The Occurrence of the Great Cormorant Phalacrocorax Carbo in Sweden, with Special Emphasis on the Recent Population Growth
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ORNIS SVECICA 11:155–170, 2001 https://doi.org/10.34080/os.v11.22854 The occurrence of the Great Cormorant Phalacrocorax carbo in Sweden, with special emphasis on the recent population growth HENRI ENGSTRÖM Abstract The population of Great Cormorants Phalacrocorax carbo tion continued to grow at a high rate. With growing sinensis in Sweden has increased considerably in size cormorant numbers, conflicts with human interests, main- during recent decades and currently Sweden holds about a ly fishery, have increased. Hunting and egg pricking have quarter of the total Northwest European population. In frequently been used as methods to reduce cormorant 1999, the population contained an estimate of 25,600 densities locally and to solve fishery related problems. It pairs, distributed over about 154 colonies. The increase seems, however, as these measures, in most areas, only was particularly strong between 1986 and 1994 (mean have had limited effects in terms of stabilising or reducing annual increase 31%), and the population grew from 1800 population size of cormorants. to 15,500 pairs. After the mid-1990s, the population in- crease within most of the core area appear to have levelled Henri Engström, Dept. of Population Biology, Evolution- off, while now fluctuating in size. However, in some ary Biology Centre, Uppsala University, Norbyvägen 18D, northern breeding areas (including the coasts of Söder- SE-752 36 Uppsala, Sweden. manland, Uppland, Gotland and several lakes) the popula- Email: [email protected] Received 20 February 2001, Accepted 8 June 2001, Editor: A. Hedenström Introduction from, mainly, fishery organisations to reduce cor- In many countries in Europe the Great Cormorant morant numbers. The main areas of conflict are Phalacrocorax carbo sinensis became extinct in the related to (1) effects on natural fish populations 19th century or remained at low numbers (Zijlstra & leading to decreases in fishery yields (e.g. Keller van Eerden 1989, Lindell et. al 1995). In the early 1995, Suter 1995b, Dekker 1997, Engström 2001), 1970s only a handful of colonies existed in the (2) predation and damage to fish in standing fishing Netherlands, Sweden and Denmark and the total gears (Bildsoe 1998, Engström 1998), (3) fish con- population in Europe consisted of no more than a sumption at fish farms (Osieck 1991), and (4) effects few thousand pairs (Bregnballe 1996). However, on the vegetation at breeding and roosting sites. due to several protective measures taken in various In the management recommendations for the Great countries in the period 1965–1980 the population Cormorant in Europe, it is suggested that each coun- slowly started to recover. From about 1980 and in the try, or regional authority, should attempt to reduce following 10–15 years, populations of cormorants in these conflicts by local solutions (Anonymous 1997). several countries showed a spectacular population In 1995, Sweden joined the European Community development with annual increases of 10–25% (Breg- and the open hunting season on cormorants had to be nballe 1996). From about the mid 1990s, the popu- abolished. Due to increasing conflicts between cor- lations in the core areas of the Netherlands, Den- morants and mainly fishery interests, local authori- mark, Germany and southern Sweden appear to have ties have in several areas decided upon widespread became saturated. control actions during the last ten years. In addition The return of the Great Cormorant has been so to management practises approved by local author- successful that renewed conflicts have come into ities, a number of illegal actions against cormorants existence in many areas. This has led to a pressure have taken place in the same period. However, 155 Cormorant colony (17 nests) at the islet Måsgrund close to the island of Svartö in southern Kalmars Sound, 1965. In total, 71 chicks were killed that year at the colony and the cormorants did not return to the islet the following year. Tage Olsson at a hide used for duck hunting (photo Edling Olsson). Skarvkoloni (17 bon) på Måsgrund ca 1 000 m från Svartö i södra Kalmarsund 1965. 71 ungar dödades det året och skarvarna återkom aldrig mer till platsen. Skjutskåran användes vid andjakt. Fotot taget av Edling Olsson med fiskardrängen Tage Olsson i skåran. despite the disputed value of present day manage- Methods ment actions, no effort has been taken to evaluate the I gathered data on recent population estimates of the effects on local or national cormorant population size of Great Cormorant colonies, mainly from local sizes. Furthermore, no information exists on to what ornithologists. Annual number of breeding pairs are level management actions has solved or reduced based on nest counts, usually at the peak breeding fishery-related problems in Sweden. season. Normally, a group of nests was considered as The aim of this study is to give data on the a colony when separated by at least 200 m of water population