Environmental and Salmo trutta complex aboundance data in the Tiber River basin (central Italy)

Salmonids inhabiting Mediterranean rivers are of particular concern for
biodiversity conservation, as they are threatened by various stressors,
including habitat alterations, overfishing, climate change, and
introgressive hybridisation with alien species. In the Tiber River basin
(Central Italy), genetic introgression phenomena of the native Salmo
cettii with the non-native Salmo trutta hinder the separate analysis of
the two species, which are both included in the S. trutta complex. Little
is known about the factors currently limiting the trout populations in
this area, particularly with respect to climate change. With the intention
of filling this gap, the aims of the current study were to: i) quantify
changes in the climate, and ii) analyse the distribution, status, and
ecology of trout populations, in the context of changing abiotic
conditions over the last decades. Fish stock assessments were carried out
by electrofishing during three census periods (1998-2004, 2005-2011, and
2012-2018) at 129 sites. The trend over time of meteorological parameters
provided evidence for increased air temperature and decreased rainfall.
Multivariate analysis of trout densities and environmental data
highlighted the close direct correlation of trout abundance with water
quality, altitude, and current speed. Climate-induced effects observed
over time in the sites where trout were sampled have not yet led to local
extinctions or distribution shifts, indicating a marked resilience of
trout, probably due to the buffering effect of intrinsic population
dynamics. Decreasing body conditions over time and unbalanced age
structures support the hypothesis that variations in hydraulic regime and
water temperature could overcome these compensatory effects, which may
lead to a severe decline in trout populations in the near future. In a
climate change context, habitat availability plays a key role in the
distribution of cold-water species, which often do not have the
possibility to move upstream to reach their thermal optimum because of
water scarcity in the upper river stretches.