Predator series: Cormorant
by Rebecca Sherratt, Features Editor, International Aquafeed
In our International Aquafeed predator series we have explored the dangers that both seals and sharks pose to the aquaculture industry, but complications do not arise just from species that dwell underwater. Many fish farmers will be familiar with the difficulties that arise when birds prey on your fish. Luckily, there are many useful methods to mitigate the damage birds can enact on your farm.
One particularly troublesome species of bird for fish farmers is the cormorant (Phalacrocoracidae). There are two specific species of bird belonging to the cormorant family, the cormorant and the shag. Both can be distinguished by their reptilian appearance, as a result of their long necks and powerful beaks. Cormorants in the Northern Hemisphere tend to exhibit dark plumage whilst those in the Southern Hemisphere are typically black and white in colour. These large water birds are very adept fishers and eagerly feast on many fish that are farmed around the world. Their powerful feel propel them to depths of up to 45 metres (150 ft) and they pluck fish from the water with lightning speed.
Cormorants can grow up to 100cm tall with an impressive wingspan of up to 160cm. They will typically weigh between 2.1-2.5kg. This species of bird can be found throughout a variety of regions, taking roost in wetlands, urban and suburban areas as well as in marine and intertidal spaces. Historically, this species was predominantly found in coastal habitats, but this shift to urban areas was exhibited as a response to the increased availability of fish closer inland. Cormorants are especially prevalent in the UK and central Europe. The great cormorant (P. carbo) and common shag (P. aristotelis) are especially common subspecies of these birds found in Europe. Cormorants can be found in most countries, with the exception of central Pacific islands.
Hunting and feeding
Cormorants have a 'generalist' diet and devour fish of all species and sizes, as well as the occasional water snakes and eels. Their diet will vary from season to season, as well as location, but typically cormorants feed on fish anywhere between 3-50cm in length. Smaller fish will be devoured almost instantly underwater, whilst larger fish are taken onshore to consume. Whilst cormorants will eat larger fish, they will typically prefer medium-sized fish of between 10-25cm in length.
Cormorants may hunt in flocks to increase their efficiency. They eat only what they need to survive, as well as what is required to feed their chicks in the nest. A cormorant requires, on average, approximately 500g of food each day.
After feeding, cormorants will often be seen holding their wings out to dry in the sun. Cormorants have preen gland secretions that some scientists suggest keeps their wings waterproof, whilst others suggest enables their wings to be water-permeable, to ensure more efficient diving.
Cormorants are such impressive fishers that some fishermen have trained their own cormorants to assist them in their fishing endeavours. Cormorant fishing still takes place in China and Japan, wherein fishermen use trained cormorants to gather fish in rivers, with a snare used to prevent them from swallowing larger fish. Historically, this was a rather popular form of fishing, although it has since seen a decline with more efficient fishing technologies being established.
Bird prevention strategies
Cormorants can prove to be serious problems for many fisheries around the world. As agile divers, they can expertly feed on fish and even those that they do not eat can often be injured as a result of cormorant feeding. Such injuries can often lead to increased stress, risk of disease and mortality. Birds can also prove to be very difficult predators to predict as their behaviour often fluctuates depending on the season, fish population levels, weather conditions and other external factors.
Physical barriers are one possible solution to mitigate the risks of cormorants or other birds from feeding on your fish. Complete enclosures exclude predators from gaining access to cultured stocks, whilst partially covered systems can produce complications, which often minimise the damage of birds feeding and possibly cause them to fly away and search for food elsewhere. This solution is especially effective against cormorants, as they feed on whatever is the easiest available species. One potential drawback to enclosed systems is the cost. Constructing these enclosures can prove very expensive and, as a result, is not a commonly adopted technique, despite the evidence that suggests a solid return on investment through reduced mortality rates.
Another low-maintenance solution, especially effective for smaller ponds or raceways, is use of overhead line or wire systems. This form of fencing can prove to be an effective deterrent against cormorants and is cheap to maintain, with a worker simply needing to maintain the tension of the wire systems at regulated intervals.
A variety of auditory deterrents are also available on the market to minimise the threat of birds on fish farms. Noise-generating scarers can prove to be cost-efficient methods to deter birds from your farm, with motion-detectors that set the system to make noise when birds appear overhead. Similar solutions also exist that make use of the cormorants' impressive vision, by deterring birds through the use of lasers and lights that make the cormorant perceive a threat nearby.
Much like many other predators that can prove troublesome for fish farms, with the right tools and knowledge, farmers can secure the safety and profitability of their farms.