5 Ways to Eliminate the Enemies of Fish: Aquatic Insect Threats Explained

The main enemies of fish in freshwater bodies are predatory insects, birds, amphibians, reptiles, mammals, and — in managed ponds and aquaculture — crabs, otters, raccoons, and invasive species. Below, each threat is explained alongside the most effective, modern ways to control it, from exclusion netting to bubble curtains and chemical treatments.

What Are the Main Enemies of Fish in Freshwater Bodies?

The enemies of fish in freshwater bodies fall into several groups: predatory aquatic insects (beetles and bugs), bird predators such as herons, amphibians and reptiles, and mammals like otters and mink. In ponds and aquaculture operations, crabs and invasive species add further pressure. Most damaging insects are aquatic bugs that are predators, attacking fish fry and juveniles in particular.

• the fringed buzzard,
• thebuzzard beetle,
• the skimmer beetle,
• water bugs,
• dragonfly larvae,
• other enemies of fish

For a wider look at aquatic life and pond keeping, our Fishing section covers related topics in depth.

Common Fish Pond and Aquaculture Predators

Common fish pond and aquaculture predators include wading birds, mammals, and aquatic invertebrates that each target fish at different life stages. Identifying which predator is present is the first step toward choosing the right deterrent, because a strategy that stops a heron will do nothing against a burrowing crab.

• Birds: Blue Herons, grebes, ducks, gulls — visual hunters that strike from the bank or shallows.
• Mammals: River Otters, mink, and Raccoons that fish at night and can empty a small pond quickly.
• Aquatic invertebrates: Crabs (family Portunidae) and mud lobsters (Thalassina) that prey on Shrimp and fry in brackish ponds.
• Insects: Diving beetles, water bugs, and dragonfly larvae that devour fry by the dozen.

In shrimp culture, competitors and predators in ponds are a recognized economic problem. Industry outlets such as The Fish Site regularly document how crabs and wild fish entering through inlet water reduce Shrimp survival, which is why water screening at the intake matters as much as bankside deterrents.

Predatory Insects That Threaten Fish

Predatory insects threaten fish mainly as larvae and adult bugs that hunt fry near the water surface and among plants. Three groups do the most damage: diving beetles and their larvae, true water bugs, and dragonfly larvae. Each hunts differently, so each is described below.

Floater Beetle and Its Larvae

The fringed floater is the diving beetle most damaging to fish juveniles — large, up to 30 millimeters, dark green with a yellow-cream ribbon edging its back. It is one of the most efficient predators of young fish in still water.

The floater beetle is an excellent swimmer, with its rear pair of legs turned into paddles. It breathes atmospheric air by exposing the back of its abdomen, where the blowholes are located. In rivers, a larger beetle sometimes occurs, the broad beetle (up to 40 mm long, 25–27 mm wide). These beetles are predators, grabbing everything they come across.

The larvae of floater beetles are the most terrible disaster for young fish. A floater larva is a body of nine segments with a round, flat head equipped with two sickle-shaped jaws. After a series of molts, the larva reaches a length of 6 centimeters.

The floater larva usually hangs near the surface of the water, exposing the posterior end of the body with a blowhole. It swims with all six legs but can move quickly with powerful leaps. In its jaws are channels through which the larva injects paralyzing poison and digestive enzymes into the victim, liquefying the prey's tissues.

Having thrust its jaws into a fish, the floater larva rises to the surface and sucks out the victim, only occasionally moving its jaws. If a larger fish manages to escape, it dies from the poison. The voracity of the larvae is phenomenal — in one experiment they ate more than 100 fish fry in a single day. Be careful: the bite of a swimmer larva is very painful.

Skomorokh Beetle

In the southern regions, the skomorokh beetle is an enemy of fish in freshwater bodies, the same size as the swimmer's beetle but with several differences. This beetle is a subject of dispute between scientists and fish breeders: some consider it a terrible pest, others deny that it harms fish at all.

The skomorokh is easy to distinguish from the swimmer beetle. The sizes are the same, but the body of the skomorokh viewed from the side is flat, while the swimmer beetle is humped. Its coloration is emerald-green — in the sun the back seems to burn with green grains — and the abdomen is light cream, against the swimmer beetle's dark abdomen.

