How Do Fish Get Into Lakes And Ponds? [Surprising Facts]


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How Do Fish Get Into Lakes?

Dove Lake
Freshwater fish can enter completely isolated lakes with the help of several external factors. Bjørn Christian Tørrissen, CC BY-SA 3.0, via Wikimedia Commons

Lakes can be landlocked or part of a greater network of wetlands. As many of them are almost fully-isolated bodies of water, connected just by small stream systems, one might wonder how fish got into them in the first place. Even more mind-boggling is how fish of the exact same genetic populations can be present in totally separate freshwater bodies. The rationales behind this phenomenon give a glimpse into the ecological connectivity of natural resources.

When lakes are connected by “liquid” highways, vigorous types of fish may independently travel between them. This is likely if the conditions in both lakes, along with their associated streams, are conserved. Lakes that are connected via estuaries and streams to coastal waters tend to have some of the most diverse communities, with salt-tolerant species shaping the community.

Strictly freshwater fish enter totally isolated lakes with the help of many external factors. Without these, they are unable to disperse on their own. The many ways in which fish dominate modern-day wetlands are enumerated below. As they enter lakes, their survival hinges on their life stages and on their unique adaptations.


1) Natural dispersal

Salmon in lake
Feeding pressures, climate change, and competition can cause fish to exit lakes and chance upon new ones. Alaska Region U.S. Fish & Wildlife Service / No copyright

As freshwater fish populations evolve over time, they naturally disperse into new areas. Feeding pressures, changing climates, competition over territories, and shifting currents can cause fish to exit lakes via a network of associated streams. If enough sexually mature males and females are able to chance upon “new” lakes, they can begin to reproduce and colonize the waters.

The odds of successful colonization are extremely thin after just one dispersal event. It usually takes several chance events, where enough fish must be present to produce fertilized eggs; the eggs must then hatch into viable fry. There’s no guarantee that the fry will eventually grow into reproducing adults, with a sex ratio that can lead to enough offspring to survive, and so forth. It can take centuries for populations to disperse and become naturalized in new bodies of water.


2) Seasonal migration

Coho salmon migrating
Coho salmon (pictured) are anadromous, meaning that they spend most of their lives at sea and only enter freshwater systems to spawn. Oregon Department of Fish & Wildlife / No copyright

Fish that seasonally migrate between lakes and marine environments can be classified as anadromous or catadromous species. Anadromous fish spend most of their lives at sea, only entering freshwater systems to spawn. Inversely, catadromous fish spend most of their lives in lakes and rivers, only swimming downstream and entering the sea to reproduce.

As these fish regularly migrate, some individuals may find themselves in fully landlocked bodies of water. Those that survive become increasingly adapted to conditions in lakes and rivers, where they evolve into subpopulations that don’t necessarily migrate in order to spawn successfully. Some have straightforward migration pathways that take them in and out of inland freshwater systems, so they are only present in lakes for a portion of the year.


3) Headwater capture

Stream bank erosion
Erosion can cause a headwater capture, which is when the original direction of the river or tributary changes. Toby Speight / CC BY-SA 2.0

Some fish are able to move into basin systems that are completely separate from their original habitats via headwater capture. A headwater, river, or stream capture is a phenomenon that occurs when the original direction of the river or tributary is diverted. This can be caused by erosion, whereby the stream banks or shoreline are worn down enough to redirect the flow of water into another nearby stream or basin. The movement of tectonic plates, glacial retreats, and the production of natural dams (e.g. by a landslide) can also result in headwater capture.

This geomorphological event can carry new fish populations into lake systems that were once wholly separate or isolated. Of course, this takes place over the course of centuries to millennia, at which point the diverted populations of fish begin to develop their own genetic signatures. New species variations and wholly new species can be produced via this manner of natural dispersal (i.e. genetic drift).


4) Floods

Flooded lake
Floods can carry fish, their fry, and their eggs, into new bodies of freshwater. Cyril Mayaud / CC BY 3.0

Considerably forceful floods can carry fish, their fry, and their eggs into wholly new bodies of freshwater. It can also cause freshwater systems to merge or intersect in more places, creating more passageways for dispersing fish populations.

Interestingly, floods can also allow separate populations of a single species to interact with one another and increase their genetic variation. Inversely, it can lead to the loss of some isolated subpopulations, which are forced downstream of their original habitats. This way, the isolated groups of fish may enter larger lake systems and interbreed with their existing conspecifics there.


