CORMORANT RESEARCH IN THE PO DELTA (NE ITALY)

Stefano Volponi

Department of Biology, University of Ferrara, Via Borsari 46, I-44100 Ferrara, Italy

The Po Delta (about 40,000 ha) is the more extended wetland system in Italy and the second delta in the Mediterranean basin. Since mid 1980s, it has represented a major area for the Great Cormorant Phalacrocorax carbo sinensis. At present, the Po Delta hosts respectively about 10% and 40% of the cormorants wintering and breeding in Italy.

In the Po Delta, research on the Great Cormorant started in the late 1980s. Stimulated by the growing complaints of aquaculture managers blaming for negative effects of cormorant predation on fish stocks and yields, research have first dealt with counts of wintering birds, monitoring of their distribution in the foraging areas, and preliminary assessment of their food (Boldreghini et al. 1993a, 1993b; Volponi & Rossi 1993). In the last five years, cormorant research have been intensified with the aims of better understanding the role of these top predator birds in the estuarine ecosystems (Volponi 1994). This note gives brief information about research on the Great Cormorant carried out in the Po Delta by the study group of the University of Ferrara.

Wintering cormorants - Counts at old and newly established roosts have been regularly carried out on fortnight or monthly basis according to season and roost size. After having increased 25-30 folds between 1982 and 1992 (from less than 200 to almost 4,800 birds in mid winter), cormorant numbers stabilised around a mean value of about 4,700 birds. The growth pattern closely followed a logistic model suggesting the carrying capacity has been reached (Volponi & Barbieri 1997).

Breeding biology - Numbers of breeding pairs and reproductive outputs have been monitored since first colony establishment in 1993. Breeding season may span over six months (egg-laying from early March to the end of July), but the bulk of nest occupation occurred during April-June. Clutch size ranged from 2 to 6 eggs (3.83 ± 0.65, n = 562) and breeding performances varied from 1.5 and 2.3 fledgling per initiated nest according to the colony and year (Volponi 1999). In 1999, a minimum number of 382 pairs (estimated on the basis of the higher number of simultaneously occupied nests) bred in four sites. In 2000, nest destruction has been carried out early in the season in the oldest and most important colony of Valle Bertuzzi. Here, however, about half of the initial number of pairs had a second very successful breeding attempt. Bird scaring and nest destruction in new and traditional nesting sites located in or close to aquaculture areas will certainly reduce the increase of breeding population but, likely, will not greatly effect the overall number of cormorants and the level of predation in the fishing valli from the late summer onwards.

Food choice - Diet composition has been mainly studied by the analysis of pellets collected at winter roosts and colonies. Additional data where gathered from freshly regurgitated fish and stomach contents. From the samples so far analysed we identified the remains of 16,000 prey belonging to 22 fish taxa (Volponi & Callegarini 1997, Volponi & Privileggi 1999). Sand smelt (Atherina boyeri) by number and Mullets (Mugilidae) by biomass formed the bulk of the Great Cormorant diet in the Delta. However, diet composition showed considerable variations during the seasons according to changes in prey distribution in the estuarine waters. Excluding cyprinids, all fish preyed upon by cormorants are euryaline species which undergo seasonal migrations from coastal sea waters, where they winter and reproduce, to the more productive brackish lagoons, which act as "nurseries" allowing better conditions for feeding and growing. Thus, during some period of the year, other species such as Sea Bass (Dicentrarchus labrax), Sea Bream (Sparus aurata), Eel (Anguilla anguilla) and Black Goby (Gobius niger) may represent a relevant proportion of the preyed biomass.

Ringing - This is a rather recent activity which started only in 1994 when the first cormorants born in the Po Delta successfully fledged (an earlier breeding attempt failed because all nests were destroyed to prevent colony establishment). In 1994-1996 we ringed about 130 nestlings with the standard metal rings, while in 1997-2000 we ringed 284 nestlings with metal and inscripted colour rings allowing individual bird recognition. Colour ringing would allow to study movements of individual birds between colonies and roosts. At present, little is known about the origin of the cormorants breeding in the Delta (are they from the inland colony of Val Campotto or from northern countries ?) or their behaviour during winter (do they stay or move from their birth colony ?). First sightings of colour ringed cormorants depicted a range of individual behaviours: while most of the birds seem to disperse locally and especially in the northern part of the Po Delta, some may undertake longer movements reaching northern Germany (soon after fledgling) or Tunisia for wintering.