The skomorokh's spiracle is small and trapezoidal (large in the floater), and its paddle-legs are densely covered with hairs that the beetle spreads apart when breathing near the surface. In the floater, the legs have sparser hairs whose tips curve over the back. All swimmer bugs can pinch hard with their jaws and secrete a murky, foul-smelling protective fluid.

Water Bugs: Gladysh, Plavt, Scorpion, and Ranatra

The second group of insect pests are water bugs: the swimmerbug, gladysh, water scorpion, and ranatra. These bugs suck their victims through a proboscis. Gladysh and plavt are excellent swimmers — the gladysh swims on its back.

The water scorpion lies flat on leaves and in the tina, well camouflaged.

The ranatra is stick-like, holding onto plant stems where it is almost invisible.

These bugs hunt from ambush. Scorpions and ranatra have the front pair of legs turned into grasping tools, while plautus and gladysh chase their victims and the plautus also has grasping front legs. The bugs bite strongly, and the gladysh's proboscis stings especially painfully. A smaller relative, the coryxa bug, sings beautifully — its song can be heard from under the water. Some coryxa species feed on plant food while others are predators, but coryxa are less harmful to fish farming than the four larger bugs and their larvae.

Dragonfly Larvae

Dragonfly larvae are the third group of insect enemies of fish, reaching up to 6 centimeters in the largest species. Their terrible weapon is the so-called mask — a special hinged lever covering the head and mouth from below that shoots 1.5 centimeters or more forward in an instant.

The brown larva (sometimes overgrown with algae) sits motionless on plants and suddenly throws out the mask to grab a fry and pull it to the mouth. Small dragonfly larvae take very small fish and, more importantly, act as food competitors. Bottom-dwelling larvae strain small animals from the substrate through the mask-bucket, again competing with fish for food.

Dragonfly larvae are safe for humans, although a dragonfly can bend and prick a finger with the spines at the very end of its body. Damselfly larvae swim slowly, wriggling their long bodies; bottom larvae run faster and can jump using reactive force; the larvae of the koromyslo dragonfly shoot through the water by drawing water into the rectum and forcefully ejecting it — a real living rocket.

Bird Predators: Herons and Other Threats

Bird predators are among the most visible and persistent enemies of pond fish, with Blue Herons leading the list. A single heron can visit at dawn and dusk and take several fish per session, working the shallow margins where fish are easiest to catch. Other bird threats include grebes, ducks, and gulls, all of which target fry and fingerlings.

Beyond birds, the broader cast of "other enemies of fish" includes:

• frogs, and to a lesser extent newts, among the amphibians;
• eels and aquatic turtles among the reptiles;
• a number of birds (grebes, herons, ducks, gulls);
• mammals such as water shrews (coutoras), mink, and otters.

River Otters and Raccoons deserve special attention in pond management because they hunt at night, are intelligent enough to defeat simple deterrents, and can repeatedly return to a productive pond. In commercial salmon aquaculture in British Columbia, operators such as Cermaq Canada Ltd, Marine Harvest Canada, and the company once known as Marine Harvest contend with California sea lions and other marine mammals, illustrating how predation scales from backyard ponds to industrial farms.

Aquatic Predators: Crabs and Competitors in Ponds

Crabs and other aquatic invertebrates are major predators and competitors in brackish and coastal ponds, especially in Shrimp culture. Swimming crabs of the family Portunidae enter through inlet water and prey directly on juvenile shrimp, while burrowing mud lobsters (Thalassina) undermine pond dikes. The Fish Site and similar industry resources note that uncontrolled crab populations measurably lower harvest yields.

Crab control and trapping is the standard response: baited traps, regular netting, and screening intake water to stop larvae and small crabs from entering. Competitors that affect shrimp ponds also include wild fish such as Tilapia mossambica, which compete for feed and oxygen, so removing them is part of routine pond preparation.

5 Ways to Eliminate the Enemies of Fish

The five most effective ways to eliminate the enemies of fish are exclusion netting, bird deterrents, bubble curtains, chemical or biological treatments, and trapping or manual removal. The right combination depends on which predators are present, the size of the pond, and whether the water holds a live crop at the time.

1. Bird Exclusion Netting and Physical Barriers

Bird exclusion netting and physical barriers are the most reliable way to stop herons and other birds, because they remove access entirely rather than relying on scaring the predator off. Pond netting installation involves stretching mesh across the water surface on a frame or cable so birds cannot reach the fish.

