome of the arapaima (Arapaima gigas) provides insights into gigantism, fast growth and chromosomal sex determination system. Scientific Reports, 9(1), 5293.
6. Fukumoto, S., Ushimaru, A., Minamoto, T. (2015). A basin-scale application of environmental DNA assessment for rare endemic species and closely related exotic species in rivers: a case study of giant salamanders in Japan. Journal of Applied Ecology, 52(2), 358-365.
7. He, F., Svenning, J. C., Chen, X., Tockner, K., Kuemmerle, T., le Roux, E., … & Jähnig, S. C. (2024). Freshwater megafauna shape ecosystems and facilitate restoration. Biological Reviews 99, 1141-1163.
8. He, F., Thieme, M., Zarfl, C., Grill, G., Lehner, B., Hogan, Z., … & Jähnig, S. C. (2021). Impacts of loss of free-flowing rivers on global freshwater megafauna. Biological Conservation, 263, 109335.
9. He, F. (2019). Diversity and risk patterns of freshwater megafauna: A global perspective (Doctoral dissertation) https://refubium.fu-berlin.de/bitstream/handle/fub188/25169/Thesis_Fengzhi_He.pdf?sequence=3 [dostęp 5.10.2024].
10. James, D., Contreras-Sanchez, W., Solomon, D. (2012). Development of More Sustainable Practices for the Culture of Gars and Other Air-Breathing Fish. Vol. 27, no. 3. Oregon State University.
11. Lang, J., Chowfin, S., Ross, J. (2019) [errata version of 2019 assessment]. Gavialis gangeticus. IUCN Red List of Threatened Species. 2019: e.T8966A149227430.
12. Yan, F., Lü, J., Zhang, B., Yuan, Z., Zhao, H., Huang, S., …. & Che, J. (2018). The Chinese giant salamander exemplifies the hidden extinction of cryptic species. Current Biology, 28(10), R590-R592.