Do cormorants eat fish?

Quick Answer

Yes, cormorants are fish-eating waterbirds that dive from the surface to catch fish underwater. Cormorants are skilled swimmers and divers, using their feet to propel themselves underwater while pursuing fish. Their body shape and feather structure are well-adapted for diving and swimming after prey. Cormorants consume a wide variety of fish species, including both freshwater and saltwater fish. Fish make up the bulk of the cormorant diet.

What are cormorants?

Cormorants are medium-to-large sized waterbirds in the Phalacrocoracidae family. There are around 40 different species of cormorants, distributed widely across the world’s coastlines and inland waterways.

Some key facts about cormorants:

• Cormorants have mainly black plumage, sometimes with white patches or colorful throat pouches.
• They have long necks and slender, hooked bills.
• Most species have partially webbed feet for powerful swimming.
• Their bodies are specially adapted for diving, with dense, waterproof feathers.
• Cormorants range in size from about 25-100 cm long with wingspans of 45-155 cm.
• They nest colonially on cliffs, in trees, or on the ground.
• Many species migrate between breeding and wintering grounds.

Some examples of common cormorant species include:

• Great cormorant
• Double-crested cormorant
• Pelagic cormorant
• Red-faced cormorant
• Pied cormorant

Physical adaptations

Cormorants have evolved a range of physical adaptations that improve their diving and swimming abilities, making them highly specialized for catching fish:

• Hydrodynamic body profile to cut through water
• Feathers structured for underwater propulsion
• Feet set far back on the body for powerful kicking
• Partially webbed feet act as paddles
• Long neck for rapid strikes at prey
• Densely packed feathers resist water penetration
• Nostrils and eyelids that close when diving

These adaptations allow cormorants to dive to depths of up to 45 meters in pursuit of fish. They are agile swimmers, using their webbed feet to propel themselves underwater.

Do cormorants eat fish?

Yes, without a doubt, fish make up the majority of the cormorant diet. Cormorants are piscivorous, meaning fish-eating, birds.

They are pursuit-diving birds, using their swimming skills and sharp vision to chase down fish underwater. Different cormorant species consume a wide range of fish species, including both freshwater and marine fish.

Some examples of the types of fish eaten by cormorants:

• Salmon
• Trout
• Eels
• Sardines
• Anchovies
• Mullet
• Minnows
• Sticklebacks

Cormorants may occasionally supplement their diet with other prey like crustaceans, amphibians, and aquatic invertebrates. But the bulk of their nutrition comes from fish.

Several adaptations make cormorants highly effective fish hunters:

• Streamlined body and flexible neck make them fast swimmers.
• Long, hook-tipped bill seizes slippery prey.
• Excellent underwater vision to spot prey.
• Ability to dive deep allows access to many fish habitats.
• Social nature means they can hunt cooperatively.

By hunting in groups and diving in sequence, cormorants can herd shoaling fish into tight balls, making them easier to catch. Their diving skills and fish diet are central to the cormorant’s ecology and allow it to thrive around waters across the globe.

How do cormorants catch fish?

Cormorants have a specialized fishing technique to catch prey underwater:

1. Dive suddenly from the surface, often from a perch or float.
2. Pursue prey underwater by propelling with its feet.
3. Chase fish using rapid bursts of speed and sudden turns.
4. Grip fish with sharp bill once within reach.
5. Return to surface to eat prey.

Some key things to note about how cormorants fish:

• Can dive to depths up to 45 meters.
• Underwater chases last anywhere from 10 seconds to over a minute.
• Use their feet and wings for steering while diving.
• May dive up to 100 times a day.
• Often work together to corral fish into groups.
• Catch a wide variety of fish species and sizes.

Diving adaptations

Several physical and behavioral adaptations allow cormorants to excel at diving after fish:

• Densely packed feathers repel water and provide insulation.
• Nostrils and ear openings that seal when diving.
• Short legs placed far back help propel dives.
• Partially webbed feet act as oars.
• Body profile reduces drag underwater.
• Ability to compress lungs and reduce buoyancy.
• Powerful legs for propulsion on dives.
• Excellent underwater vision.

Together these adaptations make cormorants swift predators capable of plunging deep after fish across a wide range of habitats, from inland lakes to rocky coasts. Their unique fishing method allows them to take full advantage of the abundant food source fish provide.

What effect do cormorants have on fish populations?

The fish-eating behavior of cormorants can impact fish populations, especially where cormorants gather in large numbers. Some of the effects cormorants may have include:

• Reduce abundance of preferred prey fish.
• Shift size distributions towards smaller individuals.
• Cause declines in recreational sport fish.
• Compete with human fisheries for prey.
• Alter community structure through selective predation.

However, quantifying the precise impacts is challenging. Outcomes depend on many ecological factors:

• Density of cormorant population.
• Availability of alternative prey.
• Overlap with spawning grounds.
• Access to aquaculture facilities.
• Presence of other predators.
• Fishing pressure by humans.
• Prey fish population trends.