• Material: High-density polyethylene nets, often with stainless steel cores, resist UV, weather, and beak damage far better than cheap garden netting.
• Placement: Suspend the net 30–60 cm above the water so herons cannot strike through it, and anchor the edges to deny access from the bank.
• Benefits: Netting also keeps leaves out, supports pond ecosystem health, and protects fish caves and other refuges built into the pond.

For a pond holding ornamental fish, the aesthetic benefit matters too — a well-tensioned net is far less intrusive than a cage and preserves the pond's appearance while still excluding predators. Pond contractors such as Sweetwater Landscape and other firms offering pond services in Windsor routinely fit exclusion netting as part of a build.

2. Heron and Bird Deterrent Strategies

Heron and bird deterrent strategies work by making the pond feel unsafe or unrewarding to visiting birds. They are most effective when combined and rotated, because predators habituate quickly to any single static deterrent.

• Predator decoys: A plastic heron decoy can deter real herons during the breeding season, but should be moved every few days so it does not become part of the scenery.
• Motion-activated sprinklers: Devices such as the Scarecrow motion-activated sprinkler fire a sudden jet of water when a bird approaches, exploiting the startle response.
• Visual deterrents: Reflective tape, mirrors, and floating eyes unsettle birds, though their effectiveness fades as birds habituate.
• Trained raptors: Active deterrence using trained raptors is used on large commercial sites to clear nuisance birds, an aggressive method reserved for high-value crops.

Predator habituation to deterrents is the central challenge: a heron that learns the sprinkler is harmless will ignore it. Rotating decoy positions, alternating deterrent types, and pairing them with netting keeps the deterrent unpredictable. Floating plants such as Lily pads, Frogbit, and water lettuce also help by giving fish overhead cover that breaks a heron's line of sight.

3. Bubble Curtains for Underwater Predator Deterrence

Bubble curtains deter underwater predators by releasing a continuous wall of air bubbles from a perforated pipe along the pond bottom, creating a visual and acoustic barrier that many predators avoid. They are widely used in larger operations to discourage diving birds and mammals and to keep fish away from intakes.

Acoustic harassment devices and underwater noise deterrents raise real environmental concerns. Studies of marine mammal management have linked acoustic harassment devices to habitat displacement, where California sea lions and other animals abandon feeding grounds. For freshwater ponds, bubble curtains are a gentler alternative because they create a localized barrier without the broad acoustic footprint of harassment devices.

4. Chemical and Biological Treatments for Pest Elimination

Chemical and biological treatments eliminate unwanted fish and pests by poisoning the water before restocking, and are used to clear ponds of predatory and competing species during preparation. The most established treatment is rotenone, a compound derived from Derris root that disrupts oxygen uptake in gilled animals.

• Rotenone: Applied to clear ponds entirely; its effectiveness depends on accurate dosing and water temperature, and it breaks down over days to allow safe restocking.
• Saponin and Teaseed cake: Saponin from Camellia teaseed cake is a traditional fish poison that selectively kills fish while sparing Shrimp, making it valuable in shrimp pond preparation.
• PCP-Na: Sodium pentachlorophenate (PCP-Na) has been used as a piscicide, though it is far more hazardous and now restricted in many places.

Selective poisoning methods exploit the different tolerances of target and non-target species — saponin's sparing of Shrimp is the classic example. Older broad-spectrum chemicals such as DDT, Chlordan, Endrin, Gusathion, and gamma BHC were once applied to ponds but are now banned or heavily restricted because of their persistence and toxicity. Salinity affects chemical efficacy: higher salinity can change how saponin and rotenone behave, so dosing must account for brackish conditions.

Lethal predator control of birds and mammals is legally restricted and ethically contentious. Animal welfare bodies such as the Royal Society for the Prevention of Cruelty of Animals (RSPCA) oppose unnecessary lethal control, and most jurisdictions protect herons and otters, so non-lethal exclusion and deterrence should always come first.

5. Trapping and Manual Removal of Insects and Crabs

Trapping and manual removal physically take predators out of the pond, and remain the most targeted option for crabs, large insects, and nuisance fish. Baited crab traps, fine nets passed through marginal vegetation, and light traps for night-flying beetles all reduce predator numbers without chemicals.

