Environmental determinants of Great Cormorant (Phalacrocorax carbo) distribution in small man-made waterbodies – a case study of gravel pits in southwest France Fre´ de´ ric Santoul 1, *, Gilles Segura 2 & Sylvain Mastrorillo 1 1 Laboratoire d’Ecologie des Hydrosyste`mes, UMR C5177 CNRS/UPS, Universite´ Paul Sabatier; 118, Route de Narbonne, F-31062 Toulouse Cedex 04, France 2 ICHTHYO SYS Environnement, 29 Alle´e de Sauveterre, 31 280 Dre´mil Lafage, France (*Author for correspondence: Tel.: +33-05-61-55 65 75; Fax: +33-05-61 55 60 96; E-mail: santoul@cict.fr) Received 20 May 2003; in revised form 26 February 2004; accepted 4 March 2004 Key words: Great Cormorant, habitat selection, fish, disturbance, gravel pits, Phalacrocorax carbo Abstract In the Midi-Pyre´ne´es region (southwest France), the increasing number of gravel pits has allowed the wintering of numerous species of waterbirds such as Great Cormorants (Phalacrocorax carbo). The debate about cormorant predation on fish stock has been sufficiently strong to have resulted in reductions in cormorant numbers by control shooting. In this context, cormorants were studied during winters 1996/1997 and 1997/1998 at two gravel pit sites in the Garonne floodplain. Human disturbances and fish densities were found to be the main parameters determining the abundance of fishing cormorants. This work will help to prompt further research and the development of a management strategy for this species. Introduction Great cormorants (Phalacrocorax carbo) are important predators on fish and they have received considerable attention in the literature. Studies on the diet and distribution of cormorants have been done in particular during the breeding period (Warke et al., 1994; Veldkamp, 1995; Kirby et al., 1996; Musil & Janda, 1997; Grieco, 1999). Data from the non-breeding period have been published for the Camargue, in the Rhone delta, France (Im & Hafner, 1984), some water bodies in Switzerland (Suter, 1995) and the lake Ketelmeer in The Netherlands (Platteuw et al., 1992). Seasonal variations in the prey species were also studied throughout the year in Bavaria (Keller, 1998). Habitat use by cormorants has been of particular interest in the context of behaviour and diet (Voslamber et al., 1995). The cormorant was sel- dom seen in Midi-Pyre´ne´es in the early 1970’s (Affre & Affre, 1978). The wintering population increased from 40 birds in January 1970 to 2886 in January 2000 (BIROE counting). The increasing number of regionally wintering birds is highly correlated with the demographic increase of the Danish and Dutch colonies (Bousquet, 1992; Marion, 1997; Bregnballe et al., 2003). The peak of cormorants arrivals is regularly noted, in the Midi- Pyre´ne´es region, at the end of October (Bousquet, 1992; Santoul, 2000). Regular roosting places scattered along the Garonne river reflect the way the birds colonize the region. However, little is known about the factors determining the number of cormorant on gravel pits. This study investigates some aspects of habitat selection by cormorants in the Midi-Pyre´ne´es re- gion. We studied the cormorants in two pits from geographically distinct gravel exploitation areas Hydrobiologia 528: 179–186, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 179