Главная Прочие Вестник охотоведения, 2014, Том 11. Номер 2. Июль
OF INVASIVE MUSKRAT IN LITHUANIA
© 2014 r. Gintare Skyriene1, Algimantas Paulauskas1, Alius Ulevicius2, Arunas Samas2
1 Vytautas Magnus University, Kaunas, Lithuania 2 Vilnius University, Vilnius, Lithuania
The changes of muskrat number, abundance and frequence of occurrence were very changeable during the whole acclimatization period (1954 - 2013) in Lithuania. The muskrat abundance indices decreased about 94% per 24 years (1989-2013).
Key words: muskrat, acclimatization, abundance, the decline of population.
The muskrat (Ondatra zibethicus) quickly colonized new areas after being introduced to Europe. However, following the successful widespread of this species, abundance of its population is very reduced during the period of the last 25 years in Lithuania. The first muskrats were introduced for financial reasons from Arkhangelsk in 1954 (82 individuals were released in the East part of Lithuania) (fig. 1). The muskrats were introduced to Arkhangelsk (1928- 1953) from Canada, Finland, England, Solovki Island, Vologda Region and others muskrats already had become local (Lavrov, 1957). The muskrat of 204 individuals (35 muskrat died during the transportation) were acclimatised in the East and South part of country from Kazakhstan after 2 years (in 1956). The animals were introduced to Kazakhstan (1944-1955) from Makushino, Kurgan Region,
Kyzylorda Region, Solovki Island, Arkhangelsk Region, Vologda Region and others muskrats already had become local (Lavrov, 1957). Totally, during the introduction of muskrat were introduced 251 animals in Lithuania. However, the muskrat origin are mixed and they are not the direct introducants from North America.
In the West part of Lithuania, the first muskrats have been reported in 1960 in Rusne, Silute district (Prusaite et al. 1988). The muskrat may immigrated from Kaliningrad Region (Slavsk and Polessk districts) in case, 114 muskrats were introduced to Slavsk district to Nemunas tributaries near Curonian Lagoon and 62 individuals to Polessk district and 26 - Ladushkin district in 1951 from Arkhangelsk (fig. 1). Totally, 202 muskrat were introduced to Kaliningrad Region (Lavrov, 1957).
Fig. 1. The introduction of muskrat into Lithuania and Kaliningrad Region, Russia (by Lavrov, 1957).
Fig. 3. Investigated long shoreline of rivers and Vistytis Regional Park
The muskrats have acclimatized well and the phase of quick exponential growth lasted until the late 1970s (Pmsaite et al. 1988). The peak number of muskrats was recorded reaching 40 thousand animals in 1971-1975 period in Lithuania. Later the population of muskrat has decreased to 15 or 17 thousand individuals (this population level remained from 1976 until 1988) and still is reducing (Ulevicius, 2001; Skyriene, Paulauskas, 2012) (fig. 2).
Fig. 2. The changes of muskrat number from the introduction to the present (Lavrov, 1957; Prusaite et al. 1988; Skyriene, Paulauskas, 2012).
The aim of this study was to examine the muskrat abundance tendencies and distribution in different regions of Lithuania.
MATERIALS AND METHODS
Muskrat abundance indices, frequency of occurence were estimated by counting activity signs (footprints, trails, faeces, nutrition squares) in 0.5 km and 1 km long river segments. The mean number of sign registrations/0.5 km of shore line was interpreted as the abundance index of muskrat. 69 rivers were assessed throughout all Lithuania. Four rivers Varene, Salcia, Sirvinta, Merkys (in the South part of Lithuania) were investigated in 5-10 km long river shoreline (Fig. 3). Investigations were carried out in spring, after spring freshets and before the beggining of intensive spring vegetation. The lodges of muskrat were counted in Vistytis Regional Park.
The retrospective study of abundance dynamics was based on Ulevicius and Balciauskas (2002), Ulevicius (2001).