Colour rings are orange with a black four letter inscription (the first letter is always an I). Please report any sightings to the Italian Ringing Scheme (Istituto Nazionale Fauna Selvatica, via Ca' Fornacetta 9, I-40064 Ozzano Emilia, BO, Italy; e-mail: infszumi@iperbole.bologna.it) which will provide detailed information about place and time of ringing.

Parasites - During stomach content analysis, the complete alimentary canal (oesophagus, stomach and intestine) of cormorants which have been drowned in fish net or shot under authorisation was searched for the presence of helminths. Preliminary data showed that thirty-one out of 35 examined birds (25 immatures and 10 adults) revealed the presence of parasites belonging to the phyla Platyhelminthes (n = 5), Nematoda (n = 27) and Acanthocephala (n = 14). In 34% of the infected birds, we recorded the presence of two (26%) or three (11%) parasitic species belonging to different phyla. Nematoda and Acantocephala were respectively identified as Contracaecum rudolphii and Southwellina hispida. The former was present in the 77% of the birds with a median intensity of infection of 79 specimens (range of 5-319); whereas the latter was found in 40% of the birds with a median intensity of infection of 13 specimens (range 1-50). Histological observations of the intestinal tract where S. hyspida was settled and inserted its trunk showed that this parasite cause several reactions of the mucosa and serious damage to intestinal tissues (Dezfuli et al. 1998). Prevalence of helminth infection in cormorants from the Po Delta is certainly high and stimulate further investigations to evaluate the role of these fish-eating birds in parasite life cycle and transmission as well as the deleterious effects of the parasite on its host.

Acknowledgement - During most of the work done in the field in the last years, our team was enthusiastically helped by several volunteers. We wish to thanks them all, especially Daniela Mengoni and Paolo Ronconi for their help in counts of wintering birds. Spandex Srl (e-mail: spandex@graff.it) courtesy provided PVC materials and Renzo Rusticali made colour rings. Nicola Baccetti (INFS, Ozzano Emilia, BO) kindly made available cormorant specimens shot under authorisation in the Ravenna district.

References

Boldreghini P., Montanari F.L., Santolini R. & Tinarelli R. 1993a. Insediamento del Cormorano Phalacrocorax carbo sinensis nell'area del Delta del Po. Supplementi Ricerche Biologia Selvaggina, 21:439-461.

Boldreghini P., R. Santolini, S. Volponi, L. Casini, F. L. Montanari & R. Tinarelli., 1993b. Variation of the use of foraging areas by a Cormorant wintering population: a case study in the Po Delta (northern Italy). Ekologia Polska, 45:197-200.

Dezfuli B.S., S. Capuano, M. Manera, C. Barbieri & Volponi S. 1998. Pathobiology associated with the acantocephalan Southwellina hispida in the alimentary canal of Phalacrocorax carbo (Aves). 3rd International Symposium on Aquatic Animal Health, Baltimore, USA.

Volponi S. 1994. Ecologia del cormorano Phalacrocorax carbo sinensis nel Delta del Po. PhD Thesis, University of Ferrara.

Volponi S. 1999. Reproduction of a Newly-established Population of the Great Cormorant in Northeastern Italy. Waterbirds, 22(2): 263-273.

Volponi S. & Barbieri C. 1997. Evoluzione della popolazione di Cormorano svernante nel Delta del Po. Atti IX Convegno italiano di ornitologia. Avocetta, 21:56.

Volponi S. & Callegarini P. 1997. Osservazioni sul regime alimentare del Cormorano (Phalacrocorax carbo sinensis) nel Delta del Fiume Po nella stagione 1993/94. Laguna, 97/1:32-39.

Volponi S. & Emiliani D. 1997. Population, habitat choice and breeding success of Cormorants nesting in the Po delta area. Suppl. Ric. Biol. Selvaggina, XXVI: 569-574.

Volponi S. and N. Privileggi. 1999. Diet and predation of great cormorants wintering in two areas along the northern Adriatic sea coast. XXIII annual Meeting Waterbird Society (Grado 8-12/11/1999). (Abstract).

Volponi S. & Rossi R. 1993. Primi dati sulla presenza e l'attività di foraggiamento del Cormorano nelle Valli di Comacchio (Delta del Po), nel corso degli inverni 1989/90 e 1990/91. Annali dell'Università di Ferrara, Sezione Biologia e Medicina, 3:1-13.

Stefano Volponi, Department of Biology, University of Ferrara, Via Borsari 46, I-44100 Ferrara, Italy (email: svolponi@racine.ra.it)