EXPERT TOPIC
CORMORANT
EXPERT TOPIC 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.
O
ne 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 feet 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 52 | August 2020 - International Aquafeed