Acta Herpetologica - 2006 - 1 - Reptiles in the diet of a «Oncorhynchus mykiss» (Osteichthyes: Salmonidae) naturalized population in Piedmont (N Italy)


Reptiles in the diet of a Oncorhynchus mykiss (Osteichthyes: 
Salmonidae) naturalized population in Piedmont (N Italy)

Franco Bernini 1, Alessandro Candiotto 1, Pietro Angelo Nardi 1, Simone Rossi 1, 
and Edoardo Razzetti 2

1 Dipartimento di Biologia Animale, Università degli Studi di Pavia, Piazza Botta 9, I-27100 Pavia 
– email: franco.bernini@unipv.it – 
2 Sistema Museale d’Ateneo, Sezione Museo di Storia Naturale, Università degli Studi di Pavia, Piazza 
Botta 9, I-27100 Pavia – E-mail: razzetti@unipv.it

Abstract. The authors report on predation by salmonid fish on some reptile species 
in two tributaries of the Tanaro river in the Alessandria province (NW Italy). The 
remains of Podarcis muralis, Anguis fragilis and of an undetermined colubrid of the 
genus Natrix were found in bromatological analyses performed on 117 Oncorhynchus 
mykiss specimens. Salmonid predation on herpetofauna once again confirms the ali-
mentary opportunism of these fishes; however, predation is an occasional phenome-
non and not a threat to the local reptile populations.

Keywords. Predation by fish, Salmonidae, Oncorhynchus mykiss, Anguis fragilis, 
Podarcis muralis.

While predation by salmonids and other fish on amphibian populations is already well 
documented (Hecnar & M’Closkey, 1997; Picariello et al., 1996), information about preda-
tion by fish on reptiles is scarce (cf. Eder et al., 1988). It therefore seems useful to report 
observations concerning this phenomenon obtained during a research about on the feed-
ing of Onchorhynchus mykiss (Walbaum, 1792) in the Northern Apennines.

The study was carried out in two tributaries of the Tanaro river, the torrents Lemme 
and Gorzente, which are entirely confined to the territory of Alessandria province.

These water bodies have a relatively small section (width 4-5 m) within the study area, 
which is located in the mountainous-hilly reach of the tributaries (300-600 m a.s.l.). The 
plenty of emerging rocks provides a tight link between the aquatic environment and the 
surrounding landscape.

Fish communities in the study area feature four species of cyprinids, one gobiid, and 
consistent salmonid populations following human introductions. In addition to brown 
trout (Salmo trutta) probably of Atlantic origin, there are two of the very few populations 
of rainbow trout (Oncorhynchus mykiss) naturalized in Europe.

Bromatological analyses were performed on O. mykiss specimens caught by elec-
trofishing in the Rio del Molino, an affluent of the Gorzente stream and in the Lemme 

Acta Herpetologica 1: 61-63, 2006



62 F. Bernini et alii

stream located at Molini near Fraconalto (Alessandria province). The samples were col-
lected from the beginning of February to the end of June 2001 and 2002. Seven fish spe-
cies were found (see Table 1). We also observed the herpetofauna present along the banks 
of the collecting place.

Andreone & Sindaco (1999) report six species of amphibians and twelve species of 
reptiles for the UTM square in which the study area is located (Table 2). We observed the 
following species during fish sampling: Bufo bufo, Rana temporaria, Anguis fragilis, Lacerta 
bilineata, Podarcis muralis, Natrix natrix and N. tessellata. R. temporaria had not previous-
ly been reported in this area; while the N. tessellata observation corroborates a historical 
record cited in Andreone & Sindacos’ atlas (1999).

A hundred and seventeen specimens of rainbow trout (75 from the Lemme torrent 
and 42 from Rio del Molino, total length 87-390 mm) were studied. Analysis of stomach 
contents revealed predation of aquatic macroinvertebrates between February and April; 
whereas in late spring (May and June) diet was mainly based on the organisms living on 

Table 1. Fish species found in the Lemme stream and in the Rio del Molino and relative abundance esti-
mates (x=present; xx=abundant¸xxx=dominant).

species Lemme Rio del Molino

Salmo trutta xxx xx
Oncorhynchus mykiss xxx xxx
Barbus caninus x
Barbus plebejus xx
Leuciscus cephalus xx
Telestes muticellus xx xx
Padogobius bonelli x

Table 2. Herpetofauna reported by Andreone & Sindaco (1999) for the UTM square in which the research 
area is located.

Amphibians Reptiles

Salamandra salamandra Anguis fragilis
Triturus alpestris Lacerta bilineata
Triturus carnifex Podarcis muralis
Speleomantes strinatii Chalcides chalcides
Bufo bufo Coronella austriaca
Rana dalmatina Coronella girondica

Zamenis longissimus
Hierophis viridiflavus
Natrix maura
Natrix natrix
Natrix tessellata (historical)
Vipera aspis atra



63Predation by Oncorhynchus mykiss on herpetofauna

the stream banks and surrounding environments. In addition to the terrestrial and ripar-
ian invertebrates which make up most of the diet, some reptiles were noticed: remains of 
Podarcis muralis, Anguis fragilis and of an undetermined colubrid of the genus Natrix were 
found in the digestive apparatus of four rainbow trout (see Table 3).

This finding confirms the already well-known fact (Pentelow, 1932; delmastro, 1981) 
that the diet of salmonids is opportunistic. The presence of reptiles in the diet of O. mykiss 
was only observed in the summer months, in correspondence with the activity period of 
these animals. The capture of reptiles by salmonids is an occasional phenomenon, not to 
be considered a threat to the populations living along the watercourse, unlike their preda-
tion on crayfish and amphibians, also noticed during our study.

REFERENCES

Andreone, F., Sindaco, R. (1999): Erpetologia del Piemonte e della Valle d’Aosta. Atlante 
degli Anfibi e dei Rettili. Monografia XXVI. Torino. Museo Regionale di Scienze 
Naturali.

delmastro, G.B. (1981): Contributo allo studio dell’alimentazione di Salmo gairdneri Rich. 
(osteichthyes, Salmonidae). Riv. Piem. Stor. Nat. 2: 71-78.

Eder, J., Eder, H., Schumacher, A., Schumacher, d. (1988): die Ringelnatter, Natrix n. 
natrix (Linnaeus, 1758) als Beute der Regenbogenforelle, Salmo gairdneri Richard-
son, 1836, und der Seeforelle, Salmo trutta lacustris Linnaeus, 1758. Herpetozoa 
1(1/2): 69-71.

Hecnar, S.J., M’Closkey, R.T. (1997): The effects of predatory fish on amphibians species 
richness and distribution. Biol. Cons. 79: 123-131.

Pentelow, F.T.K. (1932): The food of the brown trout (Salmo trutta L.). J. An. Ecol. 1(2): 
101-107.

Picariello, o., Scillitani, G., Viglietti, S. (1996): Prime osservazioni sulla predazione di 
Rana italica dubois, 1987 da parte di Salmo trutta L., 1758 nell’Appennino campano. 
Studi Trent. Sci. Nat. – Acta Biol. 71(1994): 197-200.

Table 3. data on predation by Oncorhynchus mykiss on herpetofauna (SVL =snout-vent length; TL = total 
length; W = weight).

prey SVL of prey (mm) date watercourse TL of predator (mm) W of predator (g)

Podarcis muralis 38 25/05/01 Lemme 250 177
Podarcis muralis - 02/06/01 Rio del Molino 179 56
Anguis fragilis - 02/06/01 Rio del Molino 189 70
Natrix sp. - 27/06/01 Lemme 244 175