A Floating Platform: a Solution to Collecting Pellets ...

A Floating Platform: a Solution to Collecting Pellets When Cormorants Roost Over Water Author(s): Alessandra Gagliardi, Adriano Martinoli, Luc Wauters, and Guido Tosi Source: Waterbirds, 26(1):54-55. 2003. Published By: The Waterbird Society DOI: http://dx.doi.org/10.1675/1524-4695(2003)026[0054:AFPAST]2.0.CO;2 URL: http://www.bioone.org/doi/ full/10.1675/1524-4695%282003%29026%5B0054%3AAFPAST%5D2.0.CO %3B2

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A Floating Platform: a Solution to Collecting Pellets When Cormorants Roost Over Water ALESSANDRA GAGLIARDI, ADRIANO MARTINOLI, LUC WAUTERS AND GUIDO TOSI Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi dell’Insubria via Dunant, 3 – I-21100 Varese (Italy) E-mail: [email protected] Abstract.—Wintering roosts of Great Cormorants (Phalacrocorax carbo) on Lake Como, northern Italy, are often inaccessible. We describe an easy-to-build, cheap and efficient technique to collect pellets at inaccessible roosts, which enabled us to study the diet of cormorants without causing disturbance, or killing birds. A floating platform was built using iron bars, sea-plywood panels and plastic tanks which was placed on the lake’s surface, to collect pellets ejected by cormorants using a roost on the eastern shore of the western basin of Lake Como. During two winter periods (February 2001-March 2001 and December 2001-March 2002) 106 samples were retrieved from the platform, measuring 17.5 m2. Received 30 May 2002, accepted 25 July 2002. Key words.—cormorants, diet composition, floating platform, Great Cormorant, pellets, Phalacrocorax carbo, roost. Waterbirds 26(1): 54-55, 2003

In many places, fish-eating waterbirds are involved in conflict with professional and non-professional fisheries (Dirksen et al. 1995; Carss and Marquiss 1997). Predation on fish by the growing number of wintering Great Cormorants has been suggested as a possible cause of the decline of some local fish stocks that are of economic interest in Lake Como. Therefore, a study of the food of cormorants wintering on Lake Como was initiated. For many fish-eating birds, the analysis of pellets (indigestible residue), produced by adults at colonies or roosting sites, is the most convenient way to sample their diet (Martucci et al. 1993; Dirksen et al. 1995; Derby and Lovvorn 1997). It is relatively easy to obtain regurgitated pellets on land (Martucci and Consiglio 1991; Cherubini et al. 1997), but gathering cormorant pellets when the birds roost over water can be difficult. Winter roosts are often at inaccessible locations, such as on steep cliffs or trees overhanging water. In these cases, cormorant pellets, which are produced once a day (Veldkamp 1995; Zijlstra and van Eerden 1995), fall into the water, break up, and cannot be retrieved for analysis. Here we describe an easy-to-build, cheap and efficient floating platform, suitable for collecting cormorant pellets at inaccessible locations, in order to estimate the diet composition and daily food intake. This technique has the advantage

that the diet can be studied without disturbing or killing wintering birds. METHODS At Lake Como, northern Italy, a floating flat platform (Fig. 1) was built in February 2001 to collect Great Cormorant pellets. Lake Como, because of its glacial origins, has several areas with rocky cliffs. About 160 cormorants use a winter roost in a few trees over-hanging the water, 20 m above the surface. In order to sample pellets, a floating platform of 5 m × 3.5 m (17.5 m2) was built. The materials employed were: iron bars (L section; width: 30 mm, thickness: 6 mm) welded together to produce the framework, covered by sea-plywood panels (panel size: 1.65 m × 2.40 m). The whole platform weighed about 80 kg, and was kept afloat with plastic tanks that held the platform 30 cm above the water surface. At the platform edges, a plywood border 5-6 cm high was added to prevent pellets being washed away. The draining of rainwater from the platform was allowed by spaces (0.5 cm) between each plywood panel and between the panels and the borders. To facilitate transportation, the iron framework, which consisted of two separate parts, was assembled on site. The platform was anchored about three m off the shore, under the roost trees, by two chains (6 m length), which allowed the platform to float even when water levels changed rapidly. Each chain was attached to the shore. Anchor ropes (10 mm diameter) moored the platform to the bottom of the lake. From early February to end March 2001 and between December 2001 and March 2002, the platform was visited weekly to check its position and collect pellets.

RESULTS On the day after installing the platform, bird faeces were found on it, but no pellets. 54

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of pellets (106) were collected, despite a brief, 6-month study period; (2) it did not cause disturbance to the birds that occupied the roost at night; (3) the floating and anchorage systems were stable and kept the platform steady even in a deep water basin which was often exposed to strong winds; and (4) the framework of the platform could be easily assembled and dismantled, facilitating its use in several years and allowing rapid relocation when birds change roosting sites. Finally, the costs for building and pellet collection were low, both in financial terms and labor. ACKNOWLEDGMENTS We thank Carlo Romanò (fish-biologist) and the fish- and wildlife rangers of the “Amministrazione Provinciale di Como”, that helped constructing and launching the platform. LITERATURE CITED

Figure 1. Draw of floating platform placed at the base of the cliff. The cormorant roost is about 20 meters above the platform. A system of ropes connects the platform with the bottom of the lake, at 200 m depth.

At each of the following visits, between 5-10 pellets and a total of 106 complete pellets were collected over the study period. Although the pellets were ejected from about 20 m above the platform, they remained intact, allowing us to investigate daily food intake by wintering cormorants from the remains found in each pellet. The platform was built for a total cost of 300 Euro (about US $270). It required 30 work hours to build and install the platform. DISCUSSION The floating platform was successful as a means of collecting pellets from waterbirds roosting at inaccessible locations. Its main advantages are: (1) a considerable number

Cherubini, G., N. Baccetti and M. Bon. 1997. Cormorants Phalacrocorax carbo wintering in the lagoon of Venice, Italy. Ekologia Polska 45: 31-37. Carss, D.N. and M. Marquiss. 1997. The diet of cormorants Phalacrocorax carbo in scottish freshwaters in relation to feeding habits and fisheries. Ekologia Polska 45: 207-222. Derby, C. E. and J. R. Lovvorn. 1997. Comparison of pellets versus collected birds for sampling diets of double-crested cormorants. Condor 99: 549-553. Dirksen, S., T. J. Boudewijn, R. Noordhuis and E. C. L. Marteijn. 1995. Cormorants Phalacrocorax carbo sinensis in shallow eutrophic freshwater lakes: prey choice and fish consumption in the non-breeding period and effects of large-scale fish removal. Ardea 83: 167184. Martucci, O. and C. Consiglio. 1991. Activity rhythm and food choice of cormorants (Phalacrocorax carbo sinensis) wintering near Rome, Italy. Le Gerfaut 81:151-160. Martucci, O., L. Pietrelli and C. Consiglio. 1993. Fish otoliths as indicators of the cormorant Phalacrocorax carbo diet (Aves, Pelecaniformes). Bulletin of Zoology 60: 393-396. Veldkamp, R. 1995. Diet of Cormorants Phalacrocorax carbo sinensis at Wanneperveen, The Netherlands, with special reference to Bream Abramis brama. Ardea 83: 143-155. Zijlstra, M. and M. R. Van Eerden. 1995. Pellet production and the use of otoliths in determining the diet of cormorants Phalacrocorax carbo sinensis: trials with captive birds. Ardea 83: 123-131.