Tern

Terns (genus Sterna) are seabirds belonging to the family Laridae. They are slender, graceful birds with long, pointed wings and deeply forked tails, giving them a distinct silhouette.

Often mistaken for gulls, terns are smaller and more agile in the air. They exhibit remarkable diving abilities, hovering over the water before plunging to catch small fish.

Most species display a white and grey plumage, often featuring a black cap during the breeding season. They are highly social and usually nest in colonies.

In the context of agriculture and specifically aquaculture, they are viewed as opportunistic predators that target artificially stocked fish populations.

Their persistent hunting behavior and high mobility make them challenging to control once they discover a productive feeding ground like an aquaculture pond.

Terns primarily cause economic loss in fish farms by predating on juvenile fish, particularly carp and trout fry, which are essential for future production cycles.

Continuous predation by a colony can significantly reduce the stocking density of fry, leading to decreased yields and financial instability for the farm.

Aside from direct consumption, terns often wound fish during failed hunting attempts, leaving the injured stock susceptible to fungal and bacterial infections.

These birds can also act as vectors for various fish parasites, introducing pathogens into the pond system through their presence and waste.

The constant predatory pressure creates an environment of stress for the fish, inhibiting growth rates and overall health, even for those that are not directly eaten.

The presence of terns at fish farms is most prevalent during the spring and summer months, aligning with the breeding season and the rearing of chicks.

The arrival of spring coincides with the release of fish larvae into nursery ponds, providing an ideal and easily accessible food source for the terns.

During the peak summer months, the demand for high-protein food to nourish their growing chicks drives the terns to hunt near ponds with increased intensity.

Activity gradually wanes as fall approaches and the birds initiate their migratory journey towards warmer climates to escape the coming winter.

Understanding these seasonal patterns allows farm managers to schedule prophylactic protective measures during the most vulnerable development stages of the fish.

The most obvious sign of tern activity is the persistent hovering and diving behavior over the water surface during daylight hours.

Farmers may notice groups of terns circling or congregating around specific sections of the pond, indicating high fish density in that area.

Disturbed fish populations that avoid surface swimming or hide among aquatic vegetation are common indicators of persistent avian threats.

Unexpectedly low fry counts during periodic sampling or inventory checks often confirm that predation by fish-eating birds is occurring.

• Repeated plunging and diving into the water.
• Increased bird noise around the pond banks.
• Injured fish floating on the surface with visible beak marks.

Physical exclusion is the most effective method of control; installing overhead netting over ponds prevents terns from accessing the water surface entirely.

Acoustic deterrents that broadcast distress calls or irregular sounds can temporarily discourage terns from establishing a feeding pattern at a site.

Visual deterrents like reflective ribbons, inflatable predators, or moving scarecrows can offer some protection, though their effectiveness diminishes over time.

Creating aquatic refuges using artificial floating structures allows fry to seek shelter when predators are hovering above, increasing their survival rates.

Maintaining clear pond perimeters and removing potential nesting or resting areas nearby makes the environment less hospitable and attractive to terns.