You must be kidding! How can there be sharks in the lake? !

Shark watching in the lake? It's not a fantasy.

In Lake Nicaragua, the largest freshwater lake in Central America, there live many bull sharks (Carcharhinus leucas). People once thought they were relics of ancient geological activities, but then scientists discovered that they can move freely between lakes and oceans .

A bull shark still moves freely in shallow water | Albert Kok / wiki commons

Salt water and fresh water, switch freely

Lake Nicaragua covers an area of ​​8,264 square kilometers, which is equivalent to 1.8 Qinghai Lakes. It and Lake Managua in the north were originally a bay area. Later, a volcanic eruption spewed out a large amount of magma, built a natural dam in the sea, and turned the bay into a lake.

The infusion of fresh water gradually replaced the original seawater, earning Lake Nicaragua the name "Sweet Sea". However, while it is easy to replace the water, it is extremely difficult to relocate the inhabitants of the water. Many marine creatures were trapped in the lake and either became extinct or adapted to the new environment after a long period of evolution.

Today, we can find sharks, sawfish, and bigeye tarpon in the freshwater of Lake Nicaragua— except in Lake Nicaragua, where they live almost exclusively in saltwater .

Lake Nicaragua is the largest lake in Central America. Looking out of an airplane window at Lake Nicaragua, the dumbbell-shaped island of Ometepe is made up of two volcanoes, and the narrow shore is the Pacific Ocean at the far end | Los Viajeros / Wikimedia Commons

Bull sharks in Lake Nicaragua were once thought to be a closed population, but later it was discovered that they traveled through the San Juan River to the Caribbean Sea to reproduce.

This is a remarkable ability , even more amazing than their adaptation to freshwater life. During the approximately two-week journey, bull sharks use multiple organs such as kidneys and rectal glands to continuously adjust the salt concentration in their bodies and gradually adapt to the environment. This ability to adapt to different salt levels is called euryhaline.

The geographical location of Lake Nicaragua. The San Juan River flows from the southeastern side of Lake Nicaragua, crosses the Central American continent from west to east, and flows into the Caribbean Sea | Rudyologist / Wikimedia Commons

With this ability, the bull shark has become a common freshwater shark . It is distributed in tropical and subtropical waters around the world, and also appears in the Amazon River Basin in Peru and the Mississippi River in the United States. Because of its wide distribution, it has its own names in many regions - it is called "Nicaragua shark" in Central America, "Zambezi shark" (named after the Zambezi River) in South Africa, "bulldog shark" in French, and "sardine shark" in Spanish.

Bull shark range | Chris_huh / wiki commons

Dangerous "scapegoat"

For a species, a wide distribution means greater adaptability to the environment, but this also creates problems for bull sharks (and humans).

Bull sharks gather near shore at Walker Caye, Bahamas | Albert Kok / Flickr.com

Bull sharks like to haunt waters that overlap with many holiday resorts, and they are ferocious and their fighting power should not be underestimated. Adult bull sharks are usually 1.5 to 2.3 meters long, and the largest individual on record is 4 meters. Scientists suspect that most shark attacks on humans may be caused by bull sharks.

Among sharks that can be identified as species-specific, bull sharks rank third in attacks on humans, behind great white sharks (Carcharodon carcharias) and tiger sharks (Galeocerdo cuvier).

The three most vicious sharks (from top to bottom): great white shark, tiger shark, bull shark | Elias Levy, Albert Kok / wiki commons, David Snyder / Florida Museum

But for non-professionals, it is actually very difficult to identify sharks in the wild , especially when an attack occurs and people are panicking. It is conceivable that the wrong identification of sharks will happen from time to time when filling out records. At this time, the famous guys are more likely to suffer losses.

Due to the popularity of the film series, the story told in "Jaws" is deeply rooted in people's hearts, and the low-key bull shark has probably put the blame on those big guys a lot .

Sharks attack humans just because of poor eyesight?

In fact, humans are not on the shark's "regular menu" . Bull sharks mainly feed on fish (including other smaller sharks), but they will not miss other sources of protein. Their food records include turtles, shrimps and crabs, squids, seabirds, dolphins, and even dogs and their pups.

A popular view is that sharks mainly rely on their sense of smell to find food, and their occasional attacks on humans are probably due to poor eyesight .

A study on great white sharks found that people paddling on surfboards look a bit like seals when viewed from above. When young great white sharks start to learn to hunt larger prey, they may make mistakes and attack humans. Tiger sharks and bull sharks may mistake humans for turtles .

From a shark's perspective, a person paddling on a surfboard looks a bit like a seal | Elias Levy / theconversation.com

Compared with the threat posed by sharks to humans, humans pose a greater threat to sharks.

Bull sharks are currently rated as Near Threatened (NT) , but they and many other sharks are affected by the shark finning industry, illegal trade and bycatch (i.e. accidental capture when fishing for other fish), and their overall survival is not optimistic. A statistical study published in the journal Nature shows that due to a significant increase in fishing pressure, the abundance of sharks and rays worldwide has dropped by more than 70% since 1970.

Concepcion Volcano on Ometepe Island in Lake Nicaragua | Adrian Sampson / Flickr

Protecting bull sharks is also about protecting tropical waters with extremely high biodiversity , as well as Lake Nicaragua and surrounding lakes of all sizes.

Nicaragua is known as the "Land of Volcanoes". Every time a volcano erupts, it encloses the bay into a lake, like burying a "time capsule" in the course of evolution. We can use this to observe what kind of evolutionary path a group of marine organisms will take if they are isolated in different historical periods.

References

[1] Pillans RD et al. (2005) J. Comp. Physiol. B 175 (1): 37-44

[2] Florida Museum | Discover Fishes: Carcharhinus leucas

[3] Florida Museum | International Shark Attack Files: Species Implicated in Attacks

[4] Ryan LA et al. (2021) JR Soc. Interface 18 (183):20210533

[5] Pacoureau N et al. (2021) Nature 589 :567-571

[6] Elmer KR et al. (2014) Nat. Commun. 5 :5168

Author: Maya Blue

Editor: Old Cat

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