development of the Great Cormorant in from other clumped breeding pairs. In rare cases, Sweden, with special emphasis on the last fifteen data from an established colony were missing in one years with rapid population growth and extensive year. In those cases, I chose a value in between the geographical expansion. This paper also reviews the most immediate accurate sampling years. Data on historical distribution of the cormorant in Sweden. management actions were collected from County Moreover, data on management practises to control Administration Boards, and data on illegal actions cormorant numbers and illegal persecution are pre- mainly from persons doing the inventories. sented, and the roles of these actions are evaluated in The geographical division of the data is based on relation to population development. provinces. Data from Lake Vättern, Lake Vänern, Lake Hjälmaren and Lake Mälaren (the four largest lakes of Sweden) are treated separately. Data on cormorant historic distribution were collected from the literature. 156 Results tions similar to present day levels. For example, the colony at Lake Krageholm contained some 50 pairs Archaeological data after some years of existence. Two Ph. c. sinensis The history of the Great Cormorant in Sweden after shot at Lake Krageholm in April and May 1881 the last glaciation has recently been examined by (stored at Lund Zoological Museum) suggest that Ericson & Hernandez Carrasquilla (1997), and re- breeding cormorants in Skåne and Blekinge be- sults from their study are summarised here. Cormo- longed to the subspecies Ph. c. sinensis. At Lake rant remains, found at archaeological excavations, Krageholm, and probably Skåne as a whole, the have been recorded from more than thirty places in cormorants seem to have disappeared at around southern Sweden, including the provinces of Bohus- 1881 due to destruction of nests and hunting (Nean- län, Skåne, Blekinge, Öland, Västergötland, Gotland der 1918). However, Ekman (1922) mentioned breed- and Uppland. The oldest bone remains are ca 9000 ing at Lake Krageholmssjön still in 1887. When the years old. Some of the remains are from nestlings cormorant disappeared as a breeding species from and juveniles (i.e. from Öland, Gotland and Upp- Blekinge is unclear, but colonies may have existed land) which indicates the existence of breeding until the early 20th century. Information from Vagn- populations at that time. Based on bone sizes, the sö (Hoby parish, Blekinge) (Neander 1918) suggests prehistoric Baltic cormorants appear all to have that cormorants may have bred there in the cavity of belonged to the nominate subspecies (Ph. c. carbo). an old oak-tree until 1909. Nevertheless, probably no breeding occurred in Sweden after 1909, at the latest, and the cormorant did not reappear as a Recent history breeding species in Sweden until the late 1940s. In the literature, the presence of cormorants in Swe- den was mentioned, probably for the first time, in 1555 (Olaus Magnus). Possibly, cormorants (sinen- Present distribution sis?) bred in Sweden at that time. Later, in the 1690s, The current distribution of Great cormorants in the famous naturalist Olof Rudbeck the Younger Sweden, as presented below, covers the period from observed cormorants on several occasions at an the 1940s through 1999. Some data are also given for expedition to northern Sweden (Brusewitz 1985). A 2000. If not otherwise stated, population size data cormorant in breeding plumage was also shot by this are from 1999. The number of breeding pairs for the expedition in the Bothnian Bay. Neither Olof Rud- different regions are presented in Table 1 and colony beck (1660–1740) nor Linnaeus (1707–78) men- sizes in Table 2. Table 3 shows the number of tioned any presence of breeding cormorants from disturbed and deserted colonies per province. Legal their journeys in Sweden, and most likely, cormo- disturbance of colonies usually means egg pricking rants did not breed in Sweden during the 17th and while illegal disturbance smashing of eggs, nest 18th centuries. destruction and killing of young. The size distribu- In the 19th century, cormorants are known to have tion of deserted colonies is presented in Table 4 and inhabited some lakes and coastal areas in the prov- the number of shot cormorants reported to the Coun- inces of Skåne and Blekinge (Ekman 1922, Anders- ty Administration Boards is shown in Table 5. son et. al 1984 and references therein). In Blekinge, colonies were known from Saltärna (Ronneby archi- Status in different provinces pelago), Dragsö (Hoby parish) and Tromtö (Karls- krona), and possibly they bred also at other sites. It Bohuslän and Halland is unknown when cormorants became established in Blekinge, but probably it occurred in the early 19th Along the Swedish West Coast cormorants com- century. The distribution of cormorants in Skåne menced breeding in 1995. Three small colonies are seems to have been somewhat more restricted than in currently known to exist: Flatskär (Askims fjord, Blekinge, and colonies are only known from Lake Bohuslän, 70 pairs), Soteskär (Bohuslän, 7 pairs) Krageholm and Lake Snogeholm.