Climate change will affect the brown trout

The Brown trout (Salmo trutta) of the Iberian Peninsula is found in the southern boundary of its natural distribution and thus, it is more susceptible to thermal changes. In order to study how these changes can affect the survival of this species, an interdisciplinary study was carried out and climatological, hydrological and ecological aspects were studied with the aim of developing an accurate methodology to be easily applied at greater scale.

The Cega stream and the Pirón stream were selected for this study. They are located at the left side of the Duero basin and both are representatives in this region. Thus, researchers carried out intensive samplings of the fish communities and their habitat in these two streams, a work commissioned by autonomous community of Castilla y Leon.

The climatic scenarios used in this study is called "Representative Concentration Pathways -- RCPs," which are simulations of models to develop climatic scenarios proposed by the Intergovernmental Panel on Climate Change (IPCC). Specifically, they used RCP4.5 and RCP8.5.

The thermal thresholds of trout population in the studied streams are lower than the temperature of physiological efficiency described in other studies with more suitable conditions. According to forecasts, the average temperature of air in the centre of the Iberian Peninsula will increase in summer between 3°C and 8°C by the end of the 21 century. Consequently, this will also affect the water temperature.

The number of days in which water will exceed the thermal thresholds will increase up to 13.9 times in the Cega stream in the worst-case scenario, the number of times that the threshold will exceed 7 or more days in a row will increase up to 2.9 times and the length of the events above the threshold temperature will increase up to 9.2 times. These increases in the Pirón stream will reach 8.5, 2.0 and 7.3 times the reference value respectively. According to researchers, this data "will mean a reduction up to the 56% of the thermal habitat of the trout population in the Cega stream and the 11% in the Pirón stream in the worst-case scenario."

This situation can be aggravated and to become critical due to the flow reduction caused by climate change and water extraction for irrigated plots that has increased for the last decades. Groundwater discharges provide rivers with certain resistance to thermal change, forced by the atmosphere-water energy balance, but the discharge temperature can also be affected by global warming by raising water temperature.

Trout populations have a relative thermal plasticity in early stages of their development and this could allow them to evolutionarily adapt to climate change if the change is not very sudden since this plasticity is limited. However, this capacity can be insufficient and must be proved.