• Crabs: Set baited traps along dikes and screen the intake to stop Portunidae crabs and Thalassina larvae entering.
• Aquatic insects: Net beetle and bug populations from plant-rich margins, and remove egg-laden vegetation.
• Nuisance fish: Seine out competitors such as Tilapia mossambica before restocking.

Autonomous Aerial and Water Vehicles for Farm Protection

Autonomous aerial and water vehicles are an emerging tool for protecting fish farms, using drones and robotic surface craft to patrol large ponds and scare off birds and mammals on demand. Aerial drones can be programmed to fly patrols at dawn and dusk when herons are most active, while autonomous surface vessels disturb the water to break up diving birds' hunting runs. Because the patrols are mobile and irregular, they sidestep the habituation problem that defeats static decoys.

Cost-Benefit Analysis of Anti-Predation Methods

A cost-benefit analysis of anti-predation methods weighs upfront cost, ongoing labor, and effectiveness against the value of the fish being protected. Exclusion netting has the highest reliability and is cost-effective for small to medium ponds; deterrents are cheap but lose effectiveness over time; chemical treatment is reserved for whole-pond clearing between crops; and autonomous patrols carry high capital cost justified only on large commercial sites.

Dissolved Oxygen Management During Treatment

Dissolved oxygen management is critical during chemical treatment because rotenone and saponin kill fish by interfering with respiration and the resulting die-off rapidly consumes oxygen as carcasses decompose. Aerators should run throughout treatment and recovery, dead fish should be removed promptly, and restocking should wait until oxygen levels stabilize and any residual chemical has degraded. Treating in cooler conditions, when water holds more oxygen, further reduces the risk of a secondary crash.

Preventing Aquatic Nuisance Species: Clean, Drain and Dry Protocol

Preventing the spread of aquatic nuisance species starts with the Clean, Drain and Dry protocol, a boat maintenance routine that stops invasive species hitchhiking between water bodies. Invasive species such as Zebra mussels, Giant salvinia, Silver carp, and Northern snakeheads cause severe economic damage to agriculture, aquaculture, and public utilities by clogging infrastructure and displacing native fish.

• Clean: Remove all mud, plants, and organisms from the hull, trailer, and gear before leaving the launch.
• Drain: Remove the boat drain plugs and bilge drains and empty all water, including livewells and bait buckets, on land.
• Dry: Let everything dry completely, or use a decontamination station, before entering another water body.

The Arkansas Game and Fish Commission (AGFC) documents 36 aquatic nuisance species in Arkansas and promotes Clean, Drain and Dry to anglers statewide. As AGFC communications staff including Randy Zellers have reported, Zebra mussels have forced AGFC hatchery facility closures, halting fish production until facilities can be decontaminated — a direct illustration of how invasive species disrupt aquaculture. Illegal fish stocking compounds the problem and carries legal consequences, because moving fish between water bodies can introduce both disease and invasive species. Live bait regulations restrict which species anglers may use and require disposal of unused bait on land rather than in the water.

Anglers play a central role in environmental conservation by identifying invasive species in fishing areas and reporting sightings to authorities. The U.S. Fish & Wildlife Service and state agencies operate online reporting tools, dashboards, and smartphone apps that let users log GPS-tagged sightings of species such as Northern snakeheads or Alabama spotted bass spreading through the Coosa River Chain. Research institutions including the University of Arkansas at Pine Bluff study these species and support detection and removal. Volunteer removal programs and catch-and-release fishing programs that target invasive species give anglers practical ways to help conservation agencies.

Protecting Hatcheries from Invasive Species

Protecting hatcheries from invasive species relies on water screening, filtration, and strict decontamination of every vehicle, boat, and item of equipment entering the site. Water screening and filtration at the intake stops larvae and veligers — the microscopic larval stage of Zebra mussels — from entering hatchery ponds, while equipment cleaning and decontamination procedures between trips prevent cross-contamination across water bodies. Hatcheries place decontamination stations at entry points and require their use, and where a facility is already infested, agencies may close it for treatment rather than risk spreading the species downstream. The economic stakes are high, because a single contaminated hatchery can compromise an entire region's fish production and the public utilities, irrigation systems, and recreational fisheries that depend on it.