A rare sea bird
The Mediterranean Shag (Phalacrocorax aristotelis desmarestii) belongs to the cormorant family (Phalacrocoracidae). It is distributed solely in the Mediterranean and the Black Sea and is a distinct sea bird subspecies, breeding on rocky coasts and feeding at sea. From its cognate species, the Great Cormorant (Phalacrocorax carbo), which is observed in our country during the winter and mainly on continental waters, it is distinguished by its thinner bill and high forehead. Mediterranean Shags never occur in the interior of Slovenia. Their entire breeding population is estimated at no more than 10,000 pairs, with about half of them breeding within the boundaries of the European Union.
A summer and autumn visitor at the Slovenian sea
The Mediterranean Shag does not breed in Slovenia, but merely visits us as a summer and autumn vagrant. Its breeding colonies closest to us are located at Brioni Islands, in the Kvarner region and on the Central Adriatic islands, where its breeding population is estimated at 1,500-2,000 pairs. In our sea and elsewhere in the Northern Adriatic, Mediterranean Shags gather in the post-breeding period, from late spring onwards. In our country, more than 2,000 individuals are known to spend the summer, which is no less than 11% of this subspecies’ entire population. Each evening they gather at three different communal roost sites along the coast: at Sečovlje, Strunjan and Debeli rtič. They spend the night slightly offshore, on mariculture buoys, safe from land predators. In the morning, they head for the open sea, where they spend the greater part of the day.
An acquaintance as well as a stranger in Slovenia
Mediterranean Shags’ communal roosting sites in Slovenia have been monitored by us, ornithologists, for a number of years. For the first time they occurred here, as summer visitors, in the 1980s; from then on, their numbers have been constantly increasing. Their greatest abundance is reached in the autumn months, particularly in September and October. Before winter, most of their population returns to its breeding sites. Some individuals, however, stay in our country deep into winter. In spite of it all, not enough is known about their population in Slovenia to be able to identify the areas crucial for their survival outside the breeding season. Nothing, for example, is known about where they feed during the day, what is their population’s age structure, which fish species are most important for their diet, and which human activities pose a possible threat to them.
Northern Adriatic – a richly laid table
The Mediterranean Shag is an exclusively fisheating species. The shallow Northern Adriatic sea, rich with fish, is highly favourable for its sustenance. As these bird hunt for fish predominantly at the bottom of the sea, they spend less energy in the shallow sea of the Northern Adriatic than in deeper seas that surround their breeding sites further south in the Adriatic. They feed predominantly on commercially insignificant fish species. A daily nutritional need of adult shag is about 390 g of fish.
An extreme diver
Mediterranean Shags can dive down to 80 m deep, with each dive lasting up to a minute. During shallower dives, they use only oxygen from their respiratory tract, while during longer dives the oxygen dissolved in blood is also used. To replenish the oxygen supply between separate dives, they need from 20 seconds to a minute.
A winter breeder on remote rocky shores
Breeding sites of the Mediterranean Shags are located on remote and peaceful rocky coasts and islets. They nest in loose colonies in the shelter of rocks a few metres above the surface of the sea. Nests are built of plant material and are often used in several successive seasons. Their breeding period is allocated particularly for the winter months, although it may differ a great deal during the years. Egg laying takes place between November and March, with the actual time also depending on the geographic position of their nests. Younger birds breed somewhat later, when occupying suboptimal nest sites and thus having smaller breeding success. The clutch most often contains three eggs, with incubation lasting 30 days. The young leave their nests after about 53 days.
Owing to its limited distribution as well as relatively small and decreasing population, the Mediterranean Shag has been put on the list of priority conservation species. The subspecies appears in Annex I of the Birds Directive, which means that EU member states are obliged to adopt special measures aimed at its long-term conservation. One of the basic measures is to establish areas significant for its protection, i.e. Natura 2000 sites. The Mediterranean Shag is subjected to several threat factors, the influence of which, however, cannot be always told with accuracy. These are human disturbances, especially at its breeding sites, pollution of the sea with oil, loss of habitat, unintentional catch, overfishing, predation by introduced predators, chemical pollution, competition with other species, and illegal hunting. Some of these factors also pose potential threat to the Mediterranean Shags spending the summer in the Slovenian sea. Here, the Mediterranean Shag has not been subjected to any protection at all, the same as its habitat, i.e. the Slovenian sea, with the exception of a few smaller localities on the coast.