Climate warming drives population trajectories of freshwater fish

Abstract

Climate change has emerged as a key threat to biodiversity, leading to broad-scale shifts in distributions of marine and terrestrial species as they attempt to track thermally suitable habitat. By contrast, our understanding of climate responses of freshwater species is relatively undeveloped, limiting our knowledge of whether projected warming will lead to freshwater biodiversity loss. Here, we linked a multicontinental database of riverine fish population abundance time series collected from 1958 to 2019 to temperature data from the same period. Across the sampled localities, waters warmed by 0.21 °C per decade (annual maximum of monthly temperatures). We tested whether fish responded to this change by i) increasing abundance at the cooler poleward limit of species distributions-predicted if warming has opened new opportunities-and ii) decreasing abundance toward the equatorward limit of distributions-predicted if temperatures have exceeded tolerance thresholds. We found that observed population trends were consistent with both of these expected patterns from climatic warming and that the trends were more pronounced in time series covering the longer time periods of 30+ y. The responses consistent with climate change were most evident in species with larger body sizes, higher trophic levels, river-sea migratory behavior, and more widespread distributions. Moreover, positive abundance responses to warming were more likely at higher altitudes where conditions tend to be cooler. These findings indicate that projected future warming will likely lead to widespread shifts in riverine community structure, including abundance declines at the trailing edge of species distributions.

Keywords: abundance; extinction; global heating; riverine; timeseries.

Fig. 1.

(A) The global distribution of the time series analyzed, (B) The taxonomic diversity of the population abundance time-series data analyzed; (C) change in average of the monthly maximum temperatures in a year (T_avg) and (D) change in maximum of the monthly maximum temperatures in a year (T_max), for sampled localities across the time period. Shaded areas indicate 95% CI. Images are from PhyloPic (https://www.phylopic.org/). The Lota lota image is by E. Edmonson and T.J. Bartley (https://creativecommons.org/licenses/by-sa/3.0/).

Fig. 2.

Abundance changes over the study period in response to increases in (A and B) the annual average of monthly maximum temperature (T_avg), and (C and D) the annual maximum of monthly maximum temperatures (T_max), using time series of different durations. (A and C) Modeled predicted probability of abundance change in response to warming temperature from linear mixed models, with shaded areas indicating 95% CI. (B and D) Density plots indicating the latitudinal distributions of populations showing an overall positive and negative response to warming during the time series. Gray shading shows an equal number of time series undergoing abundance increases or declines in response to warming; green or black shading indicates where abundance increases or declines in response to warming dominate, respectively.

Fig. 3.

Effect of elevation, body length, trophic level, river–sea migratory behavior, and geographic range on the population responses of freshwater fish populations to increasing stream temperature. (A–E) responses to T_avg – the average of monthly maximum temperatures in a survey year – for time series 10+ y in duration. (F–J) responses to T_max – the maximum of monthly maximum temperatures in a survey year – for time series 10+ y in duration. The three lines show the relationship between position in range and abundance change for the median value of the secondary variable and one quartile either side. Shaded areas represent 95% CI.