Urban drainage systems as important bat hibernacula in Poland

The results of winter checks in drainage systems of three cities in northern Poland are presented. The analysis o f importance of this type of roosts for hibernating bats is conducted. In total 7524 bats were counted in the winter 2013: 3412 individuals in Olsztyn, 3403 in Piła and 709 in Koszalin. Higher number of bats than in rainwater sewers of Olsztyn and Piła was observed in Nietoperek Reserve only. Six bat species were recorded in checked rainwater sewage systems: Myotis nattereri, Myotis daubentonii, Myotis myotis, Plecotus auritus, Barbastella barbastellus and bats from Myotis mystacinus complex. Urban sewage systems are important especially for Myotis nattereri. This species dominates in all checked objects, with 3314 individuals in Olsztyn, 2532 in Piła and 597 in Koszalin. Given the sheer size o f the drainage systems, this type of roosts may belong to the most important hibernation sites, particularly for M. nattereri and M. daubentonii.

Anthropogenic transformation of the landscape makes bats adapt to deal with different types of objects in urban areas (Lesiński 2006), becoming an integral part of the cities fauna (Haensel 1982, Mickleburgh 1987, Legakis et al. 2000, Lesiński et al. 2001).Numerous aggregations of these mammals were found mostly in desolated military installations, cellars or mines (e.g.Mitchell-Jones et al. 1999, Rehak & Gaisler 1999, Hutson et al. 2001, Lesiński et al. 2004).These types of objects provide them with appropriate microclimatic conditions and are the main subject of interest for bat workers.Recently, hibernation of bats in underground drainage systems was increasingly observed in Poland (Grzywiński & Kmiecik 2003, Wojtaszyn et al. 2008, 2010, Koziróg et al. 2013) and in Ukraine (Godlevskaya 2007).
Drainage systems so far have not been considered as important for hibernating bats and then not covered by regular checks.The aim of tins paper is to show the results of the inventory carried out in drainage systems of three cities in northern Poland and to document the importance of tins type of roosts for bat hibernation.

S t u d y a r e a a n d m e t h o d s
The data used in the study were collected during the winter 2013 in urban drainage systems (rainwater sewers) of three cities in northern Poland: Koszalin (21.02.2013), Pila (25.01.2013) and Olsztyn (16.01.2013) (Fig. 1, Table 1).They refer to already published data about overwintering of bats in tins type of roosts in Poznań (Grzywiński & Kmiecik 2003), Koszalin (Wojtaszyn et al. 2008), Pila (Wojtaszyn et al. 2010) and Olsztyn (Koziróg et al. 2013) as well as to unpublished data from Pila.

D i s c u s s i o n
Results of tins study indicate that drainage systems are important objects for bats wintering in Poland.Till the winter 2013, hibemacula with the most numerous populations of wintering bats were: Nietoperek Reserve in Międzyrzecz Fortified Front -max.37693 individuals in 2008 year (Kokurewicz 2013), Michałowska Bastille (part of the Modlin Fortress) in Mazovia -max.3363 (Lesiński & Olszewski 2013), Szachownica Cave on the Wieluń Upland -2902 (Ignaczak et al. 2011) and the Citadel in Grudziądz -2435 individuals (Kasprzyk & Leszczyński 2008).Inspections made in January 2013 showed that objects in Olsztyn and Pila (Table 2) are the second and the third hibemaculum with the highest number of wintering bats in Poland.Rainwater sewage systems are important hibernation sites mainly for M. nattereri, and to a lesser extent, for M. daubentonii.The higher number of M. nattereri than in the drainage system in Olsztyn was reported only from the largest Polish bat hibemaculum in Nietoperek Reserve (T.Kokurewicz -pers.coimn).Rainwater sewer system in Pila is the fourth biggest hibemaculum of tins species in Poland, exceeded only by Nietoperek Reserve (mentioned above), Olsztyn (tins paper) and Michałowska Bastille (G.Lesiński, A. Olszewski -pers.coimn).In other big Polish bat hibemacula -the Citadel in Grudziądz (Kasprzyk & Leszczyński 2008) and the Szachownica Cave (Lesiński et al. 2011) less than 2000 individuals of M. nattereri were counted (Fig. 2).In explored sewer systems other species were observed much less frequently and accounted for a small percentage of bats (Table 2).Also data from sewer system in Poznań, suggest its importance only for M. nattereri and M daubentonii (Grzywiński & Kmiecik 2003).In contrast to objects surveyed in Poland, in the drainage system in Kiev (Ukraine), Godlevskaya (2007) found significant dominance of M. daubentonii -98.49%.Species composition of wintering bats in this kind of objects probably depend on the local structure of bat fauna, geographical location, and different microclimatic conditions.
Also in old factories, in installations similar to urban drainage systems, hibernation of bats was reported.In Police near Szczecin numerous bats overwinter in former factory of synthetic fuels in brick drainage collectors -where it was found maximum of 1682 individuals belonging to seven species (Dzięgielewska et al. 2007) and in the main collector of former ammunition factory in Krzystkowice near Zielona Góra -up to 1885 individuals, mainly B. barbastellus (Wojtaszyn et al. 2005, Wojtaszyn et al. in print -a).
Our results show that such systems belong to the most important sites of wintering for bats particularly for M. nattereri and M daubentonii.Hibernation of bats in many other urban drainage systems in Poland and Europe is very likely.
In terms of bats conservation, it is very important to take into account the presence of these mammals during the repair and maintenance of drainage systems.At the premises inhabited by bats, works should be done beyond the hibernation period and should not cause changes in physical conditions.We would like to thank all those who participated in fieldworks: Radosław Jaros, Dorota Wiewióra, Tomasz Potek, Bartłomiej Dryl, Kamil Kryza, Artur Stanilewicz, Rafał Ruta, Karolina Jamska.Separate thanks to Grzegorz Lesiński, Ireneusz Ruczyński and Eliza Głowska for their valuable comments and suggestions.

Fig. 2 .
Fig. 2. The maximal number of individuals o f Myotis nattereri in major hibemacula o f this species in Poland (data from drainage systems in black).
A c k n o w l e d g e m e n t s

Table 1 .
Human population, length of inspected rainwater sewer systems and location (UTM) of the cities where the study was conducted.

Table 2 .
Species composition, number o f individuals (N) and dominance (D% -Chiroptera indet.not included) o f bats hibernating in urban drainage systems o f three cities in northern Poland in 2013.