Only local assynchronic populations of muskrat are observed which maintain the total number of population at a comparatively low level. Fourteen positive sites (out of 70) with muskrat activity signs were found. Only in three rivers muskrat abundance indices were relatively high, probably indicating local residential populations of muskrat. In the rest of positive sites (11 rivers), presence of muskrat was confirmed just by solitary signs, probably left by migrating non-residential animals. The occurrence and abundance of muskrat has decreased with time. In 2012-2013, the frequency of occurence was 20%, whereas in 1989-1993 this index was 70%. The abundance index showed a similar tendency: 0.5 in 2012-2013 and 8.6 in 1989-1993. The muskrat abundance has decreased by 94%.
Some local populations of muskrat once considered as relatively strong became extinct in the later years. Our data show that after more than 20 years the muskrats became extinct in the river of Salcia (relative abundance decreased from 16.8 tracks/0.5 km (in 1991) to 0.4 tracks/0.5 km (in 1999) to 0 tracks/0.5 km (in 2012)). Analogous changes were observed in the Varene river, where the abundance of muskrat was 17.8 tracks/0.5 km in 1991, 10.9 tracks/0.5 km in 1999 (the muskrat population decreased 61%) and no muskrat activity tracks were found in later years (2012-2013). In the river of Sirvinta, the muskrat abundance decreased by 94% after more than 20 years (table).
The muskrat abundance by counting muskrat lodges were monitored in water areas in Vistytis Regional Park, South Western Lithuania. The animals live in swamps in this territory.
Only 2 lodges were counted in 1996. After 4 years the muskrat abundance increased by 98% - 94 houses were counted.
Changes of abundance indices of muskrat in four rivers in Lithuania
However, after 13 years the muskrat abundance decreased by 86%.
In recent years, muskrats were abundant only in some rivers and their abundance has decreased with time. There is no common opinion what are the main factors determine shaping the abundance of muskrats. The muskrats are sensitive to changes in water level - hydrologic regimen (Brzezinski et al. 2010). This might be one of the main factors limiting muskrat abundance. In Lithuania, the last decade is observed high water levels in rivers which can damage the muskrat houses, flooded nests, reducing plant growth and forced to migrate. All of this may reduce the survival of muskrats, increase mortality and the fluctuation of population abundance. But if water level markedly drop then the animal becomes far easier accessible to predators. The main predator of muskrat is American mink (Holmengen et al. 2009) which live in the same habitats and can destroy the whole families of muskrat. Predation by mink can explain the number fluctuation of muskrat not only in North America but also in Europe (Pmsaite et al. 1988; Brzezinski et al. 2010). Recently, in Poland, the population abundance of muskrat has become very low, and in some areas close to extinction. It is believed that the main reason is American mink. American mink was introduced to Poland from 1980 and since then is observing decline of muskrat (Brzezinski et al. 2010).
The otter is also considered to be one of the muskrat predators (Priisaite et al. 1988). Otters are widespread in Lithuania (Baltmnaite et al. 2009) and can influence the muskrat population fluctuations.
The red fox can be important factor influencing the populations of muskrats in whole Europe (Danell, 1996). In Sweden, the foxes and minks are the main predators of muskrat (Danell, 1996).
Also, the factors limiting muskrat populations include availability of food, diseases, parasites, predators, intraspecific competition (including fighting) and exploitation of resources. Moreover, humans cause significant mortality of muskrats. These factors vary widely by area, season and muskrat population density present in the area (reviewed in Skyriene, Paulauskas, 2012).
This study was supported by the Research Council of Lithuania (grant No. LEK-14/2012).
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СНИЖЕНИЕ ЧИСЛЕННОСТИ ИНВАЗИВНОЙ ОНДАТРЫ В ЛИТВЕ
Gintare Skyriene1, Algimantas Paulauskas1, Alius Ulevicius2, Ariinas Samas2
1 Vytautas Magnus University, Kaunas, Lithuania 2 Vilnius University, Vilnius, Lithuania
Показатели численности, обилия и частоты встречаемости ондатры претерпевали значительные изменения в течение всего периода акклиматизации (1954 - 2013 гг.) в Литве. В течение 24-х лет (1989-2013 гг.) индекс численности ондатры снизился на 94%.
Ключевые слова: ондатры, акклиматизация, обилие, снижение численности.
УДК 591.69-82.252.2 (470.22)
Поступила в редакцию 03.07.2014 г.