US trout population set to decline

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Fly fishermen in the western United States have much to worry about, as the trout population there is expected to markedly decline by 2080 due to a mean habitat loss of nearly 50%.

The study is to be published in a forthcoming issue of the Proceedings of the National Academy of Sciences of the United States of America.

Seth Wenger and coauthors carried out the first study of its kind to examine the probable impacts of climate change on the common trout species of the western United States.

The authors used mathematical models and data on habitat characteristics and fish from 9890 locations in the Rocky Mountains and Great Basin to forecast the effects of climate change (increased temperatures and altered flows) by the 2040s and 2080s on four species of trout in the interior western United States (covering an area of approximately 1 million square kilometres).

The four trout species examined were the Brook trout (Salvelinus fontinalis), Brown trout (Salmo trutta), Cutthroat trout (Oncorhynchus clarkii), and Rainbow trout (Oncorhynchus mykiss).

After factoring temperature, flow regime (particularly flood seasonality), and biotic interactions, as well as topographic and land use variables, the authors obtained predictions for the four species that are far from rosy.  

Under the A1B emissions scenario, the authors forecast a further habitat loss of 58% for the Cutthroat trout by 2080 due to an increase in temperatures beyond its optimal range and continued encroachment from non-native species.

The habitats for the non-native Brook and Brown trouts are predicted to decline by 77% and 48% by 2080, respectively, driven by increases in temperature and winter flood frequency caused by warmer, rainier winters.

The habitat for the Rainbow trout is projected to decline the least (35% by 2080) because the negative effects of temperature are partly offset by altered flow regimes that benefit the species.

From a purely human point of view, this decline is expected to have significant socioeconomic consequences, as recreational trout fisheries are valued at hundreds of millions of dollars in the US alone. The ecological consequences of a shift in the dominant fish species also cannot be ignored, cautioned the authors.

For more information, see the paper: Wenger, SJ, DJ Isaak, CH Luce, HM Neville, KD Fausch, JB Dunham, DC Dauwalter, MK Young, MM Elsner, BE Rieman, AF Hamlet and JE Williams (2011) Flow regime, temperature, and biotic interactions drive differential declines of trout species under climate change. Proceedings of the National Academy of Sciences doi:10.1073/pnas.1103097108

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