5) Hitchhiking eggs

Otters drinking from river
Fish eggs may stick to migrating animals’ feet as they stop to take a drink along the shores of a stream, lake, or river! Photo by Kieren Ridley

Just because freshwater fish are aquatic organisms doesn’t mean they can only move from one place to another via water. One of the most important ways fish are naturally introduced into isolated bodies of water is through terrestrial and avian vectors. These come in the form of furry mammals, moist reptiles, large-footed waterfowl, and more!

Just as these vectors may carry microbes, seeds, and plant fragments, they may inadvertently bring around fish eggs. These little hitchhikers can usually survive in damp fur and feathers for several hours to days, allowing them to remain viable until they can be re-submerged in water. For example, as migrating animals must stop for a drink along the shores of a stream, pond, or lake, fish eggs may stick to their feet. These eggs remain there until they are dislodged by water at the animals’ next stop. Fish species can travel remarkably long distances in this manner.

Hitchhiking eggs are often how vernal ponds and new streams become occupied with fish. As long as traveling animals are able to interact with the water before moving to another location, aquatic materials get transported. Some types of fish eggs (e.g. that of the killifish and common carp) are so hardy that they can survive being eaten and expelled by some birds. They can literally fall from the sky and land in a new lake!


6) Survival in droughts

Nothobranchius fish
Nothobranchius eggs can survive for months in dried-out lakes, ponds, and rivers, as they enter a special form of hibernation called ‘diapause’! Mahomed Desai / CC BY 4.0

In some cases, fish are able to colonize isolated bodies of freshwater because they were already there to begin with! Yes, this means that they were present in one form or another in the “dry” basin. Some types of fish eggs (and a few species of freshwater lungfish as well) are able to survive through periodic droughts.

Nothobranchius eggs, for example, can survive for months in lakes, ponds, and streams that have dried out. They do so by entering a special form of hibernation called a ‘diapause’. During this period, their developmental stages are temporarily arrested and the eggs become resistant to hypoxia. Once the ephemeral bodies of water are filled with rain, the eggs exit diapause, resume development, and soon hatch.

Native to eastern and southern Africa, the Nothobranchius genus likely had to evolve to survive the markedly dry conditions outside of the monsoon season each year. Their eggs are able to persist in some of the most isolated freshwater bodies, including those that are separated from other wetlands by vast deserts. They are also transported on the fur or wings of animals that interact with ephemeral systems.

Air-breathing freshwater lungfishes, found in the drought-susceptible regions of Africa, South America, and Australia, are another group of fish that can survive for years in dried lakes and ponds. They do so by burrowing into dry earth and creating cocoons (made of their mucus secretions) that harden around their bodies. They stay alive by breathing the air that seeps into their tough burrows. Once their basins become filled with water, they re-emerge.


7) Active or passive introduction

Fish farm
Fish that escape from fish farms, especially those that are close to natural waterways, can eventually find their way into lake systems. Artur Rydzewski / CC BY 2.0

Humans have played a large part in the introduction of freshwater fish into both artificial and natural bodies of water. Hardy exotic species, which are highly adaptable and prone to quickly colonizing wetland systems, are often actively or passively introduced via anthropogenic activities. Artificial vectors, such as boats and fishing gear, may passively collect fish eggs and redistribute them to other locations.

Escapees from aquaria and fish farms, especially those which are located close to natural waterways, can eventually find themselves in lake systems. They can easily disrupt the balance in ecologically stable waters by producing new populations of opportunistic fish. This is why it’s immensely important, if possible, to stock ponds and fish farms with native species.

Occasionally, fish are deliberately stocked into lake systems in order to rehabilitate their populations or to meet the demands of recreational anglers and commercial fishers. This is usually done, with special permits and sanctions from the relevant state wildlife departments, for the maintenance of economically important and ecologically threatened fish species.


A Combination of Natural and Artificial Means

The ways by which fish enter lake systems are more complex than they may first appear. It seldom takes just one method of introduction as many factors can impede their successful settlement. The most rapidly-occupied waterbodies are those which have fish that arrived by both natural and artificial means. Even terrestrial animals aiding in the spread of fish travel through natural pathways may be affected by human activity.

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