Introduction of exotic fish species and decline of native species in the lower Po basin, north‐eastern Italy

Abstract

1. Freshwater ecosystems worldwide are experiencing native fish losses with severe threats to the conservation of freshwater biodiversity and ecosystem functioning, and the debate on whether the cause is biotic or abiotic disturbance is still open. 2. Temporal variation in fish assemblages was analysed over an 18 year period in 14 waterways of the lowland backwaters of the PoRiver in north-eastern Italy, which are important feeding, spawning and nursery sites for native fish. 3. In 1991, 14 native and eight exotic species were collected. In less than 20 years 10 native species underwent local extinction, three of which – Rutilus pigus, Rutilus aula, and Chondrostoma soetta – were endemic to the Padano-Veneto District in northern Italy. 4. Ordination of the data (MDS, CLUSTER, ANOSIM, SIMPER) showed a clear temporal gradient in fish community structure. After the establishment of the exotic predator Silurus glanis, some native species significantly declined in abundance and biomass (i.e. Alburnus arborella and Scardinius erythrophthalmus) or disappeared (i.e. Rutilus aula and Tinca tinca). Moreover, exotic species Cyprinus carpio, Ameiurus melas, and Carassius auratus from previous introductions, underwent significant changes in their abundance and biomass. No correlation was found between fish community structure and water quality parameters (BIOENV). 5. The success of exotic species, particularly S. glanis which thrived in this degraded habitat, seems to have led to the decline of native fish fauna in the canals of the lower portion of the Po River basin. Conservation strategies focusing on the containment of exotic species and habitat restoration are recommended

Long-term fish monitoring underlines a rising tide of temperature tolerant, rheophilic, benthivore and generalist exotics, irrespective of hydrological conditions

“…Besides being numerous, exotic species were particularly widespread and were previously found to be one of the most relevant factors negatively affecting the abundance and distribution of native fish communities of the lower Po River plain and its canal network (Castaldelli et al, 2013;Lanzoni et al, 2018;. However, due to the hydraulics of the siphons at Berra basin, some of these exotic fish have not been recorded in our data.…”

“…the deliberate or accidental release of an exotic species in a new area) have been recorded (Bianco, 1998(Bianco, , 2014 but the dynamics and outcomes of their invasions have been rarely thoroughly analyzed (with few exceptions, see e.g. Castaldelli et al, 2013;). Yet Italian freshwaters could be a prime model to investigate aquatic invasions: recent studies have underlined that hotspots of exotic fish invasions are present in the Po River drainage in Northern Italy and that in this area exotic species play a strong role in shaping the abundance and distribution of native communities even after several years from initial invasion .…”

Exotic species, rather than low flow, negatively affect native fish in the Oglio River, Northern Italy

“…glanis is a generalist predator and a successful invader thanks to adaptability to a wide range of prey and habitat conditions (Copp et al, ). Adverse effects on native fish populations in artificial or highly modified environments have been reported in the literature (Castaldelli et al, ), although these do not necessarily extend to all waters (Guillerault et al, ). It is widely documented that C .…”

“…Although longitudinal connectivity is an undeniable necessity for the fish community (Bunn & Arthington, ), its interruption can have counterintuitive effects. In the Oglio River, interrupted longitudinal connectivity paradoxically favoured some native species by blocking the invasion of exotics migrating upstream from the highly invaded Po River (Castaldelli et al, ). This is most evident in Figure , where the exotic community overtakes the native one downstream of the Isola Dovarese dam (the first impassable barrier, the second being a few km upstream, in Pontevico).…”

When alien catfish meet—Resource overlap between the North American Ictalurus punctatus and immature European Silurus glanis in the Arno River (Italy)

“…In the Florentine stretch of the Arno River, S. glanis has been reported by anglers since 2004 (Gualtieri & Mecatti, 2005). The increased investigation into invasive S. glanis revealed considerable positive economic (Cucherousset et al., 2017;Rees et al, 2017) but negative environmental impacts (Castaldelli et al, 2013;Copp et al, 2009). The increased investigation into invasive S. glanis revealed considerable positive economic (Cucherousset et al., 2017;Rees et al, 2017) but negative environmental impacts (Castaldelli et al, 2013;Copp et al, 2009).…”