In some cases, cormorant predation may play a role in declining fish stocks. But it is rarely the only factor. More research is needed, but management plans should consider all aspects of system ecology.

Aquaculture impacts

Cormorants can negatively impact aquaculture facilities and commercial fisheries. Some documented effects:

• Reduce stocked fish at aquaculture ponds.
• Injure or kill fish not actually eaten.
• Damage equipment and infrastructure.
• Spread disease between ponds.
• Increase costs for fish farmers.

Many aquaculture producers view cormorants as a pest species. Deterrence methods are frequently used to try minimizing losses. But impacts likely remain significant at many facilities with large cormorant populations.

Positive effects

While cormorants are often perceived negatively, they may also confer some ecosystem benefits:

• Cull weak or diseased fish.
• Nutrient redistribution from sea to land.
• Population control of prey fish.
• Indicators of ecosystem health.
• Help sustain ecological balance.

More research is needed on quantifying the net effects of cormorant predation across different habitats. In some cases, their fish consumption may play an important role in sustaining healthy ecosystems.

Cormorant population trends

Many cormorant populations underwent significant declines in the mid-20th century from human persecution and DDT pesticide use. But recent decades have seen recoveries in most species:

• Bans on DDT and legal protection allowed populations to rebound.
• Increasing aquaculture provided abundant food sources.
• Reduced human disturbance at nesting sites.
• Greater adaptability to human activities than expected.

For example, the double-crested cormorant showed dramatic increases in North America:

Year	Estimated Population
1970	500-1000 pairs
2000	1 million birds
2020	4.8 million birds

But some localized declines have occurred where control measures are in place, such as near aquaculture facilities.

Overall cormorant population trends remain variable across different regions. Some key factors influencing numbers:

• Expansion of fish farms providing food.
• Ongoing persecution at aquaculture sites.
• Reproductive rate.
• Carrying capacity of environments.
• Winter mortality rates.
• Climate change impacts.

Careful monitoring of regional populations and ecology is important for managing cormorants effectively.

Cormorant conflicts

The dietary needs of cormorants often bring them into conflict with human interests, especially at aquaculture facilities and recreational fisheries.

Some major conflicts include:

• Impacts on aquaculture stock and profits – Cormorants may gorge on stocked fish, spread disease, and damage infrastructure.
• Reduced recreational angling catches – Predation may negatively impact popular sport fish like trout, salmon, and bass.
• Competition with commercial fisheries – May deplete stocks of commercially valuable fish.
• Ecosystem alteration – Selective predation on some species may change community structure.
• Vegetation loss – Nesting colonies increase soil nitrogen and can kill surrounding vegetation.

Attempts to reduce these conflicts sometimes involve:

• Habitat modification to exclude birds.
• Lethal control such as culling.
• Non-lethal harassment/scaring techniques.
• Egg-oiling to reduce reproduction.

However, control measures are often costly, temporary solutions. Preventing damage while conserving cormorant populations remains a difficult balancing act in many areas.

Case Study: Lake Ontario

Declines in the Lake Ontario yellow perch fishery in the 1980s coincided with increasing numbers of double-crested cormorants in the region.

Researchers sought to quantify cormorant predation effects on the perch population. Some key findings:

• Bioenergetics modeling estimated cormorants annually ate 24 million yellow perch.
• Cormorants preferentially targeted small (6-9 inch) perch.
• Size distribution shifted towards smaller perch between 1980-1997.
• Cormorants congregated near river mouths used for perch spawning.
• Perch mortality increased for juveniles but not adults.

The evidence suggested cormorant predation contributed to yellow perch population declines in Lake Ontario.

Management efforts were implemented to reduce cormorant numbers:

• Egg-oiling and nest/egg destruction at breeding sites.
• Habitat alteration to exclude nesting.
• Culling of adults at roosting sites.

These interventions successfully reduced the local cormorant population and slowed the perch fishery collapse. But additional factors such as overfishing likely also played a role.

Key lessons

The Lake Ontario case study highlights some important considerations:

1. Cormorants can locally impact preferred prey fish populations.
2. They may selectively target specific sizes/age classes.
3. Control measures can reduce but not eliminate conflicts.
4. A holistic view of overall ecology is needed.
5. Multiple factors usually contribute to fishery trends.

Research into cormorant-fishery interactions continues across many regions to find an optimal balance.

Conclusion

In summary, cormorants are specialist fish-eating birds well adapted to pursuit diving and catching prey underwater. Fish constitute the bulk of their diet across all life stages. The abundance of cormorants, with their voracious appetites, can negatively impact recreational fisheries and aquaculture operations through reduced catch and direct damage. Thus, cormorants often come into conflict with human interests despite playing an important ecological role. Ongoing research aims to quantify cormorant impacts on fish populations and determine appropriate management strategies. But in most habitats, cormorants continue to thrive as effective fish predators. Their unique fishing skills and adaptations will likely ensure cormorants remain a common sight patrolling shorelines and waterways into the future.